Condiciones del PozoPROBLEMAControl del Pozo Sobrepresin y formaciones debiles TemperaturaFormaciones PermeablesRemocin de lodo Presin de Friccin Mezclabilidad/bombeabilidad Prdida de circulacinCondiciones anormales y especiales:Retrodegradacin Espumas Etc. DensidadTiempo BombeableEstabilidad del Fluido Control de filtrado Taponamiento/puenteantes Densidad Productos de Hidratacin Capacida de estabilizacin Tendencia a espumar PARAMETROS LECHADAReologaExtendedoresAgentes de pesoAceleradores Retardadores FLACDispersantes Agentes Gelificantes Silica Agentes espumantes & estabilizadores Antiespumantes LCM Extendedores CATEGORIA DEL ADITIVOSOLUCIONES{

Aditivos de cementacionAceleradores y RetardadoresCambian el tiempo de fraguadoAlteran la velocidad del desarrollo del esfuerzo compresivoExtendedoresReducen la densidad de la lechadaIncrementan el rendimiento de la lechadaAgentes de pesoIncrementan la densidadDispersantesMejoran la remocion del lodoMejoran la mezclabilidadReducen la presin por friccinControl del FiltradoMateriales para perdida de circulacion Especiales: Agentes despumantes/antiespumantes Agentes de Adherencia Aditivos expansivos Aditivos de control de mig. de gas, etc.Sistemas tixotrpicos

ConductoresPrevienen el derrumbamiento del Hoyo bajo el equipoEl tiempo prdido en el equipo de perforacin es ms corto Cemento de fraguado rpido - rpido desarrollo del esfuerzo compresivoLechadas aceleradas, e.g.:Cemento Puro + NaCl (D44) 3-5%Cemento Puro + CaCl2 (S1) 2-4%Cemento Puro + agua de mar

AceleradoresUsados para acortar las periodos I y II y acelerar los perodos III y IV - la hidratacin de los principales componentes del cemento se incrementa mas un cambio en la estructura del C-S-H gel Pueden usarse para compensar los efectos de retardacin de otros aditivos.S1 - CaCl2 - 1 a 4% BWOCChecar el tipo de CaCl2 - S1 es 77% puroCuando se mezcla en agua, se calienta.D44 - NaCl -

Cloruro de Sodio

Casing superficialesZonas no consolidadasSoporte de los BOPGrandes volumenes de lechadasLechadas ligerasLechadas pesadas en la zapataLechadas extendidas con lechadas de amarre , e.g.:Bentonita prehidratada 2-3% 12.8 ppgCemento Puro + 0.5-1% S1 15.8ppg

Densidad de la lechadaCambio en la densidad de la lechadaLigeraMAS AGUA* ABSORBENTEMATERIAL LIGERO BAJA DENSIDAD * D124 ES UNA EXCEPCION COMO EXTENDEDOR TANTO COMO EL CEMENTO ESPUMADOCem. Puro15.6 Class A 15.8 Class G 16.4 Class H

Extendedores de CementoBentonitaLITEPOZ 3 D35LITEPOZ 7 D61TXI Cement D911Trinity Lite-Wate Cement D49Diacel D D56Expanded Perlite D72Gilsonite D24KOLITE D42Sodium Metasilicate D79Sodium Silicate D75LITEFIL D124Foamed CementMicrosilicaDensidad de la lechada (lb/gal)Extendedor oSistema ligero6 7 8 9 10 11 12 13 14 15 151214.710.813.61214.213.711.91114.510.613.8121515121114.514.511.5912156151111.5

Clasificacin de los ExtendedoresExtendedores base aguaAguaArcillas (Bentonita) - D20, D128Extendores qumicos (Silicatos) - D75, D79

Solidos de baja densidadPuzzolana (Fly ashes) - D35, D56, D61, D602Kolite y gilsonite - D42, D24Perlita expandida - D72Ceniza silica (Microsilica) - D154, D155

Materiales de muy baja densidadNitrgeno - CEMENTO ESPUMADOMicroesferas de cermica - D124 (LITEFILL)

Extendedores Bentonticos

Extendedores QumicosSilicatos de Sodio y MetasilicatosReaccionan con los cationes en el sistema del cemento (Ca2+, Mg2+)Forma viscosa, silicato gelatinoso - Capaz de ligar agua extra - Baja separacin de agua libreReologas bajas para flujo turbulentoMejores propiedades y mezclabilidad que lechadas bentonticasNo controla la prdida de filtrado (usar D112 FLAC)Bajas concentraciones requeridas relativamenteCa - El silicato formado actua como aceleradorUsar retardadors , D110 (or D109)

D79, METASILICATO DE SODIO - seco ( 0.2-3.0% BWOC)D75, SILICATO DE SODIO - liquido ( 0.15 -0.60 gal/sk)

Agregados LigerosPUZOLANAS: Tierra Diatomasea (D61, D602) & Ceniza ( D35)Reaccionan con el hidrxido de calcio en el cementoResistencia a salmueras corrosivas ( Sulfatos )Baja PermeabilidadResistencia Termal KOLITE (D42) y GILSONITE (D24):Materiales a base de Carbn de piedra (D42) y Asfalto(D24) Materiales efectivos para prdida de circulacinKolite (D42) es inertePERLITA EXPANDIDA (D72)material inerte y no afecta en tiempo de bombeoNormalmente aadida 2 - 6% BWOC la bentonita previene la flotacionReduce la permeabilidad del cementoAcciones de puente a altas concentracionesMICROSILICA (Silica Fume, D154, D155):Material puzolanicoBuenas propiedades de la lechada

Extendedores UltraligerosMicroesferas de cermica, LITEFIL D124Microesferas de cermica o vidrioInertesRango de densidad: 8.5 lb/gal a 14.5 lb/galSistema de cemento espumadoNitrgeno inyectado en la lechada con espumantesMuy bajas densidades, > 6.0 lb/galBuenas propiedades mecnicas

Agentes AntiespumantesPorque usar agentes antiespumantes ?Prevenienen la gelificacin de las lechadasPrevienen la cavitacin de las bombasObtener la densidad real de la lechada mezclada a ser bombeadaPara mayor efectividad deber ser:Insolubles en el fluido espumadoSer ms activos superficialmente que el fluido espumado Mecnismo de accin:Se extiende sobre la superficie de la espuma y baja la tensin superficialEntra en la espuma reduciendo la pelcula y rompe la burbujaTipos de antiespumantesPolyglycol ethers Slido : D46 (0.2 lb/sk) Lquido : D47 (0.05 gal/sk)Silicones Lquido : D175 (0.01 - 0.02gal/sk) Lquido : M45 (0.05 gal/sk)

Casing IntermediosSecciones trabajables13 3/8 hasta 3000 ftCementaciones en 2 etapasBajo CostoLechada de relleno y de amarre, e.g.:Bentonita prehidartada 2-3%D75 0.28 gps + 1% S115.8 ppg lechada de amarreTodas las lechadas retardadasLechadas de amarre con control de filtrado.

Retardacin de los sistemas de cementoAplicacionesCasings intermedios y de produccinCementaciones forzadas y tapones de cementoAlta temperatura y profundidad

Clases de Retardadores QumicosLignosulfonatos(D13, D81, D800, D801)Acidos Hydroxycarboxylicos(D109, D110)Componentes Inorgnicos(D93, D74)Derivados Celulosos(D8)Retardadores mezclados(D28, D150, D121)Materiales especiales D177, D161 UNISLURRY(Mezcla de retardadores)

Mecanismos de RetardacinFactores que afectan el mecanismo de accinNaturaleza qumica del retardadorComposicin qumica del cementoTeoras de los mecanismos de accinTeora de AdsorcinTeora de PrecipitacinTeora de NucleacinTeora complexacinPosibles efectos negativos en las lechadasGelacinDispersinIncremento en el filtradoIncompatibilidad

Retardadores de cementoD13/D81D13/D81 con dispersanteD800/D801D800/D801 con D93/L10D110D110 con D93/L10D28/D150D28/D150 con D121D28/D150 con D93D74 - para RFC solamenteD161BHCT oFRetardador100 200 300 400 140100185125250250310300175300375220300350300400300100140450250100Fresca Mar 37% NaClXXXXXXXXXXXXXXXXXXXX

XX

XXX

Filtrado en la lechadas de cementoDefinicin:Filtrado (solucin acuosa) prdida en la formacinEnjarre depositado en la cara de la formacinPartculas de cemento dejado en el anularPorque el cemento pierde agua:Presin diferencialMedio Permeable (formacin)Agua/cemento relacin > a lo necesario p/hidratacinEtapas de la prdida de Fluido ( filtrado ):Prdida de filtrado dinmicaPrdida de fluido esttica

Efectos de la prdida de fluido en la lechadasTiempo bombeable y punto cedente vs concentracin de aguaDaos de la formacin por filtradoMigracin de gas a travs enjarres delgados y a travs de pobre calidad del cementoOtras propiedades: nRendimiento de la lechada nAgua Libre nTiempo Bombeable nSedimentacin nAdherencia nEncogimiento nEficiencia de remocin de lodon n n n n n nSe reducenSe incrementan

nHidrosttica(psi/ft) nDensidad lechada nViscosidad Plstica nPunto cedente nEsfuerzo compresivo

n n n n n

Tiempo de frague vs DensidadTIEMPO DE FRAGUEPunto cedente1604016.415.6Tiempo de Frague (min)Punto CedenteDensidad de la lechada (ppg)

Mecanismos del control de filtradoReduccion de la permeabilidad del enjarre del cementoPartculas de material rellenando los porosPartculas de polmeros taponando los porosPelcula de polmero sobre la partculas del cemento/porosCambio en la distribucin de las partculas del cemento con dispersantesIncrementando la viscosidad de la fase acuosaAdicin de polmeros solubles al aguaSolamente reduce la permeabilidad del enjarreEfectos pobres comparados con la reduccin de la permeabilidad

