09.Politecnico Di Torino Cementing Programme 2011

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    09. CEMENTING PROGRAMME

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    CHAPTER 9

    FUNDAMENTALS OF THECEMENTING

    OPERATIONS

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    30" CP

    20" CASING

    9 CASING

    7 CASING

    13 CASING

    26 HOLE

    16 OR 17 HOLE

    12 HOLE

    8 HOLECEMENT

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    Portland !"!nt# are for sure the most produced and most commonlyused hydraulic binders not only in the Construction Industry but also in the

    Oil Industry, where for their properties, availability and cost are a basicmaterial in well cementing operations.

    The hydraulic binders, as the cement, are capable to set and developmechanical strength also in presence of water as a consequence ofchemical reactions taking place during the hydration process between the

    mi water and the components present in the cement itself. Once set, thecement maintains its properties, that is high compressive strengthand lowpermeability, even if eposed to aggressive waters for very long periods oftime.

    9.1. INTRODUCTION

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    The ra% "at!r&al#, from which !ortland cements are obtained, contain 'o(r!##!nt&al o")on!nt#, that is"al&(" o*&d!#CaO$,

    #&l&a#%iO&$,

    al("&na#'l&O($

    '!rr& o*&d!#)e&O($,

    and several &")(r&t&!# such as"magnesium oide #*gO$,

    potassium oide #+&O$,

    sodium oide #a&O$,

    lead oide #!bO&$,

    -inc oide #nO$,

    sulphur trioide #%O($,

    phosphates #!O/0($, etc.

    The impurities have not to eceed a certain concentration otherwise they can havenegative effects on the cement performances.

    9.2. PORTLAND CEMENTS COMPOSITION

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    'mong the four main components, only one, that is CaO, acts as a base, while thethree othersbehave as anhydrides1 consequently, it is easy understandable that duringthe calcination process of the raw materials in the kiln, calcium salts of the three

    anhydrides will appear one after the other as temperature increases and are"

    T!traal&(" Al("&no'!rr&t!, Ca/'l&)e&O23 or more simply C/')

    Tr&al&(" Al("&nat!, Ca('l&O4or simply C('

    D&al&(" S&l&at!, Ca&%iO/or shortly C&%

    Tr&al&(" S&l&at!, Ca(%iO5or shortly C(%

    C/') and C(' are the first products to appear and melt in the kiln and for this reasonthey are called 6melting compounds71 they form the liquid phase of the clinker,

    fundamental for the subsequent reactions which lead to the formation of the silicatesand, in particular, of C(%, from which the mechanical strengths of a cement depend.

    9.2. PORTLAND CEMENTS COMPOSITION

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    In order to simplify the representation of !ortland cement components and theirreactions, a special chemical notation has been adopted by cement chemistsaccording to which the various components are epressed as a sum of their oides,

    which have been further abbreviated as follows"- Cal&(" O*&d! CaO / C- Al("&n&(" O*&d! Al2O3/ A- S&l&a S&O2/ S- F!rr& O*&d! F!2O3/ F- Man!#&(" O*&d! MO / M

    - at!r H2O / H- Sod&(" O*&d! Na2O / N- Pota##&(" O*&d! 2O / - L&t&(" O*&d! L&2O / L- Po#)or& O*&d! P2O+/ P- F!rro(# O*&d! F!O / '

    - T&tan&(" O*&d! T&O2/ T- S(l)(r Tr&o*&d! SO3/ 4- Car5on D&o*&d! CO2/ 6

    9.2. PORTLAND CEMENTS COMPOSITION

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    Two basic ra% "at!r&al# are necessary to prepare a miture from which !ortlandcements are obtained, that is"

    calcareous materialscontaining or producing calcium oide#lime$. 8ime is presentin natural rocks such as" sedimentary and metamorphic limestones, corals, shelldeposits, 6cement rock7 #found on the island of !ortland, 9+, which has a compositionsimilar to !ortland cements and from where the name of the !ortland cementsderives$ and also in artificial materials" precipitated calcium carbonates and other alkali

    wastes from various industrial processes1

    argillaceous materials, which provide alumina, silica, ferric oideas well as manyother minerals, present as impurities. The most used of such materials are" clays,shales, marls, mudstones, slate, schists, volcanic ashes, all of natural origin1 blastfurnace slag and fly ashes come from artificial sources.

    9.3. MANUFACTURING OF PORTLAND CEMENTS

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    Once the raw materials are available, the "an('at(r&n o' Portland !"!nt#requires the following steps"1.the basic minerals are crushed, blended in such a proportion to obtain a miture of

    oides corresponding to the specific chemical characteristics of the cement one wantsto manufacture1

    2.the miture is, then, ground to the desired fineness by adopting one of two possibleprocesses, that is"the dr )ro!##the %!t )ro!##In the dry process, the minerals are blended and ground in a dry state1 the drying isperformed in rotary driers and the subsequent grinding in rotary mills which areequipped with steel balls or other grinding devices. That portion of the miture thatreaches the desired fineness is transported by a stream of air into storage silos, whilethe coarser particles are returned to the mill for further milling.The wet processdiffers from the dry process for the fact that water is added to the

    miture of minerals prior to its sending to the mill1 the slurry, thus obtained, is moreeasily ground to the planned fineness and, again, the particles, which pass acalibrated vibrating screen, are collected in storage pits, while the bigger particlesreturn to the mill.

    9.3. MANUFACTURING OF PORTLAND CEMENTS

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    3.once the miture has been properly ground and homogeni-ed, it is sent to the :&lnwhere the manufacturing of cement takes place. The kiln is made by a rotatingcylinder, :3 m to 243 m long with a diameter of &.50/ m and slightly inclined #with aslope of &50;5 mm every 2 m$1 it rotates at 430&/3 revolutions per hour and thisrotation and the slope of the kiln makes the miture moving along the kiln itself. Thetemperature of the kiln is progressively increased #&330;330223302(3302533oC$ andthen decreased #;33oC$ promoting a series of reactions which transform the rawmaterials into l&n:!r.

    $. to obtain the '&n!d !"!nt, the clinker is blended and ground with a certainamount of )#(", Ca%O/=&$ in percentages about 2.505?, whose mainfunction is to avoid a phenomenon known as 6flash-set71

    +.once the cement comes out from the grinder, it is #tor!d &n lar! a&rt&t #&lo#toprotect it from humidity and carbon dioide action, which can alter its characteristics

    and performances.

    9.3. MANUFACTURING OF PORTLAND CEMENTS

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    9.3. MANUFACTURING OF PORTLAND CEMENTS

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    In order that !ortland cements develop their functions, it is necessary that the !"!nt 5!"&*!d %&t %at!rin well defined ratios depending on cement type.

    @hen water comes in contact with the cement a series of highly eothermic reactions

    take place which cause the progressive set and hardening of the slurry"the #!t occurs quite quickly, that is in few hours after water addition1the ard!n&n is a very long processlasting also several months and more.

    It is worth to remind that during the first period of set and hardening, only the superficiallayer of the cement grains react #after several months from miing, the hydration interests

    only 40A Bm inside the grains1 considering that :3? of cement grains have diameters inthe 230233 Bm range, it is very easy to understand that the complete hydration ofcements requires very long times$.

    's said, !ortland cements are formed by four main components" C$AF, C3A, C2S andC3S. 's these compounds react with water, they pass from their original anhydrousstate

    to a hydrated state1 because the anhydrous compounds are much more soluble in waterthan the hydrated forms, complete hydration of all the cement components ultimatelyoccurs. To understand how a !ortland cement hydrates, the usual approach is to studythe behaviour of each single phase and then transform these single behaviours into a6multicomponent behaviour7.

    9.$. CHEMICAL REACTIONS OF PORTLAND CEMENTS

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    9sually the component which is taken as model is the tr&al&(" #&l&at!, C3S, dueto its abundance #a cement is formedby silicates for the ;3? and the C(% representsthe :3? of the silicates$ andreactivity. enerally, when the silicates are contacted bywater, they hydrate formingal&(" #&l&at! drat!and al&(" dro*&d!#alsocalled 6portlandite7$ according to the followingreactions"

    2C3S ; ,H < C3S2H3; 3CH

    2C2S ; $H < C3S2H3; CH

    The compound C3S2H3has a quasi0amorphous aspect and for this reason is usuallyindicated as 6C-S-H gel7. On the contrary, CH, Ca#O=$&, is crystalline and appearsas heagonal plates. The C0%0= gel constitutes around the :3? of fully hydrated

    !ortland cements at ambient conditions and is considered as the main binder ofhardened cements.

    9.$.1. H=DRATION OF THE SILICATE PHASES

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    In order to better eplain what is happening when a cement hydrates, the drat&on o' t! "od!l)a#! C3Sis generally subdivided into five main stagesorperiods, that is"

    Preinduction Period" during this period, which lasts only few minutes, a large increase in

    temperature is observed, because the hydration reaction is highly eothermic. The anhydroussurfaces of the C(% grains start to be hydrated and transformed into C0%0= gel, which, reachingvery quickly critical supersaturation conditions, precipitate on the same C(% grains. On thecontrary, the concentration of C= continues to increase in solution because its saturation does notreach the critical value.

