68 Oilfield Review A Sound Approach to Drilling Jeff Alford Roger B. Goobie Colin M. Sayers Ed Tollefsen Houston, Texas, USA Jay Cooke Helis Oil & Gas Houston, Texas Andy Hawthorn John C. Rasmus Sugar Land, Texas Ron Thomas PPI Technology Services Houston, Texas For help in preparation of this article, thanks to Ron Blaisdell, New Orleans; Lennert den Boer, Calgary; Joaquin Armando Pinto Delgadillo and Egbonna Obi, Youngsville, Louisiana; Nick Ellson and Dale Meek, Sugar Land, Texas; and Ivor Gray, CJ Hattier and Sheila Noeth, Houston. APWD (Annular Pressure While Drilling), CDR (Compen- sated Dual Resistivity), FPWD (Formation Pressure While Drilling), PERT (Pressure Evaluation in Real Time), sonicVISION, StethoScope and TeleScope are marks of Schlumberger. Of the many decisions drilling engineers make, selecting an optimal mud weight is one of the most challenging and far-reaching. Today, sonic logging- while-drilling tools are instrumental in making these decisions. Generations of drilling engineers have struggled to visualize the dark and formidable downhole drilling environment. Today, engineers and geoscientists rely on increasingly sophisticated sensors to gather data from deep beneath the Earth’s surface, understand subsurface lithology, identify geologic features, locate hydrocarbons and make a host of drilling and completion decisions. Even though our sense of sight is highly developed, it has its limitations. So, early in the 20th century, scientists began development of technologies that would allow visualization of environments that could not otherwise be seen. In 1906, Lewis Nixon invented the first sound navigation and ranging, or sonar, device, as a way of detecting icebergs. 1 Early sonar devices were passive; they could only listen. However , between 1914 and 1918, W orld War I accelerated interest in and develop- ment of active sonar tools for submarine detection. The first active sonar technology transmitted a sound, or ping, through water. Multiple receivers called transponders detected the returning sound echo, providing data on the relative positions of static and moving objects. Today, advanced acoustic technologies have many uses in areas such as medicine, military applications and oil and gas exploration and production (E&P). Acoustic-based logging-while-drilling (LWD) tools provide data that help reduce uncertainty and allow engineers to make effective and timely drilling decisions. Data from sonic LWD tools not only help establish pore-pressure gradients, but also help define porosity and permeability , detect and type hydrocarbons, evaluate borehole stability, interpret lithology changes, monitor fluid-flow effects in the borehole and pick accurate casing-setting depths. 2 More importantly , these data are available in real time to help engineers and geoscientists make critical decisions that affect drilling cost and efficiency (see “Acting in Time to Make the Most of Hydrocarbon Resources,” page 4). In this article, we describe how advanced sonic tools and interpretation techniques are helping to better define the safe mud-weight window, drill deeper and optimize casing-setting depths. Field examples from the Gulf of Mexico and offshore Australia show how operators are using real-time acoustic data and wellsite-to-shore telemetry systems to limit risk and uncertainty while reducing well cost. A Pressing Need for Pressure Prediction Key to the well construction process is an understanding of the subsurface pressure environment. 3 Changes in the normal pressure gradient affect drilling safety, casing design and setting depths, and in particular, the mud- weight window. Engineers restrict the mud-weight range to sustain borehole stability, control downhole pressures and optimize casing-setting depth. Most often, the mud weight is maintained above the formation pressure—at a level required to control formation stress and prevent kicks or influxes that can lead to costly well-control problems—and below the fracture gradient to prevent the formation from breaking down and losing returns. Wells are also sometimes drilled with the static mud weight below formation pressure, or underbalanced. 4 1. For more on the development of sonar devices: http://www.absoluteastronomy.com/reference/sonar (accessed February 6, 2006). 2. For more on sonic logging: Brie A, Endo T , Hoyle D, Codazzi D, Esmersoy C, Hsu K, Denoo S, Mueller MC, Plona T, Shenoy R and Sinha B: “New Directions in Sonic Logging,” Oilfield Review 10, no. 1 (Spring 1998): 40–55. 3. Barriol Y, Glasser KS, Pop J, Bartman B, Corbiell R, Eriksen KO, Laastad H, Laidlaw J, Manin Y, Morrison K, Sayers CM, Terrazas Romero M and Volokitin Y: “The Pressures of Drilling and Production,” Oilfield Review 17, no. 3 (Autumn 2005): 22–41. 4. For more on underbalanced drilling: Bigio D, Rike A, Christensen A, Collins J, Hardman D, Doremus D, Tracy P, Glass G, Joergensen NB and Stephens D: “Coiled Tubing Takes Center Stage,” Oilfield Review 6, no. 4 (October 1994): 9–23.