Aditivos para el control de FiltradoPartculasGeles / microgeles D20 / D300, D500Latex FLAC D600G (MT,AD,L), D700 (D134)Polmeros solubles en aguaDerivados de la celulosa D60, D59(MT,ND,S), D112 (MT,LD,S)Polmeros sintticos no inicos D159(LT-MT,AD,L), D160 (LT-MT,AD,S)Polmeros sintticos aninicos D603 (MT,ND,L), D143, D158 (MT-HT,HD,L), D156 (LT,AD,S)UNIFLAC (D167, D168) LT= Baja temperatura hasta 130 FMT=Media Temp. 130-230 FHT= Alta temp. sobre 230 FLD= Baja Densidad menor 15 ppgND= Densidad normal 15 a 16 ppgHD= Alta densidad sobre 16 ppgAD= Cualquier densidad 12.5-18 ppgL= Aditivo LiquidoS= Aditivo Solido

Efecto de los Dispersantes en la PFMecanismos de accin Dispersan los granos de cemento y mejoran el empaque --> reducen permeabilidad Floculado con sal ---> accin de taponamiento SIN DISPERSANTECON DISPERSANTEENJARRE EMPAQUE AL AZAR ALTA PERMEABILIDAD EMPAQUE ORDENADO BAJA PERMEABILIDAD

Limites de perdidas de filtradoValores Tpicos: (API, 1000 psi)

Prevencin de canalizacin de gas 30 - 50 ml/30 minCementacin de Liners < 50 ml/30 minCementacin de Casing 200 - >300 ml/30 minCementacin pozo horizontal < 50 ml/30 minCementaciones forzadasFormacin con K < 1 md 200 ml/30 minFormacin con K > 1 md < 100 md 100 - 200 ml/30 minFormacin con K > 100 md 35 - 100 ml/30 minLechadas de alta densidad < 50 ml/30 min

Casing de ProduccinAislar zonas de produccinCasing de dimetro pequeoCosto poco importanteBuena adherenciaBuen control de filtradoAltas presiones de friccinRemocin de lodo importante15.8 ppg densidad o masTodas las lechadas retardadas

DispersantesReologa de las lechadas de cementoVolumen de las partculas / Volumen totalInteraccin entre partculasReologa de la fase acuosa Cambiados por los dispersantes

Porque los dispersantes ?Reducen la viscosidad y el punto cedenteFacilitan alcanzar el flujo turbulentoReducen las presiones por friccinMejoran la mezclabilidad de la lechadaReducen el agua en las lechadas (densidades arriba de 18.0 lb/gal)Mejoran la eficiencia de los FLACs

Dispersantes Tipos: Superplastizados D65 D80 D604MD145 Plastizados Lignosulfonatos Cement retarders (D13 , D 81 , D800, D801) Adelgazadores de lodo Sales Inorgnicas y cidos D45, D121

Accin del dispersantePOLYCEMENTO C2SH- + Ca + - 03SC2SH - + Ca + - 03SDISPERSANTE MOLECULA La cantidad del dispersante absorbido depende de la concentracinLa superficie de los granos de cemento se convierten uniformemente negativosLas seales se repelen uno a otro ---> dispersin SO33SO33

Rango de trabajo del dispersante D80 en Cementos ETD vs DTDYP (lb/100ft2)2015105510152030510152030BAJODISPERSO250.000.050.150.250.200.10D80 (gal/sk)25SOBREDISPERSOYpAGUA LIBREVISCOSIDADVISCOSIDAD PLASTICA (cp)15AGUA LIBRE (%)D80NOTE: CON CEMENTO ETD A 186F

Densidad de la lechada LESS WATERCAMBIOS EN LA DENSIADA DE LA LECLIGERASPESADASMAS AGUA*MAS AGUAABSORBENTESMATERIAL LIGERO MATERIAL PESADODISPERSANTEBAJA DENSIDADALTA DENSIDAD* D124 IS AN EXCEPTION AS AN EXTENDER AS IS FOAM CEMENTCemento Puro15.6 Class A 15.8 Class G 16.4 Class HAGUAMENOS

Agentes de PesoRequerimentos Alta gravedad especfica Tamaos de partculas compatibles y distribucin (sedimentacin) Baja absorcin de agua (eficiencia) Disponibilidad y costo aceptable Pureza y consistencia del producto InerteAgentes de Peso Comunes CodigoAgenteSGAgua Adicional

D31BARITA4.220.024 gal/lbD76HEMATITA4.95 0.0023 gal/lb(D907CEMENTO3.200.0529 gal/lb)

DegradacinCerca de 230 oF BHST el cemento sufre:Una reduccin de su esfuerzoUn incremento en su permeabilidadDebido a cambios estructurales en C-S-H gelPrevenido por la adicin de 35 - 40% BWOC la silica reduce C/S proporcin de C-S-H gel)

D30 Arena Silica y D66 Harina Silica Tamao Partcula -US Mesh Adicin de agua Gravedad Especfica Aplicacin:: Alta Densidad Baja Densidad Problemas de sedimentacion Problemas de mezcla (Reologa) Usar cerca de 300F NOMBRED30D66ARENA SILICAHARINA SILICA70 - 200 10% 1.12 gal/sk 2.63PreferidaAlternativaAlternativaPreferidaAlternativaAlternativaPreferidaPreferidaAlternativaPreferida> 200 + 12% 1.34 gal/sk 2.63

Agente Antisedimentante D153Control del agua Libre y /o sedimentacinCompatible con todos los aditivos Dowell y cementosNo efectos significantes con las prop. de la lechadas , excepto reologa.Mezcla-seca o prehidratada ( preferido ), agua dulce o de mar.Rango de temperatura: menor que 302 F (150 C)Rango de concentracin: 0.1 a 1.5%BWOC (dependiendo de la densidad )

Nuevos FLACs D159/D160Control de filtrado ajustableEfecto de aceleracin a baja temperatura, menor a 140 FTemperatura de trabajo en agua dulce: hasta 230 FRango de densidad: 12.5 to 18 ppgRango de concentracin: D159: 0.3 to 0.7 gps (dependiendo de la densidad)D160: 0.5 to 1.5%BWOCSal arriba de 15%BWOWIncompatible con S1; resultados errticos con D110; mejor extendido con D20

Nuevos FLACs D300Control de filtrado ajustableBajo CostoNo efectos de retardacin a baja temperaturaRango de Temperatura de trabajo en agua dulce: arriba 230 FRango de densidad : 12.5 to 15.8 ppgRango de Concentracin: 0.35 to 1.1 gps (misma p/ cualquier densidad)Sal arriba de 10%BWOWCompatible con S1; mejor extendido con D75Incompatible con D80 y D20

Nuevo HT Retardador D161Retardador de media a alta temperaturaBajo Costo - reduce el WOCControlable y predecibles tiempos de fraguePruebas de laboratorio reproduciblesRpido desarrollo del esfuerzo compresivoMenor sensibilidad a pequeas concentraciones/errores de mezclaRango de temperatura de trabajo: 250 to 450 F (agua dulce)Rango de densidad: 14 a 18 ppgRango de concentracin: 0.6 to 2.5 gpsEfecto sinergtico con D158/D159Incompatible con D600 y D134