    Induction Period" during the induction period, the hydration activity is strongly reduced because

    the C0%0= gel deposition on the C(% granules, occurred in the previous period, determines aremarkable decrease in their permeability and, therefore, in their reactivity. In this period, theprecipitation of the C0%0= gel slowly continues, while the concentration in solution of calcium ions,CaD&, and hydroyl groups, O=0, increases. @hen supercritical conditions are finally reached, theprecipitation of calcium hydroide, Ca#O=$&, begins, which allows the hydration reactions torecommence. The induction period, at ambient conditions, generally lasts a few hours. Thetermination mechanismof the induction period is not completely clear. 'ccording to one theory, itends because osmotic forces are developed within the C0%0= gel layer as hydration continues andthese forces determine the burst of the gel layer with a large release of silicates into the solutionand the formation, again, of large amounts of C0%0= gel. 'nother theory suggests that the C0%0=layer undergoes a morphological change which causes an increase in permeability to occur1 as aconsequence, water can enter again into the grains restarting the hydration process.

    9.$.1. H=DRATION OF THE SILICATE PHASES

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    Acceleration and Deceleration Periods" when the induction period ends, only a small amountof C(% has undergone hydration. The remaining part of C(% hydrates during the periods called ofacceleration and deceleration, which together forms the 6setting period7. The setting period lastsseveral days at ambient conditions.Euring the acceleration period, solid Ca#O=$&crystalli-es from the solution, being in supercriticalconditions, and the C0%0= gel restarts to deposit again into the water0filled pores. The hydratedcompounds grow in si-e, connect one with the others and a network of solid material is formed1 inthis way, the system starts to develop its mechanical strengths. 's the C0%0= gel and thehydrates deposit, the permeability of the system decreases and, consequently, the reactions areagain decelerated, because of the difficulties water and the other ionic species have to movewithin the network of solid particles #deceleration period$.

    Diffusion Period" during this period the hydration of the cement grains continuously slows downdue to the ever0decreasing porosity and permeability of the system1 the network of hydratedcompounds becomes more and more dense and its strength increases. The duration of thediffusion period, at ambient conditions, is etremely long, so that it is possible to sustain that totalhydration is never reached.

    9.$.1. H=DRATION OF THE SILICATE PHASES

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    I. Pr!&nd(t&on

    II. Ind(t&onIII. A!l!rat&onI>. D!!l!rat&on

    >. D&''(#&on

    T!r"ora" So%&n t! Hdrat&on R!at&on#o' t! Tr&al&(" S&l&at! Pa#! C3S

    9.$.1. H=DRATION OF THE SILICATE PHASES

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    The al("&nat!#, C3A and C$AF, are very reactive species, in particular C(', and their hydrationcompletes in very short times. They have a strong influence on slurry rheology#the silicates, on thecontrary, do not affect this aspect$ and on the early development of mechanical strength of setcements 's happens with C(%, also in the case of C(' #C(' is usually taken as model for therepresentation of aluminates hydration$ the first reaction occurs between water and the surface of theanhydrous solid, which very quickly leads to the precipitation of al&(" al("&nat! drat!#"

    2C3A ; 27H< C2AH8; C$AH19< 2C3AH,; 1+H

    Eifferently from the calcium silicate hydrates, those of calcium aluminate are not amorphous butcrystalline and for this reason they are not able to form a protective impermeable layer on the C

    (

    'grains surface, protecting them from continuous hydration1 therefore, in their case, no inductionperiod is observed, but hydration goes on to completion very rapidly.

    If such uncontrolled hydration is allowed to occur without taking any precaution, severe problems canbe eperienced #6flash set7$, in particular for what regards rheology. To avoid this and control the C('hydration, 2,505? of gypsum is regularly added to the cement clinker prior to its final grinding. The

    gypsum, Ca%O/

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    Ettr&n&t!, whose appearance is that of needle0like crystals, precipitates on the C(' surfacesretarding their hydration1 in other words, an 6artificial induction period7 is created. Euring this period,the gypsum continues to dissolve and ettringite to precipitate1 when the gypsum is totally consumed,

    the retardation of C(' ends and the hydration reactions restart. Fttringite becomes unstable and istransformed into al&(" "ono#(l'oal("&nat! drat!B

    C3A?3C4?32H ; 2C3A ; $H < 3C3A?C4?12H

    The C(', which has not reacted so far, forms again calcium aluminate hydrate, as epected.

    9.$.2. H=DRATION OF THE ALUMINATE PHASES

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    The reactions described above, of course, take place all at the same time, one influencing the others.)rom a chemical standpoint, the !ortland cement hydration can be considered as a sequence ofdissolution and precipitation reactions, which proceed simultaneously but at different rates1 they can

    be schemati-ed by superimposing the thermogram of the C(% with that of the C('.

    9.$.3. H=DRATION OF PORTLAND CEMENTS

    S!"at& R!)r!#!ntat&on o'Portland C!"!nt Hdrat&onR!at&on#.

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    The standards concerned with cements and their slurries are contained in the document ISO10$2, P!trol!(" and nat(ral a# &nd(#tr&!# C!"!nt# and "at!r&al# 'or %!ll !"!nt&n7,which consists of the following parts"

    !art 2" %pecification #based on '!I %pecification 23' 6%pecification for Cements and *aterialsfor @ell Cementing7, &(rd Fdition, 'pril &33&1 indicated also as '%IG'!I 23'GI%O 23/&4020&332$.!art &" Testing of well cements #based on '%IG'!I Hecommended !ractice 230& #formerly23$ 6Hecommended !ractice for Testing @ell Cements7, 2st Fdition, July &335$.!art (" Testing of deepwater well cement formulations.!art /" !reparation and testing of foamed cement slurries at atmospheric pressure.!art 5" Eetermination of shrinkage and epansion of well cement formulations at atmosphericpressure.

    'ccording to the above mentioned I%O 23/&4 !art 2G'!I %pecification 23', all '!I cements 6areproduced by grinding !ortland cement clinker, generally consisting of hydraulic calcium silicatesand aluminates and usually containing one or more forms of calcium sulphate as an intergroundaddition7.

    API !"!nt# ar! ro()!d &nto 8 la##!#, from ' to =, and in 3 t! rad!#" ordinary #O$,moderate sulphate resistant#*%H$ and high sulphate resistant#=%H$, as shown in the net slide.

    9.+. API CLASSIFICATION OF PORTLAND CEMENTS

    9 + API CLASSIFICATION OF PORTLAND CEMENTS

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    Sulphate Resistance" formation waters always contain a certain amount of salts, inparticular sodium and magnesium sulphates. These sulphates are responsible ofreactions which affect the properties of set cements. In fact, they can react, after theslurry setting, with the mineral portlandite, which is precipitated Ca#O=$&, to formmagnesium and sodium hydroides, *g#O=$& and aO=, and calcium sulphate,Ca%O/1 the calcium sulphate, in its turn, can react with the aluminates producing6secondary ettringite7. This 6secondary ettringite7 can epand too much causing thedamage and cracking of the set cement.

    To limit the attack of sulphate0containing waters, cements with low C(' content areproduced.

    =owever, this occurrence diminishes with increasing temperatures, because thesolubility of the two sulphates is low at high temperatures, so that, above 43 oC, thisrisk of sulphate attack can be considered negligible.

    9.+. API CLASSIFICATION OF PORTLAND CEMENTS

    9 + API CLASSIFICATION OF PORTLAND CEMENTS

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    CLASS ORDINAR=@O

    MODERATE SULPHATERESISTANT

    @MSR

    HIGH SULPHATERESISTANT

    @HSR

    #C(' not specified$ #(0;? C('$ #K (? C('$

    A L

    L L

    C L #ma 25?$ L L

    D L LE L L

    F L L

    G L L

    H L L

    Cla##!# and Grad!# o' API !ll C!"!nt##from '!I %pecification 23'$

    9.+. API CLASSIFICATION OF PORTLAND CEMENTS

    9 + API CLASSIFICATION OF PORTLAND CEMENTS

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    T)&al C!"&al Co")o#&t&on F&n!n!## and at!r R!(&r!"!nt# o'Portland C!"!nt#

    #from '!I %pecification 23'$

    APICLASS

    T=PICAL COMPOSITION @ LAINEFINENESS

    ATERREUIREMENT

    C3S -C2S C3A C$AF "2

    J A /5 &: 22 ; 2433 /4

    // (2 5 2( 2433 /4

    C 5( 2A 22 A &&33 54

    D &; /A / 2& 2533 (;

    E (; /( / A 2533 (;

    F L L L L L (;

    G 53 (3 5 2& 2;33 //

    H 53 (3 5 2& 2433 (;

    9.+. API CLASSIFICATION OF PORTLAND CEMENTS

    9 + API CLASSIFICATION OF PORTLAND CEMENTS

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    Currently, t! !"!nt ro(t&n!l (#!d &n En& EKP o)!rat&on# t! Cla## G!"!nt1 in fact, the last two classes of cements, Class and =, have beendeveloped in the last years to take into account the remarkable improvementsachieved in accelerating or retarding the setting times of cement slurries by chemicalmeans. The manufacturers can not add any chemicals at all to these cements,differently from what was done with the Class E, F and ) cements #these cementswere called 6retarded cements71 the retardation was due to their low content in C(%and C(' and to their coarser fineness$, in which glycols and acetates were added to

    improve the grinding process of clinker, but inducing interferences with the normaladditives used in slurries preparation.

    Classes F and ) cements are rarely manufactured and used today in cementingoperations and the idea is to cancel them as '!I cements.

