4 Oilfield Review
No More Waiting: Formation Evaluation While Drilling
Bob AdolphChris StollerPrinceton, New Jersey, USA
Mike ArcherChevronLafayette, Louisiana, USA
Daniel CodazziTamir el-HalawaniPatrick PerciotGeoff WellerClamart, France
Mike EvansSugar Land, Texas, USA
Jeff GrantHouston, Texas
Roger GriffithsAbu Dhabi, United Arab Emirates
Don HartmanGerald SirkinDevon Energy CorporationHouston, Texas
Makoto IchikawaJapan Oil, Gas and Metals National Corporation (JOGMEC)Chiba, Japan
Graham ScottNexen Petroleum U.K. LimitedAberdeen, Scotland
Ian TribeAberdeen, Scotland
David WhiteCambridge, England
For help in preparation of this article, thanks to FranoiseAllioli, Clamart, France; Sonny Auld, Emma Jane Bloor andSonny Johnston, Sugar Land, Texas; Zoila Cedeo, Ivor Gray, Bart Hughes and Russ Neuschaefer, Houston;and Chatham Grimmer, Youngsville, Louisiana.adnVISION, APS (Accelerator Porosity Sonde), arcVISION,DecisionXpress, DSI (Dipole Shear Sonic Imager),EcoScope, EcoView, ECS (Elemental CaptureSpectroscopy), ELANPlus, GeoFrame, geoVISION, GVR(geoVISION resistivity), Minitron, Orion, Platform Express,RAB (Resistivity-at-the-Bit), RST (Reservoir SaturationTool), SpectroLith, TDT (Thermal Decay Time), TeleScopeand WellEye are marks of Schlumberger.
Accurate and timely formation evaluation is an essential element of the exploration
and production business. In the past, operators had to compromise between the
real-time advantages of logging-while-drilling tools and the more comprehensive
formation evaluation of wireline techniques. A new, integrated logging-while-
drilling tool, along with powerful interpretation software, sets a new standard in
safety and efficiency, and reduces formation evaluation uncertainty.
Exploration and production companies havebeen anticipating a faster, safer and morecomprehensive way to assess the productivepotential of oil and gas reservoirs and toaccurately place productive wells using logging-while-drilling (LWD) tools. Until recently, basicformation properties, such as resistivity andporosity, in addition to drilling-relatedmeasurements, such as inclination, vibration andannular pressure, were acquired by stackingindividual measurement tools in long bottomholeassemblies (BHAs). These assemblies can takesubstantial time to make up and break downwhen tripping into and out of a well. Perhapsmore importantly, longer distances between bitand sensors cause measurement delays and force engineers and geoscientists to wait forinformation that, in many cases, could
immediately influence drilling procedures andtarget identification.
Logging-while-drilling priorities, identifiedthrough an industry survey, included reducingthe distance from the bit to LWD sensors.Reducing this distance decreases environmentaleffects on measurements and reduces waitingtime for acquisition and interpretation of dataneeded for key decisions.1 Along with improvedtool reliability and increased real-time data-transmission rates to surface, survey respondentsalso expressed a desire to eliminate chemicalradioactive sources in LWD tools.
The wait for improved capabilities is over.Scientists and engineers at Schlumberger havedeveloped an integrated LWD tool that meetsthese requirements and delivers importantdrilling and logging measurements. These include
1. Weller G, Griffiths R, Stoller C, Allioli F, Berheide M,Evans M, Labous L, Dion D and Perciot P: A NewIntegrated LWD Platform Brings Next-GenerationFormation Evaluation Services, Transactions of theSPWLA 46th Annual Logging Symposium, New Orleans,June 2629, 2005, paper H.
2. Rotary valve pressure-pulse generators, which alternatelyrestrict and open the flow of the drilling fluid, causevarying pressure waves to be generated in the drillingfluid at a carrier frequency that is proportional to the rateof interruption. Downhole sensor-response data aretransmitted to the surface by modulating this acousticcarrier frequency.
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measurements already obtained by existing LWDtools and new measurements, previously providedonly by wireline-conveyed technologies thatprovide information about formation lithologyand fluids. An innovative tool design reduces thelength of the entire measurement section to asingle 26-ft [7.9-m] collar and offers a sourcelesslogging option that reduces risk to personnel, theenvironment and the well.