Student reference is Chapter 3 "Cement Additives and mechanism of action" of the Well Cementing Manual. A selection of Key overheads will be included as a handout for this module.The references for the Instructor are Well Cementing and the ASM Cementing. Much of the information used in the Teaching Guide is from the Cementing Additives section of the ASM and some of the overheads are either taken from or modified from the ASM as are some of the handouts.This module is intended mostly to cover the general theory behind additives without discussing specific additives. As handouts, the notes pages of these slides can be given together with the Dowell Product Catalogue for Cementing. For any further details, refer the students to the Cementing Materials Manual.Porque utilizar aditivos? Para adaptar as condioes da pasta as condicoes do poo. Existem mais de 150 aditivos de cimentacao, classificados em Dowell em 8 categorias.Without showing this overhead, write three column headers on the board.Well problem, slurry parameter and additive category/solution. Start with getting students to come up with well problems only. Then move to slurry parameters that an additive could alter, followed by naming the additive category. It is important that the class understand the basic need for altering cement properties. It is not feasible to buy/store in bulk all the different systems necessary to handle all well problems encountered. All the "root" well problems must be understood. If desired, This slide may be used or leave the one students created to one side of the board if practical and summarize.8 categorias o clasificaciones e terminos generais: Aceleradores, retardadores, dispersantes, controlador de filtrado, extendedores, agentes de peso, materiales para perdida de circulacion, aditivos especiales.This OH shows how additives are broken into 8 categories. This module will cover all but special cement systems. A few special additives will be discussed. Special cement systems will be covered in Module 110.Over 100 additives for well cements are available, either in liquid or powder forms. The cement additives used today can be generally split into eight categories as shown on the overhead. They will be discussed in the following sections of this module.Cements are used at temperatures ranging from below freezing in permafrost zones to 700 F (350 C) in thermal recovery and geothermal wells. Pressures encountered can range to more than 30,000 psi in very deep wells. In addition to severe temperatures and pressures, well cements may be required to deal with weak or porous formations, corrosive fluids, and overpressured formation fluids.The use of cement additives which modify the behaviour of a cement system allows successful slurry placement under most conditions, and with the majority of cements available today.Key Ideas:Over 100 cement additivesModification of neat cement for use under wide range of conditions8 main categories of additivesFor conductor casings, short rig downtime is essential which means adding accelerators to the cement.Acelerador va a jugar co el tiempo de bombeabilidad (tiempo que tiene el liquido para ser bombeado) Cuando este tiempo disminuye, necessita mas esfuerzo de corte para bombearlo. La mezcla cemento y agua es exotermica. Acelerador/retardador juega con el periodo de inducion. Acelerador hace con que este tiempo acabe antes.En las cartas de laboratorio hay el tiempo de bombeabilida y tiempo de transicion de liquido- solido. Reacion del yeso va a formar la estringita. Aceleradores son sales y van hacer que se liberem mas iones de calcio.Periodo de pre inducion es el periodo de contacto del cemento con agua. En este periodo es formado el gel CSH en los silicatos de calcio y forma Estringita en los Silicatos de Aluminio. Los dos productos iniben la hidratacion del cemento, en el periodo de inducion. Inducen que la capa de estringita se desintegre mas rapido. Con aceleradores agregamos mas Calcio al sistema, este va a precipitar y desarollar dureza. Cemento: CSH - 80% (C2S 20% - C3S - 60%) Estringita 15% Impurezas 5%.Cacl2 con mas de 6% tienee tanto calcio que puede fraguar instantaneamente.(FLASH SET)Cacl2 esta disponivel, puede ser mezclado en agua o cemento, es barato. D77 es la version liquida del Cacl2 disuelto en agua.Accelerators are used to shorten the setting stages of the hydration process (Stages I and II) and to accelerate the hardness process (Stages III and IV). The hydration of the main cement phases are increased: gypsum is consumed faster; the formation of the ettringite is enhanced; the portlandite is precipitated earlier. There is also a change in the C-S-H structure: the permeability is increased; diffusion through the protective layer is enhanced.Inorganic salts are generally accelerators of Portland cement, the chlorides being the most well known and most efficient.The best and most efficient is calcium chloride in its solid form. S1 is 77% pure CaCl2. It is used in concentrations ranging from 1 to 4%BWOC. One precaution must be taken note of - when CaCl2 is added to water, it will heat up the water by upto 10 to 20 degrees F. Sodium chloride is another well known accelerator but below 10%BWOW after which it is neutral. D44 usually has weaker action as an accelerator. D44 en concentraciones entre 10 - 18% no es neutro. Solamente hace el mismo efecto como si estuviera a 10%. Pero continua acelerando. Concentracion normal 5% BWOW. Siempre BWOW.Seawater can be used as an accelerator as it contains upto 3% NaCl plus some MgCl2 as well as other chlorides. The consistency of the seawater must be taken into account when lab testing - especially near river mouths as the concentration of chlorides can vary very much.The liquid form of CaCl2 known as D77 is equivalent to 3.5 to 4.5 lbs of S1. It is rarely used for economic reasons.Sodium chloride ( D044) can be used as an accelerator but it is not very efficient and calcium chloride should be preferred. It effects the thickening time and compressive strength development of Portland cement in different ways, depending on its concentration and the curing temperature - see overhead. Salt acts as an accelerator at concentrations up to 10% BWOW. Between 10 - 18% BWOW it is essentially neutral, and thickening times are similar to those obtained with fresh water. The addition of NaCl above 18% BWOW causes retardation.The optimum concentration for acceleration is between 3 - 5% BWOW.Seawater contains up to 25 g/L sodium chloride which results in acceleration. The presence of Mg ( 1.5 g/L) must also be taken into account - see next pages. It should be remembered that salt concentrations are quoted as %BWOW, by weight of mix water and not the more conventional BWOC. This is probably due to historical reasons and conventional chemical terminology and allows comparison with sea water. Salt is normally dissolved in the mix water but it is common practice to dry blend it with cement in the USA (e.g.. for Saltbond cement systems).For surface casings, we are expecting to pump large volumes of slurry which means that we would want to reduce costs (this should be a low cost casing) - by using extended slurries. At the same time, some support is required which means that at least at the casing shoe there should be a strong neat slurry.Los Conductores son someros y tienen baja temperatura por eso necesitamos acelerador para acelerar ese tiempo. Offshore es caro para tiempo de espera. Cemento necesita minimo 500 psi para ser perforado como hay que ser, com peso y rotacion.The first way to reduce the density of a slurry is to simply add more water. This is also the cheapest. The problem is that this extra water tends to separate and become free water which can cause channels within the matrix of the cement.There are two ways that this extra water can be used up or bound up in such a way that it is not free: 1) by adding some sort of absorbant material which will absorb the water, for example, bentonite, silicate gel; 2) by adding light weight material which are usually absorbant as well, for example, expanded perlite, pozzolans, etc..Principal uso de extendedores: Presencia de formaciones de bajo gradiente de fractura.Hay 14 tipos de extendedores, clasificados en tres principales grupos: Extendedores base agua: Materiales quimicos que agregamos y permite agregar mas agua al sistema. (relacion agua-cemento 36-44%). Con extendedores incrementamos la relacion agua-cemento en un 60-70%. Si no colocamos aditivos, va a generar mucha agua libre. Extendedor base agua va controlar esa agua adicional. Mas comun entre eses son las argilas: Bentonita o Atalpugita (D128). Diferencia entre los dos: Bentonita absorve agua y expande su tamano hasta 400X, se estuviera usando agua dulce. Se hay calcio, sodium o K, D20 no incha. Se usar agua del mar, usamos Atalpugita (Arcilla migratoria) Va ordenar de una forma de agujas/escamas, homogeneo y paralelo, mantenendo el agua internamente entre las escamas. (D20 1-20% BWOC / D128 1-10% BWOC) Lechada con BPH es mucho mejor que en seco. Cemento en seco tiene menos resistencia compresiva, porque bentonita reacciona con calcio y no va hidratar. Usar D20 no tratada para cemento. D20 tratada es utilizada en lodo.Extendedores Quimicos: Tiene reaccion quimica en el processo de mezcla. Silicatos, son dos: Metasilicato de Sodio D79 (Solido) y Silicato de sodio D75. Silicatos reaccionan con iones de calcio y forma gel que absorve extra agua. El Cemento en la hidratacion forma el CSH y Hidroxido de Calcio. El Silicato reacciona con ese gel.Materiales de bajo peso molecular: Grano de cemento tiene SG 3.15. Podemos mantener la misma cantidad de agua y substituimos cemento por material mas liviano. Entre eses estan las Puzzolanas. Baja densidad de material en seco y puede bajar densidade hasta un 12.5 ppg.Cement extenders reduce slurry density and lower hydrostatic and placement pressures during cementing operations. This helps prevent the breakdown of weak formations and loss of circulation. It may also allow the number of stages required to cement a well to be reduced. Greater economy is also achieved by the increased yield of the standard extended slurries, but this may not be true for certain extenders.The typical density ranges available for extended cements are shown on the overhead. The use of a particular extender depends on the type of job to be performed and the results in terms of set cement properties required. Decreasing the cement density will reduce compressive strength development and increase the permeability of the set cement. They are more susceptible to strength retrogression at high temperatures so as the temperature increases, the choice of extender becomes more limited. Extended slurries are primarily used as filler materials (lead slurries) but in some cases will be used to cover and isolate zones. They may be required because the formation fracture pressure is too low to support conventional weight cements. In such cases the more expensive extended slurries may be used to ensure that good mechanical properties are achieved. Thus, for the extenders shown on the overhead, even though the possible density range is large, the range over which the extender is actually used may be limited. Often more than one kind of extender is used in the same slurry and the density of the slurry can be reduced to below that obtained with just one extender. Key ideas: Extenders reduce cement density, increase yield and reduce cost. Choose extender to give required cement properties for the particular job depending on cost and logistics.Extenders can be classified into one of three categories, depending on the mechanism of density reduction/yield increase. These are:1. Water is the most common and cheapest cement extender . However, increasing the water content has detrimental effects such as reducing compressive strength, increasing permeability of the set cement and diluting additives. Chemical and clay-like materials keep the excess of water from separation from the slurry allowing solids to remain in suspension. The most common is bentonite, a clay mineral that has the property of expanding several times its original volume when placed in water. This increases the slurrys viscosity and its ability to suspend solids. Slurry density quickly decreases as concentration increases (up to 20 %BWOC) to the detriment of cement mechanical properties. Other water extenders are based on sodium silicate. This reacts with the calcium hydroxide in the cement slurry producing a viscous gel which then allows additional water to be added to the cement system.2. Low density aggregates have a density lower than that of cement (i.e.. average 3.15 g/cm3). The density of the slurry is reduced when significant quantities of such extenders are present. The most commonly used are pozzolans, finely-divided siliceous and aluminous materials. They are obtained from volcanic ash, diatomaceous earth and fly ash from coal burning power stations. In addition to reducing the slurry density, pozzolans also improve compressive strength by reaction with the free Ca(OH)2 which is liberated as the cement hydrates.3. Very low density materials include ceramic microspheres (D124) which are small gas-filled beads with SG = 0.7. Very lightweight slurries can be obtained without the addition of large volumes of water, and good mechanical properties are preserved, especially when compared to the more conventional extenders. Foamed cements using nitrogen gas or air are used for extremely lightweight cement systems, again with excellent mechanical properties. Key ideas: Categorize extenders by their mechanism of density reduction/yield increase Water extenders - allow addition of excess water Low density materials - density lower than cement Gaseous extenders - density much lower than cementBentonite is dry blended in concentrations up to 20% BWOC but is also prehydrated in the cement mix water where 1 %BWOC pre-hydrated bentonite is equivalent to 4% BWOC dry-blended. The API recommends that 5.3% BWOC additional water be added for each 1% BWOC (dry blended) bentonite; however, lab testing is necessary to find the optimum ratio for a particular cement and system. Above 6%, the addition of a dispersant is usually necessary to reduce slurry viscosity and gel strength. The purity of the bentonite is most important (as mentioned above). Uncontrollable free water and poor fluid loss control result from poor quality bentonites.Bentonite slurries are economical are widely used as lead or filler slurries for casing cementations (e.g.. Chevrons (Gulf) 16% gel + 3% salt slurries mixed at 12.6 lb/gal). Cement densities as low as 11.5 lb/gal can be achieved but mechanical properties will not be very good. Normal bentonite extended slurries are mixed in the range 12.1 - 14.5 lb/gal. Bentonite can also be used for lost circulation cement plugs where advantage is taken of its potential for high viscosity slurries.Key ideas: Bentonite is a water-based extender Clay structure absorbs water Different Bentonites for fresh water and sea water A slurry containing 2% BWOC pre-hydrated bentonite is equivalent to one containing 8% BWOC dry-blended bentonite. The presence of high concentrations of Ca2+ ion in the aqueous phase of a cement slurry inhibits the hydration of bentonite. Thus it is necessary to ensure complete hydration in the water. Complete hydration of a good quality bentonite occurs in about 30 minutes. (If the water is too hard and gelation does not occur, this can be remedied by the addition of 1 - 5 lbs/bbl mix water of soda ash). Once hydration is complete, the mix water can be kept for a very long period without any affect on slurry properties.Bentonite can be pre-hydrated in sea water or light brine, but salt inhibits the hydration, and the slurry yield is reduced. In such cases attapulgite, D128 is recommended. Unlike montmorillonite, it has a fibrous structure, and this must be broken before complete hydration is possible; thus shearing during mixing is required. Also, it imparts no fluid loss control to slurries. These problems can be overcome by pre-hydrating the gel in 50% fresh water and then making up to the total with sea water.Pre-hydrating the bentonite should not appreciably affect the slurry properties, e.g.. thickening time and final compressive strength. They will be approximately if the same amount of water is used. The viscosity of pre-hydrated bentonite slurries is greater than the dry blended for the same amount of gel and usually the same for slurries of the same density. Key ideas: 1% BWOC prehydrated = 4% dry blended A good Bentonite will hydrate in 30 mins Hydration of D128 in sea water requires shearingThe slurry density decreases and yield increases quickly with bentonite concentration. However, this is to the detriment of mechanical properties such as compressive strength and permeability, which do not improve. Thus such cements are less resistant to sulphate waters and corrosive fluids. Settling of cement particles is prevented as the clays form an intricate frame of flocculated colloidal particles in the slurry. Free water is reduced due to the absorbing action of the bentonite. The viscosity and gel strength of the slurry can be high (with consequently good solids suspending properties) and depends on the quality of the bentonite and the ions present in the mix water. Above 5 - 6% BWOC bentonite, dispersants may be required to improve mixability and rheological properties of the slurry, especially at higher weights (i.e.. greater than 12.5 ppg). These problems may be reduced in some cements when the lignosulphonate retarders such as D13/D81 are used. Thickening times can be rather erratic and lab tests should be run for all jobs.High concentrations of bentonite tend to improve fluid-loss control. This is not true for attapulgite extended slurries. The clays can be separated from water by filtration and build up a cake in front of permeable sections thus reducing water loss from the slurry. The bentonite must be fully hydrated. Bentonite is also an effective extender at elevated temperatures.The most commonly used water-based extender is bentonite (D20) which is a clay-like material essentially consisting of hydrous aluminium silicates. Bentonite, also known as gel, contains at least 85% of the clay mineral smectite (also called montmorillonite). For salt waters the use of attapulgite (D128) is recommended (see later). Two different types of bentonite are available,1. Premium Wyoming Bentonite : approved by API as an extender for well cement. This should be ordered for all cementing operations (specify non-treated bentonite referring to API Spec 13A, Sect.5).2. Beneficiated Bentonite: also known as peptized bentonite, to which polymers have been added to artificially improve hydration. Such bentonites are forbidden in well cementing operations (for spacers also) as they can interfere with other additives.