    9.+. API CLASSIFICATION OF PORTLAND CEMENTS

    9 , MAIN CHARACTERISTICS OF PORTLAND CEMENTS

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    9.,. MAIN CHARACTERISTICS OF PORTLAND CEMENTS

    The main characteristics of a cement slurry regard"

    0 d!n#&t" the density of a slurry usually ranges between 2.(30&./3 kgG8 and is preferablymeasured by using the pressuri-ed fluid density balance or the classical mud balance1

    0o")r!##&! #tr!nt" this test is performed to evaluate set cement and cement blend slurries

    for resistance to thermally induced strength retrogression. To do this, the cement or cement blendset slurry samples are eposed to temperature and pressure for varying periods of time andobserved for changes in compressive strength. The procedure involves comparing thecompressive strength observed after some established initial periods #i.e. &/ h, /; h, :& h$ withthat measured after longer periods #i.e. &; days$. %amples that ehibit lower compressive strengthvalues after the longest periods may be considered to be subMect to strength retrogression and notsuitable for field use at that temperature and pressure. The temperature at which thermally

    induced strength retrogression occurs is commonly considered to be about 22302&3o

    C. 't thesetemperatures, the C0%0= gel undergoes a structural change, which usually causes a reduction incompressive strength and an increase in permeability of set cements. This phenomenon consistsin a transformation of the C0%0= gel into a new phase called 6alpha-dicalcium silicate hydrate7 #N0C&%=$, which is highly crystalline and much more dense than the original C0%0= gel. 's aconsequence, a reduction in volume or 6shrinkage7 of the hydrated phases takes place withdeleterious effects on the integrity of the set cement. )or this reason when cementing wells at

    temperatures 223o

    C, the silica content in the slurry has to be increased by adding to the base!ortland cement (50/3? @OC ground quart-, usually silica sand or silica flour, or by inter0grounding the !ortland cement clinker with quart- #eocem cement$ or by using speciallydesigned cement slurries #i.e. EensCrete$.

    9.,. MAIN CHARACTERISTICS OF PORTLAND CEMENTS

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    9.,. MAIN CHARACTERISTICS OF PORTLAND CEMENTS

    0t&:!n&n t&"!" thickening time tests are designed to determine the length of time that acement slurry will remain pumpable in a well under simulated downhole conditions of pressureand temperature. The slurry under testing is evaluated in a pressuri-ed consistometer, whichincorporates a rotating cylindrical slurry container equipped with a stationary paddle assembly, all

    enclosed in a pressure vessel capable of withstanding the pressures and temperatures epecteddownhole1

    0'&ltrat!" fluid0loss tests are designed to measure the slurry dehydration during and immediatelyafter the completion of the placement phase. It ranges between (302533 m8G(3P1

    0)!r"!a5&l&t" this test is used to determine the relative permeability of a set cement sample toliquids or gases1 the results can be used to enhance the design of cement slurry formulations,though they do not always provide an accurate indication of the actual permeability of set cements

    under downhole conditions. The permeability of set cements is very low10 #l(rr #ta5&l&t" the obMective of this test is to verify the static #quiescent$ stability of cementslurries, after these have been conditioned, to simulate their placement in the wellbore. Theslurries are then left in static conditions for a certain period of time to determine if any free fluid orparticle settling occurs1 both measurements are required because free fluid can be formed withlittle or no sedimentation and sedimentation can take place without any free fluid appearance1

    0r!olo&al )ro)!rt&!#" the rheology of cement slurries #viscosity, yield point, gel strengths$ is

    determined with the rotational viscometer and are used to calculate friction losses during slurryplacement and to design the hydraulic programme.

    9 7 CON>ENTIONAL PORTLAND CEMENTS SLURRIES

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    The !ortland cement slurries find application in the most varied operationalconditions, such as"temperaturesfrom below 3oC, as it occurs in 'rctic areas1up to (53oC and more as can be observed in geothermal wells or in tertiary recoveryactivities where thermal methods are applied1pressuresfrom few tens of kgGcm&as encountered in shallow wells1up to &333 kgGcm&, which can be faced in deep and ultra0deep wells1formations that can be highly fracturedwith very low fracture pressures or strongly

    overpressured1presence of corrosive and sour fluids.

    9.7. CON>ENTIONAL PORTLAND CEMENTS SLURRIES

    9 7 CON>ENTIONAL PORTLAND CEMENTS SLURRIES

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    To cope with all these very different situations, which often can be encountered in thesame well as the casing strings deepen, the slurries have to be properly designedtaking into account the actual well conditions. This is made possible because duringthe years many very effective chemicals have been developed and introduced in the

    design of cement slurries, rendering them fleible and adaptable to any situations.Therefore, the !ortland cement slurries, depending on their composition andapplication, can be classified as follows" a!l!rat!d #l(rr&!# with short thickening times and early compressive strengthdevelopment suitable for low temperature wells1r!tard!d #l(rr&!#with long thickening times as required in high temperature wells1l&t%!&t #l(rr&!#to be used in wells characteri-ed by low fracture gradients1%!&t!d #l(rr&!#to be used in wells with very high formation pressures1 d)!r#!d #l(rr&!#, which are particularly necessary in deep wells with hightemperatures and pressures where high densities but also a good rheology areessential1 r!d(!d '&ltrat! #l(rr&!# to be used in front of permeable formations and forminimi-ing formation damage1lo#t &r(lat&on ontrol "at!r&al#which allow to restore circulation in fractured orlow fracture gradients wells.

    9.7. CON>ENTIONAL PORTLAND CEMENTS SLURRIES

    9 7 1 ACCELERATED SLURRIES

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    A!l!rat!d #l(rr&!# are used when the necessity to #ort!n t! #!tt&n t&"!andJor to a!l!rat! t! ard!n&n )ro!##arises. They are generally employedfor cementing surface or intermediate casing strings in wells showing a bottom holetemperature below 53QCand are mandatory below &5QC, where normal slurries reacttoo slowly and the time spent on @OC #wait on cement$ becomes ecessively long.%ometimes, they are used to offset the set delay caused by other additives such asdispersants and fluid loss control agents.

    These slurries are obtained by adding to the cement inorganic salts, the mostcommon of which are" al&(" lor&d! #CaCl&$ and #od&(" lor&d! #aCl$1 in

    theory, other numerous salts, like silicates #i.e. sodium silicate$, sulphates #i.e.gypsum$, carbonates, aluminates, nitrates, nitrites can be added to the cement aswell as some organic compounds #i.e. calcium formate, oalic acid$, but in practicethe choice is limited to the chlorides for their easy availability and low cost.

    'cceleration can be also obtained by d!r!a#&n t! "&*&n %at!r a"o(nt1 the

    densified slurries are especially useful for cementing surface casings in cold weatherconditions and for setting cement plugs which require short pumping times and veryrapid strength development.

    9.7.1. ACCELERATED SLURRIES

    9.7.1. ACCELERATED SLURRIES

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    M!an" o' At&on" Cal&(" Clor&d!, CaCl2, is the most common acceleratorused to shorten the thickening time of cement slurries. The accelerating action of

    !ortland cement slurries to CaCl& additions involves different and comple physicaland chemical phenomena. The presence of chloride ions can affect the structure ofthe C0%0= gel coating, increasing its permeability and, therefore, the hydration rate ofthe C(% phase1 also C(' hydration is accelerated. )urthermore, CaCl& significantlymodifies the distribution of ionic species in the aqueous phase of the slurry thusfastening the hydration reactions.

    9.7.1. ACCELERATED SLURRIES

    9.7.1. ACCELERATED SLURRIES

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    CaCl2ACTION ON THICENING TIME AND MECHANICAL STRENGTHS

    CaCl2

    @ !&t on C!"!nt(&oC /3oC /5oC

    02$

    $B001B171B1+

    3B301111B02

    2B321B100B+8

    THICENING TIME @B"&n

    CaCl2

    #?$ 4 h 2& h &/ h 4 h 2& h &/ h 4 h 2& h &/ h

    3&

    /

    .%.A

    A

    /(/

    /4

    &A234

    223

    (&A

    (;

    &4:&

    ;;

    ;A2:4

    &3(

    &4:;

    A(

    5A24:

    2;3

    2&5&:;

    (2(

    MECHANICAL STRENGTHS @at" # TEMPERATURE

    at 1,oC at 27oC at 38oC

    9.7.1. ACCELERATED SLURRIES

    9.7.2. RETARDED SLURRIES

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    R!tard!d #l(rr&!#are used &n ord!r to !"!nt &nt!r"!d&at! and d!!) a#&n #tr&n# !*)o#!dto & t!")!rat(r!# %!n lon t&:!n&n and #!tt&n t&"!# ar! r!(&r!din order to completethe cementing Mob in the planned time1 they are obtained by adding to the neat cement additives,called 6retarders7, whose primary function is to tune the pumpability time of the slurry according tothe operational needs.

    The commonly used retarders belong to the following main chemical classes"8ignosulphonates=ydrocarboylic 'cids #gluconate salts, citric acid$Cellulose Eerivatives #C*=FC$Organic !osphonatesInorganic Compounds #boric acid, phosphonic acid, chromic acid, -inc and lead oides, bora$

    L&no#(l)onat!#constitute the commonly used retarders for most temperature conditions and areadded alone #for medium to medium0high temperatures$ or blended with other chemicals, such asorganic #gluconates$ and inorganic #bora, -inc oide$ acids and their salts, when very hightemperatures are encountered.