This article briefly reviews the history ofmeasurements-while-drilling (MWD) and LWDtechnologies, along with their advantages andlimitations. We introduce the new EcoScopemultifunction logging-while-drilling service anddescribe its measurements and the obstacles
overcome during its development. Case studiesdemonstrate the early impact of this newtechnology and its associated interpretationsoftware in Gulf of Mexico, North Sea and MiddleEast reservoir exploitation.
Advances Above the BitThe technological progression of takingmeasurements while drilling has been steady butsomewhat limited by the difficulties oftransmitting data to the surface in the boreholeenvironment. Commonly, analog data from LWDsensors are converted to binary form downhole.By using a flow-restricting mechanism in thedrilling-fluid flowstream, data are transmitted by
producing positive or negative pressure pulses.These pressure pulses are transmitted throughthe mud column inside the drillpipe, read at thesurface by pressure sensors, and then recordedand processed.
Another type of data transmission uses rotaryvalves with a modulator that generates acontinuous pressure wave to carry information.2
Recent developments in this technology haveresulted in data-transmission rates as high as fourtimes the industry average that are much lesssusceptible to drilling and mud-pump noise, anddownhole motor stalls. This technology is used inthe Orion high-speed telemetry platform and the
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TeleScope high-speed telemetry-while-drillingservice (above).
The first MWD tools were developed in theearly 1970s to measure drilling-relatedproperties, such as inclination and azimuth,which are essential in directional drilling.3
Important additional measurements such astorque, weight on bit (WOB) and temperatureallow drillers and drilling engineers to monitordownhole drilling performance parameters inreal time rather than infer these parametersfrom surface measurements. Generally, real-timeMWD measurements are monitored to helpoptimize the drilling process, avoid drillingproblems and monitor the well trajectory toensure that the intended target is reached.4
These early measurements improved industryknowledge of dynamic drilling processes. As aresult, drilling became more efficient, less risky,and often less expensive. For example, there arenow fewer catastrophic borehole failures thatforce companies to sidetrack or abandon wells.Borehole quality has improved, decreasingcementing costs and problems. Reducedborehole rugosity also improves the quality offormation evaluation from both wireline andLWD devices.
The first LWD measurements were developedin the 1980s to identify penetrated strata and, inmany cases, to confirm the location of the bitwith respect to the formation rather than havingto rely only on the measured depth. Thiscapability facilitated well-trajectory changes toavoid hazards and to penetrate the targetedreservoir.5 LWD also served as an alternative wayto acquire basic formation data in areas wherewireline logging proved difficult, such as inhighly deviated and horizontal wells, or in wellswith problematic boreholes. Another primeobjective of logging the borehole while drillingwas to measure formation-fluid properties beforethe drilling processparticularly invasion ofdrilling fluidssignificantly disturbed thereservoir near the wellbore.
Borehole imaging techniques have beendeployed on wireline since the 1960s. As while-drilling data-transmission rates improvedduring the last decade, similar techniques havebecome an important part of LWD operations.6
For example, real-time images from LWD toolssuch as the RAB Resistivity-at-the-Bit or the GVRgeoVISION resistivity sub are used to assessformation bedding, identify fractures, assist information evaluation, and to direct geosteering
and geostopping operations.7 As LWD measure-ments have improved and become morenumerous, they are now increasingly used to helpoperators make crucial drilling decisions anddetermine the state of stress around theborehole.8 In addition, LWD technology is playinga role in both completion and stimulation design.
Challenges While DrillingChanges in the near-wellbore environment fromdrilling time to wireline logging time andinherent differences in tool designs must betaken into account when comparing LWDmeasurements with those of wireline logs.9
However, there is usually one indisputable fact:the near-wellbore region is less disturbedimmediately after bit penetration than days orweeks later, when wireline logging occurs.
The number of LWD measurements continuesto grow, but wireline logs are still preferred inmany areas, especially where drilling-rig costsare moderate, wellbore inclination is low, andborehole conditions are satisfactory. In addition,the range and versatility of wireline measure-ment and sampling capabilities are compellingreasons to use wireline tools.
Until recently, several measurements that aidin the identification of formation fluidsgas, oiland waterwere not deployed in LWD systems.One example is the thermal neutron capturecross section measurement, which determinesthe probability that a thermal neutron iscaptured by nuclei in the formation. The captureof neutrons results in the emission of ga