Silicate extenders react with calcium and magnesium cations in the cement system or with calcium chloride to form a highly hydrated, gelatinous calcium silicate gel. This gel is capable of binding very large volumes of water and its structure provides sufficient viscosity to allow the use of large quantities of mix water without excessive free water separation. This is completely different to the mechanism of clay extenders where water is absorbed. The gel contains hydrated calcium silicate which will accelerate cement setting and thus can reduce the efficiency of other additives such as retarders and fluid loss additives. The two most common chemical extenders are D75 and D079, the liquid and powder forms of sodium silicate. Sodium Metasilicate (D79), Na2SiO3, is dry blended with cement, usually in the range 0.2 - 3.0% BWOC. This gives a density range of 11.0 - 14.5 lb/gal. If it must be added to the mix water then calcium chloride should be added to ensure gel formation. For greater yields, more than 3.0% D79 can be used but additional calcium chloride should be added, the quantity never exceeding that of D79. Higher concentrations may be required if the product has been stored for a long period as it is anhydrous and additional water may have been absorbed.Liquid Sodium Silicate (D75), Na2O.(3-5)SiO2, is added to the mix water, usually in the range 0.15 - 0.60 gal/sk. If used in fresh water, or if calcium chloride is to be part of the cement system, then calcium chloride should be dissolved prior to the addition of D75. It is possible to control the density over a wide range by adjusting the quantity of mix water used. The calcium chloride concentration depends on the required density and BHCT. The higher the D075 concentration, the higher the S001 concentration required, and usually the lower the slurry density. It is most important that the D75 be intimately mixed in the mix water and be kept agitated before use to prevent precipitation. The longer the mix water is prepared before a job, the longer will become the thickening time. There is no need to add additional calcium chloride to a sea water system to provide extra cations. It has a large accelerating effect and therefore a powerful retarder is recommended. D110/D109 high temperature liquid retarders are commonly used. In addition to its extending properties, D75 has found many applications in solving numerous other slurry problems when added in small quantities, ie. usually less than 0.10 gal/sk. It will help prevent sedimentation, reduce free water, thus stabilizing slurries and improving rheology and fluid loss.Key ideas:Extender reacts with lime in cement or CaCl 2 to form calcium silicate gelAllows addition of extra waterSolid (D79) or liquid (D75) formsWill accelerate cement slurriesFly ash is obtained as a by-product of coal burning power stations. Diatomaceous earth (D602) (or Diacel D (D56)) is a very lightweight, high silica content pozzolan. The silica phases react with the calcium hydroxide in the cement giving a cementatious material. The addition of pozzolans to cement systems will increase their resistance to sulphate attack. The reaction between silica and the calcium hydroxide liberated during cement hydration leads to the formation of a stable, cementitious compound (calcium monosilicate) which is durable. This reaction also reduces the permeability of the set cement. Fly ash has a mean particle diameter close to that of the cement. The addition of fly ash to a system containing say D66 or 5% microsilica would therefore help prevent attack by carbon dioxide, etc..The lightweight particles are inert and do not react with the cement and are classified as extenders because they increase the slurry yield. They are primarily used for lost circulation where their bridging effect is beneficial. Care should be taken that concentrations used are not too high to plug mixing and float equipment.Key ideas: S.G.'s of aggregates less than cement - reduced density Bulk volume of slurry higher - increased yield Additional water required - increased yield, reduced densityPozzolan extenders are perhaps the most important groups of cement extenders. They are siliceous or siliceous and aluminous materials with little inherent cementatious value, but when in a finely divided form and in the presence of moisture, will chemically react with free calcium hydroxide liberated during the cement hydration to form compounds possessing cementatious properties. In addition to acting as an extender, they also react and contribute to the compressive strength of the set cement.There are two types of pozzolan, (i) natural (volcanic ashes and diatomaceous earth), and (ii) artificial (fly ashes).The Ca(OH)2 liberated when cement hydrates ( 22 lbs/sack), is fairly soluble and does not contribute to the set cement strength. In the presence of a pozzolan, the free Ca(OH)2 combines with the silica to form a secondary C-S-H gel which is durable and will resist expansion. Additional mechanisms of action by which pozzolans help sulphate resistance are thought to include (i) the instability of ettringite in the presence of pozzolans, and (ii) the higher compactness of pozzolanic cements.The water permeability of set pozzolan/cement systems is usually less than 0.001 md. This, in addition to the reduced free Ca(OH)2 content, improves the set cement resistance to attack by sulphate and corrosive fluids. Alkalis are also rendered less harmful due to an ion exchange process which occurs between the encroaching alkali fluids and zeolites in the pozzolan. Key ideas:Pozzolans most important group of extenders, either natural or artificialChemically react with Ca(OH)2 in cement slurry contributing to compressive strength of cementResistant to corrosive fluids (low cement permeability)Fly Ash and Diatomaceous Earth D602 (or Diacel D, D56) are the two predominantly used pozzolan extenders due to their availability and cost and acceptable cement properties.Fly Ash is the residue from power plants which burn pulverized coal. Its surface area is approximately the same as Portland cement. Its composition and properties can vary widely but it primarily consists of a glass (silica and alumina with some iron oxide, alkalis and magnesia) and also some quartz, mullite, hematite, marnetite and combustible matter. Type F fly ash is generally used with cement. Type C contains 10% lime which give it some cementatious properties (but care should be taken when used with cement as rheology will be affected and its properties depend on its source). 2% gel is usually used to reduce free water and because of the widely varying properties, mix water is quoted as a minimum and maximum in cementing manual. Fly ash is not recommended for use at temperatures exceeding 450 F (232 C). When used alone will not give a very lightweight cement; 13.2 to 14.1 lb/gal is the usual range.Diatomaceous Earth is composed of the siliceous skeletons of diatoms deposited from fresh/sea waters. Its main constituent is opal, an amorphous form of hydrous silica. It has to be ground to a fineness similar to cement to be used as an extender. Slurry properties are similar to bentonite slurries but with lower viscosity. Cost tends to be the main disadvantage but D056 is one of the most effective extenders at temperatures above 4509 degF..Blend preparation of pozzolans is based on the Equivalent Sack which is the weight of pozzolan that has an absolute volume of 3.59 gals (i.e.. equal to that of cement). Hence different pozzolans have different sack weights. Blends are designated by a ratio, e.g.. 25:75 means 25% of an equivalent sack of pozzolan to 75% of a sack of cement. For a sack of pozzolan with a sack weight of 74 lbs, this blend would contain 74 lb x 0.25 = 18.5 lbs pozzolan, and 94 lbs x 0.75 = 70.5 lbs cement, and the blend sack weight is 89 lbs. Additive concentrations should then be determined by weight of the blend mixture, i.e.. the 89 lbs and not 94 lbs.Key ideas:Two notation systems for mixing Pozzolan cements(1) Based on bulk volume(2) Based on equivalent sackNatural pozzolansFly ash : 13.2 to 14.1 lb/gal systemsDiatomaceous Earth (D602 or Diacel D D56) :11.0 to 13.0 lb/gal systems Lightweight particle extenders reduce the density of the slurry because of their low density with respect to the cement particles. They include the following: Kolite (D42): a powdered coal with SG = 1.30. It is a coarsely ground material which is primarily used for lost circulation and is not usually used as an extender. It is inert and has less effect than other extenders on the compressive strength. Its high melting point (1000F) allows it to be used in thermal well environments. Bentonite (2 - 8 %BWOC) is normally added to improve slurry properties.Gilsonite (D24): is a naturally occurring asphaltite material with SG = 1.07 and a wide particle range (up to 0.6 cms). Low density slurries with relatively high compressive strengths can be designed due to its low water requirement ( 2.0 gal/sk). At high concentrations mixability may be a problem; up to 50 lb/sk can be used to give densities of 12.0 lb/gal. Bentonite is often used. Above 300F the gilsonite will soften so should not be used. It is primarily used as a lost circulation material. Expanded perlite (D72): a crushed volcanic glass (76% silica) which expands when heated to the point of incipient fusion. Slurries with densities as low as 12.0 lb/gal are possible due to the bulk density of the perlite (7.75 lb/cf). Bentonite (2 - 4%) should be added to prevent segregation. The perlite is crushed at higher pressures (above 3000 psi) so lighter surface densities should be mixed. Silica fume or microsilica is a by-product of the ferrosilicon industry. Particles are glassy, amorphous microspheres with a mean particle size of 0.1 - 0.2 m (which is 50 - 100 times finer than cement or fly ash) and a very high surface area (15 - 25000 m2/kg). Because of its fineness and purity, it is the most reactive pozzolan material available and is an effective extender. It has a low bulk density (15 lb/cf) allowing cement slurries as low as 11.0 lb/gal to be achieved. Its high surface area allows large quantities of water to be added without creating free water problems or requiring the addition of bentonite. Moderate fluid loss control is possible as the extremely fine particles pack between cement grains and possibly reduce the permeability of the initial filter cake deposited. Thus thickening times are not extended as a result of having to add polymer fluid loss control agents. Another main advantage is the extended cements good compressive strength. The pore-blocking nature of the microsilica is also used in certain gas migration control cement systems.It is normally used as an extender in the concentration 15 - 28% BWOC. It is expensive when compared to other extenders but its cost can be reduced by blending the cement and microsilica with a second extender such as fly ash (same concentration as the microsilica). A dispersant or cement equalizer may be required to reduce gelling tendencies and improve rheology. Good thickening times with low consistencies are achieved. Due to its very fine size, microsilica may present difficulties in bulk handling. It moves very slowly in pneumatic systems and the particles float and dust easily. It can, however, also be used as a suspension in water (50% active component) and then added to the slurry mix water instead of dry blending.Key ideas: Reactive Pozzolanic material SG lower than cement, low bulk volume and additional water requirements allow density reduction and yield increase Improves cement propertiesDry-blended or slurried with waterLightweight aggregates add to the bulk volume of a slurry. They can be mainly divided between pozzolans and lightweight particles. They have a specific gravity less than the cement, a greater bulk volume and mostly require that more water be added to the system. This allows reduced densities and greater yields to be achieved.Pozzolans are widely available and provide economical slurries. They react with the free calcium hydroxide formed during hydration of the cement slurry forming a cementatious product which aids in strength development. They do not require as much water as bentonite and therefore their densities are higher. Below 140F (60C) the lime-pozzolan reaction is slow..