    %ervice Companies commerciali-e other efficient products for etreme conditions, but very oftentheir chemical composition and mechanism of action are not completely known to the users1therefore, it results very difficult or impossible to assign them to a specific chemical category.

    9.7.2. RETARDED SLURRIES

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    To eplain the way retarders fulfil their function, many theories on their mechanism of actionhave been proposed, such as"

    Adsorption heory:the retardation is due to the adsorption of the retarder on the surface ofC

    (

    % and C(

    ' grains, thereby limiting the contact of these phases with water.

    Precipitation heory:in this case the retardation effect is due to the reaction of the retarderwith the calcium andGor hydroyls ions present in the aqueous phase of the slurry with thedeposition of an insoluble and impermeable layer around the cement grains.

    !ucleation heory:according to this theory the adsorption of the retarder takes place directlyon the nuclei of the hydration products, impeding their subsequent growth.

    Comple"ation heory: calcium ions are chelated by the retarder, thus preventing theproduction of the hydrated phase portlandite, whose appearance terminates the inductionperiod.

    It is highly probable that several, if not all, of these mechanisms are involvedcontemporaneously in the retardation of a slurry depending on the chemical nature of theretarder #very often the retarders are mitures of different chemical compounds$ and cementcomposition.

    M!an" o' At&onB l&no#(l)onat!# retard the slurry thickening and setting by acting onthe kinetics of C(% and also C(' hydration reactions, through a combination of the adsorptionand nucleation theories.

    9.7.2. RETARDED SLURRIES

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    LIGNOSULPHONATES @HR-12 EFFECT ON THICENING TIME OF CLASS D CEMENT SLURRIES

    HR-12

    DEPTH @" TEMPERATURE@oC

    FRESHATER

    SALTSATURATED

    ATER

    001+

    30$830$8

    ,2,2

    2B308B00

    2B$07B00

    001+

    3,+83,+8

    7878

    2B007B00

    1B$0+B00

    0

    01+0300+0

    $2,7

    $2,7$2,7$2,7

    97

    979797

    1B30

    1B30$B30N.D.

    1B30

    N.D.3B20N.D.

    01+030

    0+0

    $877$877

    $877

    120120

    120

    1B002B00

    $B00

    N.D.N.D.

    $B20

    080100

    +$87+$87

    1$91$9

    2B003B30

    2B00N.D.

    9.7.3. LIGHT EIGHT SLURRIES

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    L&t-%!&t #l(rr&!#are used when slurries with densities lower than normalare required, such as in case of performing a cementing Mob in formationscharacteri-ed by low fracture gradients, in particular at shallow depth, or when

    circulation losses problems are epected. To obtain light0weight slurries, fourmain classes of products are generally used, that is" water e"tenders: these products allow the addition of very high volumes ofwater with respect to neat cements. The most common additives belonging tothis category are" bentonite, sodium silicates, diatomite1 low-density materials: the density of these materials is sensibly lower than

    that of the neat cement to which they are added also at significantconcentrations. elong to this class materials such as" perlite #volcanic rockformed by aluminium silicates, dR3.2&; kgG8$, gilsonite #naturally occurringhydrocarbon, 6asphaltite7, dR2.3: kgG8$, powdered coal, ceramic or glassmicrospheres1gaseous e"tenders:nitrogen or aircan be used to prepare foamed cements,

    characteri-ed by etremely low densities, but still sufficient compressivestrengths,composite slurries" such as the %chlumberger 8iteCrete systems.

    9.7.3. LIGHT EIGHT SLURRIES

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    1. at!r E*t!nd!r#B !nton&t!!nton&t! is one of the most common materials used to decrease the density of a slurrybecause of its availability and low cost. entonite can be added to a cement in

    concentrations up to &3? @OC, but above 4? the use of a dispersant is necessary toreduce the slurry viscosity and gel strength. entonite can be dry-blendedwith the cementor can be prehydrated in water before the cement addition. 'ccording to '!IGI%Ospecifications, only pure untreated bentonite should be used in well cements.

    ormally, bentonite is used at concentrations of /0;? @OC. '!I recommends that for each2? increase in bentonite concentration, the increase in water be 5.(? @OC for all classes

    of cements1 however, laboratory testing is recommended to establish the optimum waterrequirement for each cement and bentonite sample.

    's epected, the density of the slurry decreases and its yield increases as the bentonitepercentage increases, passing, in case of a class cement, from an initial value of 2.A3 kgG8and :5.: 8G233 kg with no bentonite down to 2./& kgG8 and up to 2;A./ 8G233 kg for a &3?bentonite addition. 'lso the fluid loss improves with higher bentonite concentrations.

    The addition of entonite causes a decrease in the mechanical strength of a set cementslurry.

    9.7.3. LIGHT EIGHT SLURRIES

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    1. at!r E*t!nd!r#B !nton&t!

    ecause the presence of high concentrations of calcium ions in the aqueous phase of aslurry strongly affects the ability of a bentonite to hydrate, the etending properties of

    bentonite are greatly improved if the material is allowed to hydrate in the mi water prior toslurry miing #)r!drat!d 5!nton&t!$. ' slurry prepared with &? prehydrated bentonite isequivalent to a slurry obtained with ;? of dry0blended bentonite. The complete hydration of agood quality bentonite takes place in more or less (3P. The thickening time and the finalcompressive strength of prehydrated bentonite slurries are similar to those of dry0blendedslurries at the same density.

    The bentonite can be prehydrated in fresh water and also in sea water or light brine, but inthe last case the salts limit the hydration, reducing the yield of the slurry1 highly saline #saltsaturated$ waters further decrease the bentonite hydration and are, therefore, notrecommended. In this case, attapulgite#called 6salt gel7$, another clay mineral, is frequentlyused, but, unlike bentonite, without having any improvement in fluid loss control.

    9.7.3. LIGHT EIGHT SLURRIES

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    ENTONITE@

    ATER@LJ100:

    SLURR=DENSIT=

    @:JL

    SLURR= =IELD@LJ100 :

    02$

    ,810

    121,20

    $$1+$8,+3

    7,08,,972

    107912911+0$

    189180173

    1,71,21+7

    1+21$71$3

    7+7870983

    109,12091321

    1$3$1,,7189$

    CLASS G CEMENT @$$ ATER

    9.7.3. LIGHT EIGHT SLURRIES

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    2. Lo% D!n#&t Mat!r&al#B M&ro#)!r!#

    M&ro#)!r!# are small gas0filled beads characteri-ed by very low density#between 3.&03.A kgG8$. They are, therefore, essentially used for preparing highstrength, low permeability and very light slurries with densities as low as 2.3& kgG8

    without making use of gaseous etenders.

    The main limitation of microspheres is their tendency to crush and collapse as depthand hydrostatic pressure increase1 as a consequence the density of the slurriescontaining microspheres increases at bottom hole conditions with respect tosurface. )or this reason, microspheres are not suitable for cementing deep wellswith high hydrostatic pressures.

    Two main types of microspheres are currently available"glass microspheres1ceramic microspheres.

    In slurry preparation !ra"& "&ro#)!r!# are generally used because of theiravailability and lower cost with respect to glass microspheres.

    9.7.3. LIGHT EIGHT SLURRIES

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    2. Lo% D!n#&t Mat!r&al#B M&ro#)!r!#C!ra"& "&ro#)!r!# #called also 6cenospheres7$ are obtained from ashes produced incoal0burning power plants1 their main constituents are silica and alumina, therefore, theyehibit a certain po--olanic behaviour, in particular at high curing temperatures. The beads are

    filled with a CO&0& miture. The particle si-e distribution of ceramic microspheres is lesshomogeneous than that of the glass counterpart and ranges between &30533 Bm1 furthermore,they are heavier with density in the 3.403.A kgG8 range. )or this reason higher concentrationsof ceramic microspheres are required to obtain low density slurries. Their use is notrecommended in wells where pressures eceed (23 kgGcm&values.

    PRESSURE SLURR= 1 SLURR= 2 SLURR= 3

    :J"2 )#& :J"2 )#& :J"2 )#& :J"2 )#&

    0 0 1.08 9.0 1.2, 10.+ 1.$$ 12.0

    3+ +00 1.1, 9.7 1.37 11.$ 1.+2 12.7

    70 1000 1.2+ 10.$ 1.$0 11.7 1.+7 13.1

    1$0 2000 1.32 11.0 1.$+ 12.1 1.,1 13.$

    210 3000 1.3$ 11.2 1.$9 12.$ 1.,$ 13.7

    280 $000 1.37 11.$ 1.+2 12.7 1.,8 1$.0

    9.7.3. LIGHT EIGHT SLURRIES

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    2. Lo% D!n#&t Mat!r&al#B Foa"!d C!"!nt#

    If formations characteri-ed by etremely low fracture pressures #naturally fractured rocks,depleted reservoirs, caverns, etc. where the fracture pressure gradient could assume values

    below 2 kgGcm&G23 m$ have to be cemented, the standard light0weight slurries are not suitablebecause they still ehibit a density which is much higher than deemed necessary1 thepossibility to further lighten these systems is hindered by the fact that densities below 2.(&02.//kgG8 are reachable only by strongly increasing the water content or the concentration of the lowdensity materials, compromising the mechanical strength and the permeability of the setcements.