Transcript
68 Oilfield Review
A Sound Approach to Drilling
Jeff AlfordRoger B. GoobieColin M. SayersEd TollefsenHouston, Texas, USA
Jay CookeHelis Oil & GasHouston, Texas
Andy HawthornJohn C. RasmusSugar Land, Texas
Ron ThomasPPI Technology ServicesHouston, Texas
For help in preparation of this article, thanks to RonBlaisdell, New Orleans; Lennert den Boer, Calgary;Joaquin Armando Pinto Delgadillo and Egbonna Obi,Youngsville, Louisiana; Nick Ellson and Dale Meek,Sugar Land, Texas; and Ivor Gray, CJ Hattier andSheila Noeth, Houston.APWD (Annular Pressure While Drilling), CDR (Compen-sated Dual Resistivity), FPWD (Formation Pressure WhileDrilling), PERT (Pressure Evaluation in Real Time),sonicVISION, StethoScope and TeleScope are marks ofSchlumberger.
Of the many decisions drilling engineers make, selecting an optimal mud
weight is one of the most challenging and far-reaching. Today, sonic logging-
while-drilling tools are instrumental in making these decisions.
Generations of drilling engineers have struggledto visualize the dark and formidable downholedrilling environment. Today, engineers andgeoscientists rely on increasingly sophisticatedsensors to gather data from deep beneath the Earth’s surface, understand subsurfacelithology, identify geologic features, locatehydrocarbons and make a host of drilling andcompletion decisions.
Even though our sense of sight is highlydeveloped, it has its limitations. So, early in the20th century, scientists began development oftechnologies that would allow visualization ofenvironments that could not otherwise be seen.In 1906, Lewis Nixon invented the first soundnavigation and ranging, or sonar, device, as a wayof detecting icebergs.1
Early sonar devices were passive; they couldonly listen. However, between 1914 and 1918,World War I accelerated interest in and develop-ment of active sonar tools for submarine detection.
The first active sonar technology transmitteda sound, or ping, through water. Multiplereceivers called transponders detected thereturning sound echo, providing data on therelative positions of static and moving objects.Today, advanced acoustic technologies havemany uses in areas such as medicine, militaryapplications and oil and gas exploration andproduction (E&P).
Acoustic-based logging-while-drilling (LWD)tools provide data that help reduce uncertaintyand allow engineers to make effective and timelydrilling decisions. Data from sonic LWD tools notonly help establish pore-pressure gradients, butalso help define porosity and permeability, detectand type hydrocarbons, evaluate borehole
stability, interpret lithology changes, monitorfluid-flow effects in the borehole and pickaccurate casing-setting depths.2
More importantly, these data are available inreal time to help engineers and geoscientistsmake critical decisions that affect drilling costand efficiency (see “Acting in Time to Make theMost of Hydrocarbon Resources,” page 4). In thisarticle, we describe how advanced sonic toolsand interpretation techniques are helping tobetter define the safe mud-weight window, drilldeeper and optimize casing-setting depths. Fieldexamples from the Gulf of Mexico and offshoreAustralia show how operators are using real-timeacoustic data and wellsite-to-shore telemetrysystems to limit risk and uncertainty whilereducing well cost.
A Pressing Need for Pressure PredictionKey to the well construction process is anunderstanding of the subsurface pressureenvironment.3 Changes in the normal pressuregradient affect drilling safety, casing design andsetting depths, and in particular, the mud-weight window.
Engineers restrict the mud-weight range tosustain borehole stability, control downholepressures and optimize casing-setting depth.Most often, the mud weight is maintained abovethe formation pressure—at a level required tocontrol formation stress and prevent kicks orinfluxes that can lead to costly well-controlproblems—and below the fracture gradient toprevent the formation from breaking down andlosing returns. Wells are also sometimes drilledwith the static mud weight below formationpressure, or underbalanced.4
1. For more on the development of sonar devices:http://www.absoluteastronomy.com/reference/sonar(accessed February 6, 2006).
2. For more on sonic logging: Brie A, Endo T, Hoyle D,Codazzi D, Esmersoy C, Hsu K, Denoo S, Mueller MC,Plona T, Shenoy R and Sinha B: “New Directions in SonicLogging,” Oilfield Review 10, no. 1 (Spring 1998): 40–55.
3. Barriol Y, Glasser KS, Pop J, Bartman B, Corbiell R,Eriksen KO, Laastad H, Laidlaw J, Manin Y, Morrison K,Sayers CM, Terrazas Romero M and Volokitin Y: “ThePressures of Drilling and Production,” Oilfield Review 17,no. 3 (Autumn 2005): 22–41.
4. For more on underbalanced drilling: Bigio D, Rike A,Christensen A, Collins J, Hardman D, Doremus D,Tracy P, Glass G, Joergensen NB and Stephens D:“Coiled Tubing Takes Center Stage,” Oilfield Review 6,no. 4 (October 1994): 9–23.