Conventional lightweight cement systems extended with chemical or lightweight particles allow slurry densities as low as 11.0 - 11.5 lb/gal to be prepared. However, at such weights, the properties of the cement may not be acceptable (except with microsilica systems). Unstable slurries with high free water and long thickening times are likely and compressive strength and permeability may be unacceptable).More expensive extended systems with improved cement properties are possible using glass/ceramic microspheres Litefil, D124 or foamed cement with nitrogen/air. These systems also allow much lower densities to be achieved while maintaining good cement properties and meet the criteria for a critical zone cement.Module 110 - Special cement systems will cover Ultra Light Weight slurries. Just mention their existence for this module.Antifoam agents are used with cement slurries to prevent excessive foaming problems during mixing which lead to pump cavitation and suction problems, and can cause slurries with erroneous densities to be pumped downhole.For an antifoam additive to be effective it must (i) be insoluble in the foaming fluid/slurry, and (ii) be more surface active than the foaming fluid. They produce a shift in the surface tension and/or alter the dispersibility of solids so that the conditions required to produce a foam are no longer present. The insoluble antifoam acts by spreading on foam surfaces forming a film with a lower surface tension. The shock generated by the act of spreading is sufficient to cause foam destruction. The antifoam will also enter the foam and cause thinning of the foam's surface film due to gravitational drainage causing eventual rupture to occur.Two classes of antifoamer are used, (i) polyglycol ethers and (ii) silicones. Very small concentrations are required; usually less than 0.1% BWOC.D47 liquid, and D46 powder, antifoamers are of the poly(glycol ether) family. Powdered antifoamers such as D46 are usually high surface area, highly absorbent inorganic filler materials (e.g.. attapulgite) that have been treated with liquid antifoam materials. Both antifoamers are best when added to the system before water is mixed with the cement. When used with latex cement systems much higher concentrations of antifoam are required. (i.e.. up to five times more than normal in some cases), D47 should be well dispersed in the mix water to be effective, especially at low temperatures. They are not as effective as defoamers.D144 liquid is a silicone based antifoamer and defoamer. Such antifoamers are very effective in all cement systems and can be added to the system at any time to prevent either foaming or destroy foams which have already been formed. It is normally used for special cement systems with inherently bad foaming problems such as SALTBOND and latex cement systems.M045 is another silicone based liquid antifoam which is not as effective as D144 and will not break foams.Key ideas:Antifoam: Prevent foam before it starts (D47, D46, M45)Defoamer: Also destroy foam after it occurs (D144)Small concentrations requiredIncrease concentrations for latex and salt-based cement systems For intermediate casings, the main reason for cementing is to separate the zones into workable sections and there may be various different technical reasons for using special slurries. The casing can be cemented in two stages.Due to the volumes being quite large and with some zones being weaker, we can expect to use an extended slurry with possibly some fluid loss. Cost is an important issue in most cases. Some retardation will be necessary.Tail slurries should be used to support the casing and the shoe and any possible liners that may set later on. Tail slurries will be neat cement with some retardation and possibly some fluid loss control.The details of the mechanisms of action are discussed in the next pages. Key ideas: Act to delay hydration of cements Depends mainly on cement, density, temperature and other additives. Many retarder classes availableLignosulphonate derivatives are the most commonly used retarders, being primarily the sodium and calcium salts of lignosulphonic acids. These are polymers (MW = 20,000 - 30,000) derived from wood pulp. They are usually unrefined and contain various amounts of saccharide compounds. Refined lignins have a more predictable behaviour and are less sensitive to changes in cement composition and concentration variations. The higher temperature range retarders (D800/D801) are similar to D013/D081 but the lignosulphonates have been treated to improve temperature stability.Pure lignosulphonates have little retarding power. The retarding effect is due to the presence of low molecular weight and complex structures of carbohydrates and aldonic acids. They are effective with all classes of Portland cements; their normal usage range is 0.1 - 1.5% BWOC.Their mechanism of action is a combination of the adsorption and nucleation theories. The C3S hydration kinetics are mainly affected, and to some extent that of the C3A also. Such retarders perform best with low C3A cements.Their effective temperature range can be extended when blended with retarder aids such as sodium borate. At the higher limits of their ranges, the D13/D081 and D800/801 series may cause gelation with some cements, and dispersants will be required. This will also tend to increase the thickening time. When D800/D801 are used with salt slurries it is important that the salt be added after the mix water has been prepared or the retarder will precipitate out. Key ideas:Lignosulphonates most commonly used retardersHigh molecular weight polymers (ex-wood pulp)containing low molecular weight carbohydratesEffective with all Portland cements, low-medium temperature application. More efficient with low C3A cementsRetardation due to combination of adsorption and nucleation theoriesThe hydroxycarboxylic acid group of retarders contain hydroxyl and carboxyl groups in their molecular structures. The main retarders in this family are the gluconate and glucoheptonate salts which offer a powerful retarding action. They can easily cause over-retardation at BHCTs less than 200F (93 C). The main retarders of this group are the water solution of gluconate salts, D109 and D110; the D110 is a 50% solution of D109 to give increased sensitivity. These are classified as high temperature retarders. D109 is presently in the process of being obsolete.Their retarding action is generally attributed to hydroxycarboxylic groups which are capable of strongly chelating a metal cation such as Ca2+. Stable rings are formed which partially adsorb onto hydrated cement particles and those complexes which remain in solution poison nucleation sites of hydration products. Key ideas:Powerful, high temperature retardersHydroxycarboxylic acids (gluconate/glucoheptonate salts)Retardation due to nucleation theoryTo provide retarders for well cements which are effective at higher temperatures (i.e.. higher than 220F (104C), it is necessary to use blends of the various retarder compounds. The most well known of these is the high temperature retarder D28. It is a blend of a lignin amine and a sodium organic salt (sugar derivative). These are easily identifiable in D28 as two different coloured powders. Because it is a blend and such a strong retarder, care must be taken when using samples for lab testing as it may be impossible to get a consistent and representative sample. It is recommended that the two components be used separately when performing laboratory tests. A liquid form of D028 has been introduced, D150, where 0.25 %BWOC D028 is equivalent to 0.10 gal/sk D150.It works by the sugars and sugar acids scavenging or complexing Ca2+ ions from the cement interstitial water delaying setting. It also acts to poison hydration product nucleation sites.D121 is primarily a dispersant which has good retarding properties. It is a mixture of a lignosulphonate and a gluconate (a hydroxycarboxylic acid salt). It is widely used as a retarder at higher temperatures and as a retarder aid to improve the effectiveness of D028 in the temperature range 300 - 350F (149 - 177C).Borax, or sodium tetra decahydrate, (D93) is used primarily as a retarder aid and has the ability to extend the effective temperature range of most lignosulphate retarders up to 600F (315C). Its disadvantage is that it will adversely effect fluid loss. Why it works is still unknown but it is thought that it stabilizes the retarders at the higher temperatures. Borax is an inorganic retarder.Key Ideas:Lignosulphonate + other material blendsPowerful retardersRetard by adsorption and nucleation theoriesRetarder aids - D093, D099, D121Retarders inhibit hydration and delay setting of cement thus allowing sufficient time for slurry placement in deep and hot wells, or those applications which require long working times. In addition to retarder additives which are added to cements, retarded cements are available. These contain lignins, gums, starches, etc.. which provide the retarding action. However, these may not be compatible with cement additives and the setting times may be difficult to control consistently. Hence the development of Class G and H cements which are not allowed to contain introduced materials. Cement setting times are originally controlled when the cement is manufactured; i.e.. by grinding to a particular particle size, controlling the composition and the cooling rate of the clinker. Additional retardation required has then to be done in the field using chemicals. The technology of retarders is well developed and several types are used depending on the range of temperature application. The main groups are shown on the overhead and are discussed later in detailWhy they work is something of an enigma, although several theories have been proposed. Both the chemical nature of the retarder and the cement phases (silicate or aluminate) must be considered.Four principle theories have been proposed to explain the mechanism of set retardation of Portland cement. These are:Adsorption theory:Retardation is due to the adsorption of the retarder onto the surface of the hydration products, thereby inhibiting contact with water.Retardation is due to the adsorption of the retarder onto the surface of the C-S-H gel hydration product formed around the grains of C3S rendering it hydrophobic. It is also possible that the retarder changes the morphology of the layer giving it a more impermeable structure. The hydration process is then prolonged.The hydration products of C3A have a much stronger adsorption effect than C3S. At C3A sites the retarder concentration in solution will be reduced much quicker and there will be less in solution to retard hydration with other phases. Overall the retarder efficiency will be reduced, especially in cements with a higher C3A content.This mechanism of retarder action is applicable to the lignosulphonates, cellulose, saccharin and organophosphate families of retarder. Key ideas:Retarder adsorbed onto hydrating cement grainsContact with water is inhibitedFurther reaction between cement phases and water slows down, i.e.. retarded.Precipitation theory:The retarder reacts with calcium and/or hydroxyl ions in the aqueous phase, forming an insoluble and impermeable layer around the cement grains. Key ideas: Retarder reacts with ions in aqueous phase Impermeable layer formed around cement grains Further reaction between cement phases and water is retardedNucleation theory:The retarder adsorbs on the nuclei of hydration products, poisoning their future growth.Complexation theory: Calcium ions are chelated by the retarder, preventing the formation of nuclei.Nucleation and complexation is thought to be applicable to the hydroxycarboxylic acid family of retarders and to play a role in other retarders actions. The hydroxycarboxylic acids or lignosulphonates attach themselves to calcium anions and inhibit nucleation and growth. It is possible that all of the above effects are involved to some extent in the retardation process. The predominant factor depends on the type of retarder used and the cement phases upon which the retarder acts.Lechada tiene que tener suficente tiempo para bombear y desplazar, mas um tiempo de seguridad de 1 hora. Cemento y agua a 120 oF el TT es de 1 hora e media. Temperatura es el principal factor que afecta la hidratacion del cemento.Diferentes tipos de retardadores: Lignosulfonatos son los mas comum. (Usados e lodo como dispersantes) Ese es el efecto secundario. D800 es lignosulfonato depuess de retirado las impurezas, porisso aguanta un poco mas de temperatura. D81/D13 contienen impurezas. Todos los retardadores usados con dispersante aguantam mas temperatura.D74 es usado exclusivamente para sistema tixotropico.Celulosicos tienen propriedades de controlar filtrado.Mezcla de retardadores: Derivados de sacarina. Azucar es el mas poderoso retardador.Teoria de retardacion: Selecionar el retardador depende del tipo de cemento utilizado (composicion quimica). Depues en base a temperatura.(Ver tabla). Retardadores tienen 4 teorias para explicar como trabajan (teorias no comprovadas): 1) Adsoro, se adere al gel CSH y Estringita y el agua tiene que pasar por la pelicula de retardador y de gel. Sabemos que algunos retardadores se quemam cuando expuestos a temperatura por alto periodos, pero no est claro porque el retardador deja de funcionar (porque el cemento que retorna a superficie fragua? No sabemos2) Precipitacion: retardador reacciona con Calcio y Aluminato y se precipita. El precipitado va hacia superficie del grano de cemento inhibiendo la hidratacion. 3) Nucleacion: El retardador es absorvido por el nucleo del CSH/Estringita y va inhibir que el nucleo continuie cresciendo. Retardador va al nucleo de los sub-produtos y nucleo no cresce, assim no hay desarollo compresivo. 4) Complexacion: Retardador va sequestrar iones de calcio (nada que ver con el cemento), no dejando ellos se agruparen y no hay desarrolo compressivo, cemento no queda solido, sigue como liquido.Lignosulfonatos trabajam por adsorcion.Efectos secundarios de los retardadores: Cuidado con gelificacion, gelificacion puede ser causado por mala seleccion de retardador. Lignosulfonatos causan efecto de dispersion. Celulosico tiene poder de controlar filtrado.When a cement slurry is placed across a permeable formation under pressure, a filtration process occurs. The aqueous phase of the slurry escapes into the formation leaving cement particles behind in the annulus and forming a filter cake at the permeable formation face. This process is called fluid loss. Fluid loss is normally considered as a two-stage process, firstly the dynamic stage when the slurry is in motion and being placed in the well, and secondly, the static stage after placement (WOC or shutdowns during the job).To maintain adequate slurry performance, fluid-loss rates of 20 - 300 mL/30min are required. To reduce the fluid-loss rates of the 1,500 mL/30min normally obtained with neat slurries to acceptable levels, then fluid-loss control agents are added to cement systems. The higher the water-to-cement ratio of a cement slurry then the more likely will be fluid loss and acceptable criteria may have to be different to conventional slurry weights. Fluid loss increases almost linearly with temperature. At higher temperatures fluid loss is difficult to control and special additives must be used. Also the retarders required at such temperatures can interfere with, and damage fluid- loss control. Uncontrolled fluid loss may cause problems during or after the execution of a cementing job. If the fluid loss is high then:1. The slurry density may increase beyond an acceptable level during placement. This density increase may become very important when the area of the permeable formation is large and the contact time is long. Also, as the slurry density changes, so does its other properties (rheology, setting time, etc.).2. During static periods such as WOC, annular bridging may occur. This is a local process and is more likely to occur in a narrow annulus and/or restrictions. 