    In these conditions, the only solution is to prepare ultra0light slurries by additions ofmicrospheres, as seen in the previous paragraph, or by adopting 'oa"!d !"!nt# orspecially0designed slurries.

    Foa"!d !"!nt# are dispersions which contain a base cement #neat or light0weight$, a gas#usually nitrogen because inert or, in some cases, air$, a foaming agentand foam stabili-ers.The foam density is tuned by simply varying the concentration of gas in the slurry1 therefore,

    slurries with density between that of neat cement systems #2.;302.A& kgG8$ and that of the gas#practically close to -ero$ can be theoretically prepared, though in practical terms densities inthe 3.:302.:5 kgG8 range are usually designed.

    9.7.3. LIGHT EIGHT SLURRIES

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    9.7.$. EIGHTED SLURRIES

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    !&t!d #l(rr&!#, that is slurries with a density higher than that of neat cement slurries, aregenerally designed for deep wells, where high pressures and temperatures are encountered,or when unstable formations require very heavy systems in order to be contained. In thesecases, the slurries must be designed at a density higher or at least equal to the mud density

    used while drilling. Therefore, slurries with density above &.33 kgG8 and up to &./30&.43 kgG8are sometimes needed to maintain the control of such wells.

    To increase the slurry density, two main approaches can be followed, that is"reduction of the waterGcement ratio1addition of weighting materials with specific gravity as high as possible.

    1. at!rJC!"!nt Rat&o R!d(t&onIn this case the amount of mi water is reduced below the ratios usually recommended by '!Ifor each class of cements in accordance with the slurry density required. The main problemthis solution poses is the difficulty in obtaining at the same time acceptable rheologicalproperties and good fluid loss control. The maimum density reachable with this method isaround &.2( kgG8. The main advantage is the low cost of the solution.

    9.7.$. EIGHTED SLURRIES

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    2. !&t&n A!nt#

    The addition of very !a "at!r&al# has to be used when slurry densities well above&.2(0&.&3 kgG8 are needed. ' high density material to act as a good weighting agent mustpossess some basic characteristics, that is" compatibility with the cement and its additives. )or instance, if barite is used asweighting material, it must have a granulometry similar to that of the cement in use,otherwise settling can occur if too large particles are present or viscosity and gel increasecan be epected in case of too small si-ed particles1 their water requirement has to be as low as possible. =igh water demand reduces thedensity of the slurry and increases volumes of weighting materials and costs1

    the material has to be inertand not interfere with the hydration reactions of the slurry.

    The materials most commonly used as weighting agents are"barite #a%O/$1hematite #)e&O($1manganese tetraoide #*n(O/$1ilmenite #)eTiO($1

    sand #%iO&$.

    9.7.$. EIGHTED SLURRIES

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    $$

    2.1. !&t&n A!nt#B ar&t!ar&t! is a natural occurring mineral, universally added to increase drilling mud weight. It ismainly formed by barium sulphate #a%O/$, but other minerals #i.e. carbonates and sulphates ofheavy metals$ can be present, depending on the source and the purity. The purity of the mineral

    strongly affects the specific gravity of this material, which can in fact vary between /.&& and /.((kgG8. The water requirement of barite is rather low #around &( 8G233 kg$, but sensibly higher thanthat of hematite and ilmenite, which makes this product causing a consistent diminution ofcompressive strength of the set slurry #from 2G( up to S with respect to a neat cement slurry$.9sually barite has a particle si-e distribution around /5 Bm for ;50A3?, very similar to that of thecement.

    MATERIAL DENSIT=JL

    ASOLUTE>OLUME

    @LJ:

    COLOUR ATERREUIREMENT

    @LJ100 :

    ILMENITEHEMATITE

    ARITEMn3O$

    $$+$9+

    $33$8$-$90

    022+0200

    023$

    LACRED

    HITEREDDISH

    0020

    200

    C8'%% ' C8'%% E 'HITF @'TFH %89HH 'HITFG%89HH

    9.7.$. EIGHTED SLURRIES

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    $+

    C8'%% 'CF*FT

    #kg$

    C8'%% ECF*FT

    #kg$

    'HITF

    #kg$

    @'TFH

    #8$

    %89HHEF%IT

    #gG8$

    'HITFG%89HHH'TIO#qGm($

    100

    100100100100100100

    100100100100100100100

    -

    2++07+

    1001+0200

    -2++07+

    1001+0200

    $,0

    +17+7+,32,9080+920

    380$37$9+++2,1072+8$0

    1880

    198020,0212021,022$02300

    2000210021802220228023202380

    128

    1$01$81++1,117017+

    1$+1++1,31,917$18118,

    Cla## AB d / 31+ %J / $, a5#. ol("! / 317 LCla## DB d / 32+ %J / 38 a5#. ol("! / 308 Lar&t!B d / $2+ 5J / 23 a5#. ol("! / 23+ L

    9.7.+. DISPERSED SLURRIES

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    're named d)!r#!d #l(rr&!#, those slurries %o#! r!olo&al )ro)!rt&!# a! 5!!n!nan!d 5 t! add&t&on o' #)!&'& !"&al# all!d d)!r#ant# or al#o 'r&t&onr!d(!r#. Their use is particularly necessary in deep wells with high temperatures andpressures which require very heavy slurries1 in these wells normal slurries can not be pumped

    in t(r5(l!nt 'lo% @a 'lo% %& !n#(r!# a 5!tt!r ol! l!an&n and #l(rr )la!"!nt because of their poor rheology. The dispersants, on the contrary, improve dramaticallyviscosity, yield strength and gels of a slurry, permitting to obtain relatively low friction losseswith the high flow rates necessary to reach turbulent flow conditions.

    The most common dispersants used in cementing operations belong to the followingcategories"

    )la#t&&!r#1#()!r)la#t&&!r#.

    )or what concerns the mechanism of action, both plastici-ers and superplastici-ers behavemore or less in a similar way1 in fact, most dispersants are anionic polymers and,consequently, they are adsorbed #6adsorption mechanism7$ on the positively charged cementgrains, from the initial phase of cement hydration to the final set, favouring their mutual

    repulsion and consequently their dispersion.

    9.7.+. DISPERSED SLURRIES

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    PLASTICIQERS

    The main chemicals, used as cement dispersants and belonging to this category, are wellknown products, regularly present in drilling mud formulation, that is"

    lignosulphonatesand modified lignosulphonates1hydrocarboylic acids" citric acid, tartaric acid, salicylic acid, gluconic acid and glucoheptonicacid.The plastici-ers behave also as retardersand have been described in the paragraph dedicatedto this category of cement additives1 because of their two0fold effect, the use oflignosulphonates and hydroycarboylic acids has to be carefully evaluated to avoid undesiredsecondary effects.

    SUPERPLASTICIQERSThe products of this category represent the most commonly used dispersants for well cementsand are"polysulphonated polymers" polynaphthalene sulphonate #!%$1polymelamine sulphonate#!*%$, polystyrene sulphonate#!%%$1polycarboylate0based products.

    CLASS D 7 +J8 C & & 9 H l

    9.7.+. DISPERSED SLURRIES

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    CLASS D 7 +J8 Ca#&n &n 9 Hol!

    PNS @CFR-3

    T!"). at!r D!n#&t P> =P Flo% Rat! 'orT(r5(l!n!

    Ann(lar>!lo&t 'orT(r5(l!n!

    OC oC :Jl&tr! ) FJ100 "2 l&tr!J"&n "J"&n

    0 8 38 1.97 +0.$ $.2 18,0 11$

    0.+0 8 38 1.97 $2.8 0.0 9$0 +7

    0.7+ 8 38 1.97 39.2 0.0 8,0 +3

    0 38 38 1.97 29.$ 27.3 2800 172

    0.+0 38 38 1.97 7.9 0.2+ 3+0 220.7+ 38 38 1.97 21.1 0.0 $,0 29

    0 ,0 38 1.97 22.+ $0., 31,0 19$

    0.+0 ,0 38 1.97 1,., 0.0 3,0 22

    0.7+ ,0 38 1.97 1+.$ 0.0 33+ 20

    0 88 38 1.97 1+.7 $1.3 32+0 2000.+0 88 38 1.97 23.7 2.0 1030 ,3

    0.7+ 88 38 1.97 13.2 0.2+ $90 30

    9.7.,. REDUCED FILTRATE SLURRIES

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    @hen a cement slurry is placed in front of a permeable formation and is subMect to adifferential pressure t! 'l(&dlo## )ro!## ta:!# )la!as a result, a certain amount ofwater is lost into the formation and if this process is not adequately controlled seriousoperational consequences can arise, such as"

    the density of the slurry tends to increasebeyond safety limits if the water content decreasestoo much. This determines an increase of the hydrostatic head of the slurry and a progressiveworsening of its rheological properties with negative effects on friction losses, because higherpump rates are required to achieve the planned flow conditions1 a decrease in water content leads to shorter thickening times which can compromise thesuccess of the cementing Mob1 during static periods, i.e. during wait0on0cement, annular bridging can occur, particularly invery narrow annuli or in correspondence of hole restrictions with risks of formation fluidmovement #gas migration$.

    9sually, normal neat cement slurrieswithout any fluid0loss control agents show an'!I filtratearound 233302533 mlitreG(3P. This value can be considered acceptable in normal wells, but assoon as the operative conditions become more severe #deep wells, high differential pressuresand temperatures, very permeable formations, sensitive and reactive formations$ the API'&ltrat! a# to 5! #tronl r!d(!d !!n do%n to 20-+0 "l&tr!J30 . To reach this obMective'l(&d-lo## ontrol "at!r&al#have to be added to cement slurry formulations.

    *any additives are available which can control fluid0loss #more or less the same seen for

    9.7.,. REDUCED FILTRATE SLURRIES

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    *any additives are available which can control fluid loss #more or less the same seen fordrilling muds$1 they belong to two main categories"particulate materials, that is materials characteri-ed by very small dimensions so that theycan enter the filter cake and lodge among the cement particles decreasing its permeability.

    'dditives with this behaviour are" bentonite, carbonate powder, carbon black, asphaltenes,lattices, polyvinylalcohols. The mechanism of action of this category of fluid0loss controlagents, which are characteri-ed by small particles with a wide range of granular si-edistribution, is the physical plugging of the pore throats of the filter cake and the consequentreduction in its permeability.water soluble polymers, which operate by increasing at the same time the viscosity of theaqueous phase and decreasing the permeability of the filter cake. ' wide variety of polymers

    ehibits these properties such as" cellulose derivatives #C*=FC, =FC, =!C$$, non0ionicpolymers #uar um derivatives, !U'" polyvynilalcohol$$, anionic polymers #%!%"sulphonated polystyrene$ and also cationic polymers #!FI, polyethylene imine$. %yntheticpolymers represent today the most common fluid0loss control agents. These materials controlfluid0loss through a combination of the two mechanisms described above, though the mainaction is always due to the physical plugging of the pores of the filter cake. @ater0solublepolymers can form weakly bonded colloidal aggregates in solution, which are sufficientlystable to become wedged on the filter cake constrictions. They can also be adsorbed on thesurfaces of the grains, thus decreasing the si-e of the pores to occlude. The effect on watermobility, due to an increase in its viscosity, is by far less important than the mechanicalplugging action.

    9.7.7. LOST CIRCULATION PRE>ENTION MATERIALS

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    Occurrence of &r(lat&on lo##!# during cementing can have serious consequences on thefinal result of a Mob and are very difficult to combat at this stage. 8uckily, in most cases, the riskto incur in circulation losses is eperienced during drilling and can be anticipated by a careful

    analysis of pore and fracture pressures1 therefore, timely and appropriate countermeasurescan be adopted before starting the cementing operation, in such a way to eliminate orsignificantly reduce the problem.

    If the circulation loss can not be eliminated before the cementing Mob, some options can beadopted to limit the consequences"the first is to maintain the downhole pressure during the Mob below that caused by the mud at

    its maimum equivalent circulating density. This is obtained by reducing the density of theslurry, limiting the height of the cement column or minimi-ing the friction losses during theslurry placement. The use of light0weight slurries aids in limiting circulation losses1 the second option consists in pumping a plugging materialas a spacer in front of the slurry,add lost circulation materials, 8C*s, to the slurry itself or use special additives which impart tothe slurry thiotropic properties1the last option is to use a combination of the many techniques available, a solution, which is

    often necessary, when trying to prevent cement losses in highly fractured or vugularformations.

    9.7.7. LOST CIRCULATION PRE>ENTION MATERIALS

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    T)! Mat!r&al Nat(r! o' Part&l!# A"o(nt U#!d at!r R!(&r!d

    Gran(lar G&l#on&t! Grad!d + to +0 33

    P!rl&t! E*)and!d 1+ to ,0 +0

    aln(t S!ll# Grad!d 1 to + 1$

    Coal Grad!d 1 to 10 33

    La"!llar C!llo)an! Fla:!# Fla:!d 01+ to 2 Non!

    F&5ro(# Nlon Sort F&5r!# 03 to 12 Non!

    Gla## Lon F&5r!# 2 to 3 Non!

    *any cement additives can cause the slurry to foam during miing. E*!##&! #l(rr 'oa"&n

    9.7.8. ANTIFOAM AGENTS

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    y y g g an a! #!!ral (nd!#&ra5l! on#!(!n!# on #l(rr )!r'or"an!#such as"slurry gellation1cavitation problemsduring pumping with loss of hydraulic pressure1

    slurry density underestimation. If air is entrapped in the slurry at surface, the density of theslurry results reduced and consequently more cement is added to the slurry to respect theplanned value1 but as soon as the slurry is pumped downhole, the air becomes compressedand the density of the slurry increases above what designed. The densitometer at surfaceconditions underestimates the true downhole slurry density.

    To combat this problem, ant&'oa" a!nt# are usually added to the mi0water or are dry0

    blended with the cement. 'ntifoam agents work by causing a shift in surface tension, alter thedispersibility of solids or both things together, so that the conditions required to produce a foamare not anymore present. In general, antifoamers should have the following characteristics"insoluble in the foaming system1a surface tension lower than that of the foaming system.

    In well cementing two classes of antifoam agents are commonly used")ollol !t!r#1#&l&on!#.

    E i th l t tt ti h b id b th I d t t h th

    9.7.9. STRENGTHENING AGENTS

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    Euring the last years, more attention has been paid by the Industry to enhance othermechanical characteristics of a set cement slurry than compressive strength, in particular its'l!*(ral and #o: r!#tan!capabilities.

    %ome materials, which can be used to improve the strength of a cement, are the following"

    F&5ro(# Mat!r&al#" some fibrous materials, such as nylon fibreswith fibre lengths up to &.5mm, when added to a slurry in concentrations between 3.25? and 3.5? can increase itsresistance to the stresses caused by perforations, hydraulic fracturing and formationmovement.

    M!tall& M&ror&55on#" these products were introduced to improve the impact resistance,

    toughness and tensile strength of set cements. The concentration of the microribbons isusually about 2.5? by volume of slurry. This system is particularly effective for kick0off plugs.

    R(55!r#" rubber chops, added in concentration up to 5? @OC, considerably increase theimpact resistance and fleural strength of cements.

    's drilling technology advancements have made possible the eploration and production of

    9.8. SPECIAL PORTLAND CEMENT SLURRIES

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    hydrocarbons in ever more demanding situations, such as deep waters, cold climates,ultradeep wells and particular requirements have arisen regarding environmental safeguard,long0life well integrity, safety, etc., the need for ad0hoc cement slurries has become a priorityleading to the development of #)!&al ##t!"#, to be used when epressly necessary.

    'mong the many formulations nowadays available, the following systems are common in thesedemanding environments" t&*otro)& !"!nt #l(rr&!#, which are preferably used in case of circulation losses, gasmigration control, casing repair and remedial cementing Mobs1 !*)an#&! !"!nt and latt&!#-5a#!d !"!nt #l(rr&!# to be used for enhancing -onal

    isolation and mitigate gas migration occurrences1!"!nt #l(rr&!# 'or d!!) and (ltrad!!) %!ll#where the high temperatures can cause theretrogression of the mechanical strength of standard formulations1 !"!nt #l(rr&!# 'or lo% t!")!rat(r! %!ll# as encountered in 'rctic regions and in deepwaters1!"!nt #l(rr&!# 'or #o(r !n&ron"!nt#1 'l!*&5l! and #%!lla5l! !"!nt #l(rr&!# with enhanced mechanical performances andimproved bonding characteristics1#)!&al !"!nt# %&t !n&n!!r!d )art&l! #&! @EPS dtr&5(t&onto be used in particularlyproblematic cementing Mobs #CemCrete and Cem%tone systems$.

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    9.9. CASING EUIPMENT

    9.9. CASING EUIPMENT

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    %everal devices are installed or attached to a casing string to fulfil different purposesand functions. )or eample" the lower end of a casing string is protected by a (&d! #o!, whose aim is tofacilitate its running into the hole1at a certain distance from the casing shoe, one or few more Moints above it, a ollar#floating or automatically filled$ is installed to provide also a seat for the cementingplugs and to avoid the flowback of the slurry1 on the outside of the casing, !ntral&!r#, #rat!r# and #to) ollar# are

    attached in critical sections to assure acceptable casing centrali-ation and enhancemud removal operations.

    9.9.1. CASING SHOES AND COLLARS

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    GUIDE SHOE

    G(&d! #o!# are a very simple andeconomic devices. They are installedon the lowermost Moint of a casingstring and have a rounded nose, whosefunction is to guide the casing throughdog0legs and restrictions in theborehole.

    The round nose and internal

    accessories are constructed withdrillable materials such as cement oraluminium1 the eternal case, on thecontrary, is made usually by +055 or 0;3 steel grade.

    STANDARD

    GUIDE SHOE

    DON-ET

    GUIDE SHOE

    Float !(&)"!nt consists of specially

    9.9.1. CASING SHOES AND COLLARS

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    FLOAT SHOES AND COLLARS

    Float !(&)"!nt consists of speciallydesigned a#&n #o!# and ollar# whichcontain check valves #ball valves, flappervalves, poppet valves$ which prevent wellbore

    fluids from entering into the casing string.Consequently, as the casing is run into thehole, the casing is filled from the surface withmud. The frequency of filling is usually onceevery 5023 Moints, but this decision has to bebased on the casing collapse resistance.@hen the casing reaches the bottom, mud is

    pumped in order to completely fill the casingMoints and circulation is established to cleanthe well.Euring casing Moints addition and once thecement slurry has been displaced, the floatvalve installed in the casing shoe orGand in thecollar allows preventing the mud or the cementslurry to flow back into the casing string.