3. Although the contact time of cement slurries compared to drilling fluids is short, damage to sensitive zones can occur if fluid loss control is not achieved. The filtrate has a high pH (12 - 12.5) and contains many ions (70% SO2-, 15% OH- and 12% Ca2+) which can be responsible for permeability impairment especially in shale and clay mineral formations. When water filters into a formation, some of the additive will do so too. It is thought that retarders and dispersants can be harmful. This may be a problem in fractured formations where the filtrate can enter deep into the formation. If the depth of invasion of the filtrate is short enough that perforations will extend beyond it, then no problems should be expected; this is normally the case.Cement particles do not endanger the formation permeability because even highly porous formations are able to retain enough particles to build a filter cake rapidly.Key ideas:Filtrate lost, filter cake remainsDifferential pressure, permeable mediumTwo stages, dynamic - vs - staticThe result of fluid loss affects all cement slurry properties. The change in the water to cement ratio affects many properties of a cement slurry and the set cement. Fluid loss control is important to maintain the water to cement ratio as close as possible to that designed in the lab.The influence on thickening time and yield point is described in the next slide by the graph.Some formations can be damaged by the high ph filtrate.Gas migration could occur through the very highly permeable filter cake that is formed by a high fluid loss slurry as well as through the poorer quality set cement.The change in water to cement ratio is mainly due to the reduction in the water content of the slurry as filtrate to the permeable zone(s). Under dynamic conditions, when the filter cake does not grow, solid particles will remain in the moving slurry and thus increase the solid content. The slurry volume is therefore decreased and density increased (sometimes called slurry densification). This in turn affects hydrostatic pressure calculations and perhaps will require that an increased slurry volume be pumped to ensure coverage.Plastic viscosity and yield point increase, raising friction pressures and BHPs. This may require a reduction in pump rates with the adverse consequence that turbulent flow will not be achieved and mud removal impaired. In principle the increase in solids content will improve settling tendencies and compressive strength. Bulk shrinkage is decreased by an order of magnitude.Some FLACs have a strong retarding effect on setting times. They adsorb onto cement grains and inhibit the cement hydration process. If fluid loss is not well controlled then the thickening time will be reduced due to a decrease in the water-to-cement ratio.Lead slurries will experience the highest fluid loss (they contain the most water, must deposit the filter cake, and will be exposed for a greater length and time to the formation). At first the solid fraction may not increase as filter cake is being deposited but once equilibrium is reached and the cake stops growing, then setting times may be affected as the solids content increases. This may increase the chances of annular bridging. These reasons are a strong argument for the use of fluid-loss additives in lead slurries.Key ideas:Main effect during dynamic stageWater - to - cement ratio reducedMust maintain slurry properties, especially for critical jobsAs the slurry loses water, the density increases. It is well known that a higher density slurry will have a shorter thickening time.As for yield value, an increase in density will cause an increase in yield value. The influence may be small at lower increase in densities but can become dramatic at higher densities.The exact mechanisms by which fluid-loss control additives work are not fully understood but several processes are known to occur. Once fluid loss starts across a formation, a filter cake of cement solids is deposited on the formation surface. Fluid-loss additives decrease the filtration rate by:Increasing the viscosity of the aqueous phase which reduces the rate of filtration through the filter cake according to Darcies Law. This primarily applies to the water soluble polymer type fluid loss control agents but is not the main factor affecting fluid loss.Reducing the permeability of the filter cake. The fluid loss control additives work by blocking the pores of the cement filter cake thus sharply reducing its permeability. This can be due to either:mechanical blockage of pores by particulate materials (e.g.. bentonite and lattices), orblockage of pores by hydrated polymers which are adsorbed on filter cake particles. Above a minimum FLAC concentration the molecule chain concentration is sufficient to allow overlapping and interaction of chains. Therefore the blocking and plugging of pores is more efficient once the minimum concentration has been exceeded.Dispersion of the cement slurry improves fluid loss. Some fluid-loss control additives require that a dispersant be present in order for them to be effective and others have their efficiency much improved. Slurry deflocculation allows smaller cement particles to be formed while they are still in suspension and then when they are deposited in the filter cake they are packed closer thus reducing the permeability.Foams or emulsions are a special case and exhibit low fluid loss rates, the permeability reduction being due to a reduction in the relative permeability of water and the presence of 3 phases (solid, gas and water for foam).Two principle classes of fluid loss additives exist: finely divided particulate materials and water soluble polymers. Key ideas:Improve fluid loss control of slurries by:Increase viscosity of interstitial waterReduce permeability of cement cakeImprove ordering of moleculesFLAC additives requiredThe most commonly used classes of fluid loss control agents are from the family of cellulose derivatives.D8 was the first to be used (in the late 1950s). It is a slightly anionic carboxylethyl hydroxyethyl cellulose (CMHEC). It is primarily used as a cement retarder for special applications. It viscosifies interstitial water and its carboxyl groups adsorb onto cement grains blocking pores in the filter cake. The performance of such fluid loss additives in salt slurries is improved by the addition of a hydroxycarboxylic acid such as tartaric acid.D60, D59 and D112 are hydroxy ethyl celluloses (HEC). They are basically the same but the D112 has a higher molecular weight thus allowing it to be used effectively for lower cement slurry densities. D59 is designed for use in salt water slurries and D112 can be used in both fresh and salt water systems. D60 is a mixture of D59 and the PNS dispersant D65 which is added to counteract the viscosifying effect of D59.FLAC properties are due to the viscosifying of the interstitial water and adsorption onto cement grains and blocking of pores by the molecule chains.All cellulosic fluid loss control additives share various disadvantages: a) they are effective water viscosifiers. At higher concentrations slurry mixability will be affected and ultimately undesirable viscosification of the slurry may result; b) up to 150 degF (65 degC), they are effective retarders so care must be taken to avoid over retardation of the slurry; c) their efficiency decreases with increasing temperature and they are not recommended for use above 235F (113C) BHCT; d) can create slurry foaming problems so higher than normal antifoam concentrations may be required. They will also foam when mixed in mix water.Dispersants are generally required to improve fluid loss control. Synthetic polymers are effective fluid loss control agents particularly for low temperature applications, i.e.. less than 130F (54C).D127 Low Temperature FLAC is used where the BHCT ranges from 80 - 130F (25 - 55 C). It has no retarding effect and is compatible with calcium chloride accelerated slurries. The calcium chloride will tend to thin the slurry whereas Superplasticizer -type dispersants are not effective and will thicken slurries. A critical minimum concentration of the additive is required to obtain good fluid loss control. There is a sharp threshold effect associated with this additive; within a very short concentration range (0.3 - 0.4% BWOC), the fluid loss rate falls from 500 to 20 mL/30min. The additive can produce unpredictable results and is really only effective when partially hydrated and stabilized so careful and consistent laboratory testing before use is required.Fluid loss control is achieved through adsorption of the polymer molecule chains on cement grains and the blocking of pores in the filter cake thus reducing its permeability.D603 liquid FLAC is effective in the temperature range 77 - 248F (25 - 120 C) BHCT in fresh and salt waters. It is important to optimize its concentration with respect to dispersant and is very sensitive to cement type and quality.D143 High Temperature FLAC is a high molecular weight polymer fluid loss control agent similar to D603 designed to give good mixability (low viscosity) at low temperatures. It gives good fluid loss control at temperatures above 180F (82C) up to 400F (205C) BHCT and in salt slurries with greater than 18% BWOW NaCl. The polymer structure is modified above 180 degF (82C) and high pH due to partial hydrolysis. It is the partially hydrolyzed polymer which is responsible for giving the fluid loss control. Its liquid equivalent is D158 which is used in a concentration range of 0.2 to 1 gps. 0.7 gps is equivalent to 1% D143D73.1 liquid FLAC is a high molecular weight polyethyleneimine (50% active) polymer in water and is classified as a cationic fluid loss control agent. The polymer is effective over a molecular weight range of 10xE3 - 10xE6 and is highly branched. The higher the molecular weight, the more effective the fluid loss control. Intermingling of polymer chains in cement pores leads to a sharp decrease in free space and reduction in permeability.PNS-type dispersants (e.g.. D80 and D65) must be used to give significant fluid loss control. If used alone then D073 may give worse fluid loss control that just the neat cement due to a reduction in the filter cake permeability. With a dispersant an insoluble association is made between the two polymers and divalent cations in the slurry system to create particles which block cement filter cake pores reducing permeability.The main advantage is that such FLACs are effective at high temperatures, providing excellent fluid loss control up to 435F (224C). They also do not appreciably affect the thickening time or compressive strength. However, they tend to promote sedimentation due to the reaction with anionic particles/molecules also present in the cement system. Higher mass agglomerates are formed with a greater settling tendency. Obtaining an optimum slurry design can be difficult. They are most effective in fresh water and can be used in salt slurry systems with less than 15% BWOW NaCl but must be well dispersed in all mix waters.LT=Low temperature range - upto 130 FMT=medium temperature range - 130 to 230 FHT=high temperature range - over 230 FLD=low densities - less than 15 ppgND=normal densities - 15 to 16 ppgHR=high densities - over 16 ppgAD=any densities - 12.5 to 18 ppgL=liquid additiveS=solid additiveCement slurries containing FLAC polymers must be well dispersed to obtain optimum fluid-loss control. Sulphonated aromatic polymers (D80, D65, D604M) or salt are almost always added in conjunction with FLACs.Deflocculation of the cement slurry is enhanced; the cement particles are smaller due to de--agglomeration, etc., and are also freer to move around. Thus packing of the cement grains in the filter cake and perhaps also the polymer aggregates, is improved. This leads to a reduction of the permeability of the filter cake. Care should be taken that over-dispersion does not result in settling and then give artificially improved fluid loss results when tested in the API test apparatus.Key ideas:Good high temperature FLACMust use with dispersant to be effective (forms complexes)Little is found in the literature to justify the levels of fluid-loss control required to achieve a good cement job. Reasons for controlling fluid loss are generally quoted as:1. To prevent primary cementing failures because of:excessive increases in slurry density which may increase friction and hydrostatic pressures (i.e.. placement pressure) or lead to premature setting of the slurry.annulus bridging as a result of excessive fluid loss into permeable zones. This then leads to either a sharp increase in pump pressures which may exceed fracture gradients, or the loss in hydrostatic pressure transmission to lower, high pressure zones.2. To reduce formation invasion by cement filtrate which can be damaging and deleterious to production.3. Maintain constant cement slurry properties while the slurry is placed and the setting process starts, either to,maintain full transmission of hydrostatic pressures to lower zones (especially in thin annuli and in potential gas migration situations), orfor cement squeezes into low permeability formations.Attributing primary cement job failures to a lack of fluid-loss control is very difficult. This is why it is generally difficult to justify the use of fluid-loss additives except in the most complex situations. Fluid-loss control requirements depend considerably on the clients preference and the type of job and cost. FLACs greatly increases the cost of a cement slurry. Dowell's recommendations are shown on the overhead.Some clients (e.g.. Mobil) do not call for fluid loss control whereas others (e.g.. Chevron) insist on very efficient control. If mud filter cake is removed, i.e.. by using scratchers, etc., then good fluid-loss control is mandatory to ensure good bonding with the formation. Fluid loss control is much better when a thin, impermeable mud filter cake is present.Under dynamic conditions, the amount of filtrate lost should not exceed that which would impair slurry properties which are very sensitive to the water-to-cement ratio. When static, the filtrate loss from the cement slurry should be less than that which would cause bridging of the annulus due to filter cake growth. The lighter the slurry then the less strict should be the fluid loss requirements.Key ideas:Control required tailored to application and well conditionsMost important for liners, gas migration and horizontal wellsResults will also depend on cement and mix water. Production casings or liners are used to isolate production zones and have usually fairly small annular spaces making friction pressure losses important, therefore, making the use of dispersants important. Higher formation pressures may be encountered as well in the pay zones requiring weighted slurries. Since the slurry volumes are relatively small, cost does not become an important issue. Fluid loss control is usual.