    FLOATCOLLARS

    FLOAT SHOES

    A(to"at& '&ll-() #o!# and ollar# contain checkvalves #flapper and poppet valves$ similar to those

    9.9.1. CASING SHOES AND COLLARS

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    AUTOMATIC FILL-UP SHOES AND COLLARS

    valves #flapper and poppet valves$ similar to thoseinstalled in float equipment, but which have been modifiedto maintain an open position during casing running toallow filling and reverse circulation. The casing is

    continuously filled while lowered into the well thus savingtime and reducing the pressure surges risks as commonwith float equipment. The valves are generally designedin such a way to reduce casing overflow by regulating thefill0up rate as a function of the casing run0in speed1 if therun0in is, for instance, one Moint per minute, the fluid levelinside the casing should remain one or two Moints below

    the annulus level. If run0in problems arise, also reversecirculation is permitted to eliminate bridges and any otherrestriction. @hen required the system can be convertedinto a conventional float equipment #by dropping a ball$which impedes fluid flowback1 conversion is generallyperformed when the casing is in place, but can be alsodone while running to control overflow or limit the hookload. 'uto0fill equipment is recommended when the hookload is not a big problem.

    AUTOMATIC FILL-UP >AL>ESBFLAPPER @LEFT POPPET @RIGHT

    D&''!r!nt&al '&ll-() #o!# and ollar# combine the

    9.9.1. CASING SHOES AND COLLARS

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    DIFFERENTIAL FILL-UP SHOES AND COLLARS

    advantages of floating and auto0fill equipment. They aredesigned to automatically fill and regulate the fluid levelinside the casing string. Eifferential fill0up equipment isoften used when running in hole very long strings toreduce surge pressure risks and formation damage. Thetypical differential valve regulates fill0up through the actionof a floating differential piston. The piston slides upwardsto open the valve and downwards to close it and isdesigned in such a way that the upper pressuri-ed area isapproimately 23? larger than the lower. The forcesmoving the piston are produced by hydrostatic pressures

    acting on the upper and lower surfaces. @hen thepressure above #hydrostatic pressure inside the casing$eceeds A3? of the pressure below #hydrostatic pressureinside the annulus$, the piston will slide down closing thevalve and halting the filling1 if, on the contrary, the reverseoccurs and the pressure below eceeds A3? of thepressure above, the piston moves up opening the valve

    and the filling is resumed.

    DIFFERENTIAL FILL-UP SHOE @LEFTAND COLLAR @RIGHT

    M(lt& #ta! !"!nt&n tool# can be used in the following circumstances"

    9.9.2. MULTI-STAGE CEMENTING EUIPMENT

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    M(lt&-#ta! !"!nt&n tool#can be used in the following circumstances"

    in wells where the hydrostatic head due to the slurry can eceed the fracture formationpressure with high risks of fracturing the formations1

    in deep, high temperature wells where the time to pump the desired quality and quantity ofcement slurry is not sufficient1

    where only portions of the wellbore require to be cemented1

    when different blends of cement must be safely pumped1 in hori-ontal wells where the curved section of the well requires proper cementing.

    The two0stage cementing, which is the most practised technique, can take place according to

    two methods, which require slightly different tools"a$ regular two0stage cementing1

    b$ continuous two0stage cementing.

    oth techniques require the use of"

    #ta! ollar#1

    #!t# o' a))ro)r&at! )l(#.

    The stage collars are devices installed in

    9.9.2. MULTI-STAGE CEMENTING EUIPMENT

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    STAGE COLLARS AND PLUGS

    The stage collars are devices, installed inthe casing string at planned depths,which are hydraulically opened andclosed using free0fall darts or pump0downplugs to select and shift the appropriateinternal sleeve.

    C!ntral&!r# are one of the simplest but yet more beneficialdevices used in primary cementing. They are attached to the

    9.9.3. CENTRALIQERS

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    CENTRALIQERS

    p y g youtside of a casing string and are designed to maintain it asmore centred as economically feasible in the hole, providingthe following benefits"

    reducing drag and differential sticking risks while the casingis run into the hole1 improving"

    0 mud removal efficiency10 cement placement, by creating a uniform sheath ofcement around the casing Moints10 performance of the other casing attachments, such

    as scratchers and cement baskets.The most frequently used types of centrali-ers can be groupedinto three standard main categories, while a fourth categoryincludes recently developed tools, that is" rigid centrali-ers1 semirigid centrali-ers1 spring0bow centrali-ers1 new devices #ceramic centrali-ers$.

    Sto) ollar# or r&n ollar# are simple devices having thefunction to hold or limit the movement of eternal casing

    9.9.$. STOP COLLARS SCRATCHERS

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    STOP COLLARS SCRATCHERS

    attachments such as centrali-ers, scratchers, cement basketsalong the casing.

    Srat!r# are eternal devices designed to facilitate theremoval of mud cake and gelled mud from the wellbore wallsby means of pipe movement. This action provides betterbonding surfaces for the slurry and helps -onal isolation. They

    also reinforce the cement column and induce locali-edturbulence during slurry placement. These devices are veryeffective when the casing is well centrali-ed and moved beforeand during the cementing Mob. To prevent build0up, scratchersshould be spaced in such a way that an area worked by ascratcher be overlapped by adMacent scratchers.There are two general types of scratchers" reciprocating type1 rotating type.

    C!"!nt&n )l(#are used with the following scopes" to wipe out the mud film from the interior of the casing string1

    to separate cement slurry from spacers and muds1

    9.9.+. CEMENTING PLUGS

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    CEMENTING PLUGS

    to separate cement slurry from spacers and muds1 to prevent overdisplacement of the cement slurry1 to indicate when the cementing Mob is completed by an increase indisplacement pressure1

    to allow casing to be pressure0tested immediately after cement slurryplacement and before floats are checked.The plugs used during a cementing Mob are generally two" the 5otto")l( and the to) )l(. In most operations both plugs are used, butsometimes only the top plug can be launched into the casing string.

    otto" and to) )l(# are available in many configurations and theirchoice depends on the particular situation that has to be faced. Top and

    bottom plugs are quite similar in eternal appearance, but they differ ininternal design1 to avoid confusion between them, they can be differentlycoloured. ottom plugs precede the slurry and consist in a hollow,plastic or metallic #once wooden$ core with a thin elastomeric membranearound it, which can be ruptured. Top plugs are pumped immediatelyafter the slurry and before the displacement mud and, once in placeabove the bottom plugs, indicate, through an increase in pumpingpressure, the end of the slurry placement. They have no rupture diskand can resist very high pressures without breaking.

    TOP PLUG AND OTTOM PLUG

    NON-ROTATINGTOP PLUG AND OTTOM PLUG

    C!"!nt&n !ad# called also plug containers are

    9.9.,. CEMENTING HEADS

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    CEMENTING HEADS

    C!"!nt&n !ad#, called also plug containers, areinstalled on top of the casing string before starting thecementing Mob to allow the drop of the plugs without,or partially, opening the casing.

    *any styles of plug containers are made availablefrom different manufacturers, such as" standard and compact single and double plugcementing heads1 subsurface release #%%H$ plug containers foroperations from floating vessels1

    remote control cement plug container systems.

    STANDARS SINGLE-PLUG @LEFTAND DOULE-PLUG @RIGHT

    CEMENTING HEADS

    9.10. PRIMAR= CEMENTING DESIGN

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    ' Cement !rogramme is usually reali-ed in two stages"

    a )r!l&"&nar d!#&n is shown in the eological and Erilling!rogramme1

    the '&nal d!#&n is completed at the 6la#t "&n(t!7, only when thelaboratory receives the actual data of the Must drilled hole section, and inparticular the data concerning temperatures, =% hole si-e, pressures,

    deviation.

    The design of a cementing Mob, even if it is only a preliminary eercise, requires all the same thecollection and interpretation of a huge amount of data, if all the various factors playing a role in

    9.10.1. PRELIMINAR= DESIGN

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    the operation are to be considered.In particular, it is necessary to perform a thorough and comprehensive acquisition of allpertinent data, such as"

    planned depth of the well #vertical and measured$ and its nominal dimensions #well profile,planned inclination and a-imuth$1 epected wellbore geological and mineralogical characteristics #i.e. stratigraphy, lithology,water0 and hydrocarbon0bearing levels, salt -ones, permeable formations, reactive formations,etc.$1 predicted wellbore physical characteristics #i.e. pore pressures and gradients, fracturepressures and gradients, static temperature$1theoretical top of cement #according to the '!IGI%O recommendations the top of cement is tobe located at a depth where the static temperature at that particular depth is higher than the

    bottom hole circulating temperature, =CT$1planned drilling mud data#density, rheology, filtrate, composition$1 planned casing string characteristics #i.e. si-e, thickness, mechanical properties, length,weight, composition$1 epected problems #i.e. circulation losses, hole instability, gas migration, sour fluids,

    mechanical strength retrogression$1 parameters, which can affect the integrity of the cement sheath and the efficiency of itshydraulic isolation during the production life of the well and after abandonment #i.e. sour fluids,tectonic movements, mechanical stresses$.