Well cements are highly concentrated suspensions of solid particles in water. The solids content can be as high as 70%, and as such, they will exhibit high rheological values. The rheology of cement slurries is related to:The solid volume fraction: the solids content is a direct function of the slurry density. The higher the density the greater the number of cement particles in a given volume and the higher the rheology. The less dispersed the solid particles and the greater the aggregate sizes, the higher the rheology. Inter-particle interactions: which depend primarily on the surface charge distribution, and also the number of particles and ionic species present in the system. The higher the concentration of solid particles, the greater the interactions between particles and the higher the rheology.The rheology of the aqueous phase: The interstitial fluid of a cement slurry is an aqueous solution of many ionic species and organic additives whose rheology will be different to that of water. The higher the viscosity of the base fluid, the higher the viscosity of the slurry. This has a little effect compared to the solid volume fraction.Cement dispersants are solutions of negatively charged polymer molecules which are attracted to the positively charged sites on the surfaces of hydrating cement grains. There, the original positive charge is inversed and the system becomes increasingly negatively charged as the concentration of dispersant is increased. The resulting repulsive charges break up the cement particle aggregates into individual particles, and the system is dispersed.This allows turbulence at lower pump rates, and the generation of less friction pressure at any given pump rate. They allow a reduction in the water content of a slurry while maintaining pumpability. This allows high density, reduced water slurries to be mixed without the addition of weighting agents. Cement slurries also have to be properly dispersed to allow fluid loss additives to work efficiently. Key ideas:Cement slurry rheology defined by:Yield point (Ty) force which causes slurry to flow (lbf/100ft2)Plastic viscosity, (PV) : force required to move slurry at a given velocity (cP)The harder it is for cement particles to be moved and be kept moving (increased number particles/total volume, higher inter-particle forces, etc..), the higher the rheologyDispersants act to reduce inter-particle reactions and improve their mobility so to decrease rheology. Ty and PV will thus be reduced Key ideas: Three main types of dispersantPNS type sulphonatesLignosulphonatesOrganic acidsPNS type sulphonates mainly use for well cementsD65 and D80 are the most commonly used TIC dispersants. They are both PNS-type dispersants. D65 is totally soluble in water; D80 is a water solution (38%) of D65 and can be prepared by the addition of 1.0 lb D65 to 1.5 lbs fresh hot water (0.1% BWOC D65 = 0.03 gal/sk D80). They are compatible with most cementing additives and in salt solutions up to 18% BWOW NaCl. Since they are obtained from different sources, small variations in performance may be seen in the field. D121 is a high temperature dispersant (> 200F [93C] BHCT) with secondary properties of retardation and fluid loss control. It is recommended for use in reduced water systems with densities greater than 16.4 lb/gal. It is a very active dispersant and can also be used in salt cement systems. It is also an effective retarder aid when used with D028 and is also a good latex stabilizer.D604M is used for ETD cements where overdispersion would occur if D80 was used (in fresh water). D604M reduces free water development or sedimentation. It is compatible with most additives but when used with D109 retarder, free water may occur. (NB: D604 was replaced by D604M. They are the same chemicals but the chemical responsible for the formation of microgel has been changed to an environmentally safer product).D145 is a PMS-type dispersant which has a different mechanism of action to the other dispersants and only 20% active matter. It therefore does not affect the slurry thickening times. It degrades at temperatures above 180F which limits its applications. It is a useful dispersant for microsilica slurries.D080A and D604AM are used as dispersants/FLAC additives in salt rich (above 18% BWOW NaCl) cement slurries, i.e... Saltbond. They are modified versions of D80 and D604M and give much better results than the previous D059/D045 salt FLAC/dispersant systems. Generally D80A is used for DTD cements and D604AM for ETD cements. A critical concentration is required to ensure good fluid loss. Most cements will work better with D604AM than D080A, even when DTD and then just the fluid loss needs to be checked.D45 was designed specifically for salt systems with over 18% BWOW NaCl where D65 caused gelation problems. It has a very strong retarding affect and calcium chloride should be used at temperatures over 140F. Care should be taken as false set and excessive viscosification may occur, even for large D45 concentrations. D45 is citric acid and is in the process of being obsolete at the time of writing.Key ideas:TIC notation applies to those additives whose main friction is dispersionChoice of dispersant governed by cement type, application, availability and costSeveral types of dispersants are used in the cement world. They are known as superplasticizers in the construction industry. 1. Sulphonates are the most common cement dispersants. These are solutions of highly branched polymer backbones. The most common are the polynaphthalene sulphonate (PNS) type dispersants, the dispersing ability of which is highly variable depending on the cement. D80 is typical of this family. Other sulphonates are the polymelamine (PMS) type. These are mainly used in the construction industry and in drilling fluids but are also effective in cement slurries. D145 is a typical example which was developed specially for microsilica cement systems as it has a much less retarding effect than D80/D65. 2. Lignosulphonates are mainly used as drilling mud thinners. They are also effective in cement slurries but will give extended thickening times. They are normally used as cement retarders (D13, D81, etc..). Their performance depends greatly on the cement quality and may cause gelation.3. Non-polymeric chemicals such as hydroxycarboxylic acids have strong dispersing properties and are also very strong retarders. Typical examples are D121 and D45 (citric acid). The trademark TIC is attached to chemical codes for additives whose primary function is as a dispersant. Other additives can also be effective dispersants but their primary use is for other purposes. Each of the dispersants mentioned above will be discussed in more detail later.