    The '&nal d!#&n, which will be effectively applied in the real well, is postponed until all thedetailed information pertaining to that particular hole section is made available Therefore the

    9.10.2. FINAL DESIGN

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    detailed information pertaining to that particular hole section is made available. Therefore, thecementing operation is planned few days and, often, only few hours before the commencing ofthe Mob. This implicitly underlines the criticality of this operation and the impact that even smallvariations in the value assumed by the parameters involved can play on the entire operation.

    The final design of any cementing Mob must carefully consider the following points"verification of the datapreviously used in the preparation of the preliminary programme1evaluation of the characteristics and composition of the drilling mudin hole1calculation of the slurry volumes, based on the actual hole si-e measurements and casingdimensions1

    control of the pore and fracture pressuresdevelopment versus depth1 verification of the static temperature profile of the well and calculation of the bottom holestatic temperature, bottom hole circulating temperature and temperature differential betweenthe bottom and top of the cement column1evaluation of the consequences of the epected hole problems1slurry designwith definition of its properties and composition1

    design of the required chemical washes and spacersin terms of volumes, characteristics andcomposition1

    eecution of the required laboratory workto fine0tune the characteristics, composition andperformances of slurry and preflushes with evaluation of their mutual compatibility and that

    9.10.2. FINAL DESIGN

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    performances of slurry and preflushes, with evaluation of their mutual compatibility and thatwith the drilling mud1selection of the casing centrali-ationscheme1

    design of the hydraulic programmewith selection of the recommended flow rates and flowregimes to use during slurry displacement1selection of the most appropriate placement techniqueof the slurry1selection of the surface equipment.

    >ERIFICATION OF THE DATA USED IN THE PRELIMINAR= DESIGN

    9.10.2. FINAL DESIGN

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    This group comprehends data easily verifiable, because immediately available from the analysisof the drilling operations carried out in that specific hole section. To this group belong the databelow described.

    At(al Pro'&l! o' t! !ll" this information is very important and very easy to acquire. It showsdepth, length and nominal si-e of the casings run in the hole till that moment and the basiccharacteristics of the open hole, in which the net casing will be lowered and cemented.The depth and si-e of the last casing, open hole and net casing determine the volume of theslurry to prepare for the cementing Mob and serve, with other considerations, as a guide for thepositioning of the cement top.

    L&tolo" from the analysis of the *aster 8og and the logs recorded in the open hole section,Must prior to the cementing Mob, eventual changes in the epected lithology can be envisaged.9nless of large differences in the lithological sequence between what predicted and what found#for instance the occurrence of not epected salt -ones$, the slurry design is not too muchaffected by minor lithology variations.8ithology can essentially influence the slurry type and its composition.

    >ERIFICATION OF THE DATA USED IN THE PRELIMINAR= DESIGN

    9.10.2. FINAL DESIGN

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    Dr&ll&n M(d Carat!rt and Co")o#&t&on" this represents another important piece ofinformation easily available. =ow the mud characteristics #i.e. density, rheology, filtrate, p=, etc.$and composition #i.e. @*, O*, %O*$ have varied while drilling the open hole section can

    be derived from a careful analysis of the 6Eaily *ud Heports7, which also indicate the maMorproblems the mud posed while drilling.The knowledge of the mud type in hole, its composition and properties is essential to plan andreali-e an adequate mud removal and to prevent slurry contamination.Chemical washes and spacers can be used for this scope and their composition strictly dependson the type of mud in hole.

    D!&at&on Data" the knowledge of the actual traMectory followed by the well is also critical due toits influence on volumes, casing centrali-ation, mud removal efficiency and well control. @hilemost of these data are available in the case of deviated wells, this is not always possible for theso0called 6vertical wells7. @hen planning a centrali-ation scheme in such vertical wells, acommon recommended practice is to assume a minimum inclination of (o to account foruncertainties in the well traMectory.Of course, if during the drilling of a certain hole section the inclination and a-imuth data have

    been acquired because *@E or 8@E tools have been used, the problem does not eist.

    >ERIFICATION OF THE DATA USED IN THE PRELIMINAR= DESIGN

    CALCULATION OF ASIC PARAMETERS

    9.10.2. FINAL DESIGN

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    CALCULATION OF ASIC PARAMETERSOnce verified the data as above described by comparing the predicted with the actual situationof the well, the net step consists in calculating the basic parameters necessary to plan the Mob

    and define the slurry properties and composition. These parameters are also important forplanning the laboratory tests and are" effective hole si-e1 temperature profile1 pressures regime.

    HOLE SIQE E>ALUATION's well known, a well is rarely in gauge, as a consequence of the interactions betweenformations, drilling fluids and drill string1 these chemical and physical interactions can causeboth hole enlargement or tightening. The variations in the hole shape and si-e influence manyparameters of a cementing Mob, first of all the volumes, but also the hydraulics #velocity of thefluids in the annulus, flow rates, mud removal efficiency, friction losses$ and the centrali-ationschemes.

    'll possible efforts have to be made in order to obtain a clear picture of the hole si-e and shapevariations with depth1 this obMective can be reached or by means of comparisons with similar

    wells in the same area #usually the volume of slurry required is increased of a certain percentagedepending on the lithological characteristics of the area$ or, better, by recording caliper logs.

    >ERIFICATION OF THE DATA USED IN THE PRELIMINAR= DESIGN

    TEMPERATURE REGIME

    9.10.2. FINAL DESIGN

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    TEMPERATURE REGIMEThe knowledge of the different temperatures at which the slurry will be eposed is anotherfundamental piece of information for a correct and reliable design of a cementing Mob and forreali-ing a successful operation.The temperatures, which are necessary to determine with a high degree of accuracy, are" the ottom =ole %tatic Temperature, =%T1 the ottom =ole Circulating Temperature, =CT1 the temperature differential, VT, between the bottom and the top of the cement column.

    OTTO* =O8F %T'TIC TF*!FH'T9HF, =%TThe ottom =ole %tatic Temperature, =%T, is the undisturbed temperature at the bottom of awell and represents the temperature of the formations located at that depth.The =%T can be" calculated, once the geothermal gradient of the area and an assumed surface temperature areknown. eothermal gradients are easily derived from previous wells drilled in the area of interestor from other sources, while the surface temperature is assumed equal to a certain value.

    '!IGI%O 23/&40&"&33( in 'ppendi C suggests a value of &:oC #;3o)$, but other surfacetemperatures can be used at the discretion of the Operator1

    estimatedfrom previous wells drilled in the area1 directly measured in the well of interest by means of temperature sensors during logging ortripping.

    >ERIFICATION OF THE DATA USED IN THE PRELIMINAR= DESIGN

    The ottom =ole %tatic Temperature affects the following properties of a slurry"

    9.10.2. FINAL DESIGN

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    The ottom =ole %tatic Temperature affects the following properties of a slurry" the rate at which a slurry develops its compressive strengthonce placed into the wellbore. Onecritical point to consider, when evaluating the development rate of the compressive strength of acertain slurry at =%T conditions, is the time required by the temperature to raise from the=CT, observed at the end of the cementing Mob, to the =%T. This is, in fact, the temperature atwhich the compressive strength is measured in the laboratory. )or very cold wells the timerequired to pass from the =CT to the =%T can be as long as &/ h1 this time can have seriousconsequences on the development of the mechanical properties of a set cement slurry1 the durability of the set cementduring the lifetime of a well.

    Temperature simulators can assist in predicting more realistic static and circulating temperaturesand temperature profiles inside the casing string and in the annulus, which more reliablyrepresent actual well conditions.

    >ERIFICATION OF THE DATA USED IN THE PRELIMINAR= DESIGN

    OTTO* =O8F CIHC98'TI TF*!FH'T9HF, =CT

    9.10.2. FINAL DESIGN

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    ,The ottom =ole Circulating Temperature, =CT, is the temperature the slurry will theoreticallyencounter while being placed in the hole and can be predicted once the =%T has beendetermined, as previously seen.The =CT influences, together with the depth and the volumes to be pumped, importantcharacteristics of a slurry, that is" the thickening timethe slurry should possess to safely perform the operation1 the rate of compressive strength development, which depends from the time required by theslurry to pass from the =CT to the =%T1 and, consequently, its composition #i.e. type and concentration of accelerators, retarders,miing water, etc.$.

    The methods, normally used to predict the =CT, are essentially" the '!IGI%O methodology, as published in the Eocument I%O 23/&40&"&33(, 'nne C6'dditional Information Helating to Temperature Eetermination7 and 'nne F 6Cementing%chedules71 numerical simulators, normally developed and provided by %ervice Companies.

    >ERIFICATION OF THE DATA USED IN THE PRELIMINAR= DESIGN

    TF*!FH'T9HF EI))FHFTI'8 FT@FF CO89* TO! 'E OTTO*

    9.10.2. FINAL DESIGN

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    The determination of the Temperature Eifferential between the bottom and the top of the cementcolumn is also critical, because it can happen that a slurry, retarded to remain fluid for a certainperiod of time when eposed to the =CT predicted for the bottom of the well, could remainunset or develop its compressive strength very slowly when placed at shallower depths, asoccurs in correspondence of the cement top, where the temperature is surely lower.The practice usually adopted is to fi the top of cement at a depth where the static temperatureat this depth be higher than the =CT1 if these conditions are not reachable, a solution consistsin performing a multistage cementing Mob or in making use of two slurries with differentcomposition, the lead slurry being, for instance, less retarded than the tail slurry. If this lastpractice is applied, care has to be given to avoid any stops during the cementing Mob, otherwisethe