The rheology of a cement slurry is due to the formation of hydrates (C-S-H gel and ettringite) when the anhydrous cement powder is added to water. Both chemical and electrostatic interactions take place during the initial and induction stages of the hydration process. The hydrolysis of C-S-H gel results in a surface negatively charged with silicate hydrate ions. Free calcium cations (Ca2+) in the aqueous phase are attracted to these negatively charged groups on the C-S-H gel surfaces and react with them inducing a positive charge. The Ca2+ ions can react with negative groups on the same grain surface or on adjacent or different grain surfaces. The latter results in the bridging of the two grains due to their mutual attraction for the Ca2+ ion. This occurs due to the large cement surface area and the competition for Ca2+ ions between adsorption sites. The overall effect is that portions of cement grains will be positively charged and other portions negatively charged. There will be an interaction between the oppositely charged patches. If no bridging occurred, the cement grains would be uniformly positively charged leading to spontaneous dispersionThe degree of particle interaction will determine the rheology of the slurry. The yield value is related to the force required to break the interactive bonds between particles and then produce movement. Plastic viscosity is related to the mechanical friction between particles. To disperse a cement slurry then either the electrostatic interactions must be modified to promote repulsion of particles, or the slurry must be mechanically sheared. Key ideas:Surface ionization of cement particles results in surfaces becoming oppositely chargedInter-particle attraction requires a force to overcome it The greater the attraction the higher the yield and viscosityAttractive forces changed by addition of dispersantsIt has been seen that to disperse a cement slurry, it is necessary to disrupt the particle aggregates either by shearing or by the addition of chemicals called dispersants. Both actions release a portion of the entrapped water in the aggregates effectively decreasing the volume of the dispersed phase and reducing the slurry viscosity.The electrostatic charged network existing on and between cement grain surfaces must be broken to obtain this dispersion.Dispersants contain negatively charged molecular groups which when added to the slurry, adsorb onto the positively charged sites (Ca2+) on the cement grains and thus suppress charged particle inter-actions. The polyanion dispersant molecule brings several negative charges. The cement particles become uniformly negatively charged, repel one another, deflocculate and the slurry is dispersed. (The amount adsorbed varies with the concentration of the dispersant). As more dispersant is added, the surfaces become more and more negative and thus more dispersed; the yield point will consequently decrease. Optimum dispersant concentration is reached when all the positive adsorption sites are saturated. At this optimum level the homogeneity of the cement structure is improved and permeability and shrinkage is reduced.The cement hydration process would be impaired if polycations were added to achieve dispersion by interaction with the negatively charged sites. Key ideas:Dispersant molecules are negatively chargedAttracted to positive sites on cement grains suppressing particle interactionsMore dispersant--> more negative sites --> more repulsionParticle aggregates deflocculated and slurry dispersedThe fact that different cements have different responses to dispersants lead to the classifications of Easy-to-Disperse (ETD) and Difficult-to-Disperse (DTD) cements.The overhead shows the response of yield point, free water and plastic viscosity of a Class G cement to varying concentrations of D80 dispersant at 185 F. The plastic viscosity remains relatively unaffected, decreasing slightly with increasing D80 concentration. The Ty value drops sharply to zero over the range 0.05 to 0.10 gal/sk D80. Free water increases sharply once the slurry is over-dispersed (i.e... when Ty = 0). This behaviour is typical of ETD cements; the D80 over disperses the slurry causing sedimentation and/or free water.It can be seen that the workable, and controllable, range of D080 is very small, i.e... low values are only possible within a small range without the slurry developing free water (this range is represented by the shaded column on the overhead graph). For this reason, alternative dispersants were developed for ETD cements. Key ideas:ETD cements can be easily over-dispersed with D080 resulting in slurry instabilityResults in small concentration range in which Ty and FW can be optimized.To determine cement type:Mix according to API Spec. 10 and measure rheology and free water at 185 degF. Then plot FW -vs- Rheology and look at results. FW and Ty indicates and ETD cement.To increase the density of the slurry, two methods can be used: 1) by adding heavier weight materials, for example hematite or barite; 2) or by adding more cement to the same quantity of water and by adding dispersant which will make the slurry more pumpable.WEIGHTING AGENTSHigh cement slurry densities are commonly required to allow well control to be maintained or for special applications (e.g... kick-off plugs). Densities of up to 18.0 lb/gal can be obtained using reduced water slurries. Higher densities require the use of weighting agents which are dry blended with the cement. The weighting agents have a higher specific gravity than the cement and have