W
> Predicting pore pressure in the Gulf of Mexicowith seismic data. In this example, the initialvelocity model based on conventional stacking-velocity analysis (above left)tt predicts thepresence of overpressure (black circle).Although pore-pressure predictions based ontthis information are not sufficiently accurate fordrilling, a higher degree of seismic-velocityresolution can be obtained by using tomographicanalysis and checkshot datat to refine the velocitymodel (above right).tt Further data processingallows construction of a three-dimensional (3D)pore-pressure cube (bottom right).tt
The optimal mud-weight range is frequentlynarrow andw difficult to define; this is especiallytrue in tectonically stressedy regions and indeepwater environments. Within this narrowmud-weight window, engineers balance severalfactors, including the minimum flow ratewrequired for hole cleaning, downhole motor andtelemetry operationsy and equivalent circulatingand static densities. Drilling fluids such as oil-base and synthetic-base muds frequently exhibitythermomechanical and compressibility propertiesythat vary withy depth, making it difficult tooptimize drilling efficiency whiley maintainingmud weight.
Operating within the mud-weight windowallows engineers to improve drilling efficiencyand set casing at the best possible depth. Ifcasing is set too shallow, well-construction costtypically increases,y well depth is limited,production rate may bey compromised and, insome cases, the target may noty be reachable.
Maintaining the mud weight within a specificwindow reliesw on accurate determination andprediction of anomalousf changes in formationpressure. The analysis of shalef resistivity usingywireline log data is one of thef oldest methods fordetecting abnormal pressure.
Formation resistivity dependsy on porosity, thetype of thef fluid within the pore space and itsionic strength. Under normal compactionconditions, an increase in shale resistivity withydepth corresponds to a reduction in porosity(left). An anomalous change in formationpressure is usually associatedy with a shift in thenormal compaction trend, indicated on anelectric log by ay reduction in resistivity associ-yated with an increase in porosity.
For the purpose of maintainingf safe mudweight while drilling, information aboutabnormal pressure needs to be available whiledrilling. Although formation resistivity isy one ofthe most common LWD measurements, severalfactors can have a significant effect on the data,potentially masking changes in the normalcompaction trend and hindering the detection ofabnormal pressure.5
Changing temperatureg in the borehole withdepth alters the resistivity ofy formationf water,while the presence of hydrocarbonsf considerablyincreases resistivity. Large deposits of organicfmatter mayr alsoy increase resistivity, obscuringundercompaction indicators. Changes in thecondition of thef borehole, such as an increase inborehole diameter due to washout or caving,further increaser resistivity measurementy error.Although many ofy thesef effects can be
compensated for, reliance on resistivity datayalone for pore-pressurer prediction significantlyincreases drilling risk.g
Geoscientists can often identify abnormallyypressured formations using seismic velocities.For a given lithology, acoustic velocity usuallyydepends on porosity: the greater the porosity, thelower the acoustic velocity. In normally com-ypacted sediments, compaction increases withdepth. Porosity, in turn, decreases with depth,and so the velocity ofy sonicf and seismic wavestraveling through the formation generallyincreases with depth (below). Deviations fromthis trend can often be attributed to layers ofsediments that have not compacted, signalingabnormally highy pressure, called overpressure.However, uncertainties in seismic velocitiescommonly resulty in depth errors, making itdifficult to define exact distances to drillinghazards and geological targets.
Velocity modelsy created from seismic data canabe improved by addingy high-resolution informa-tion from sonic measurements obtained whiledrilling (next page). Today, geoscientists andengineers combine while-drilling and wireline
> Electric log analysis to predict pore pressure.In normally compacted sediment, electricalresistivity will increase with depth along anormal trend line (red). A deviation in resistivityfrom the normal trend may indicate abnormalformation pressure.
R i ti itiResistivityResistivityiiR i ti itideviationdeviationii
Winter 2005/2006 71
> Defining mud-weight windows. Sonic velocity can be used to predict changes in the normal compaction trend that is often anindicator of abnormal pressure (top left). Unlike resistivity measurements, sonic velocity is unaffected by changes in boreholetttemperature and salinity. Real-time compressional-slowness measurements from sonic LWD tools are used to predict porepressure and help define kick and borehole breakout limits (top right). Adding sonic shear measurements (tt bottom), available infast formations, helps determine kick and mud-loss potential, fracture limits, and the safe mud-weight window shown in white(Track 4). Various types of shear failure can also be defined (Track 5).
8,800
9,000
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Rate of Penetrationft/h
Depth,ft050
Gamma RaygAPI 1500
Phase Shift Resistivityohm.m 2.00.2
Attenuation Resistivityohm.m 2.00.2
Sonic LWD ∆tµs/ft 50150
Onset ofoverpressure
Normal compactionNormal compactionNormal compactionpt dtrendtrendtrend
50150
DivergenceDivergenceDivergence
O dO erpress redOverpressuredOverpressuredpzonezonezonezone
∆t
Gamma Ray Depth,gAPI ft15030
Vp/Vs Ratio60
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Poision’s Ratio0.5( )
( )
0
Minimum Stress5,000psi0
Friction Angle50deg0 25lbm/galUS5
Tensile Strength6,000psi0
Diameter25in.5
Caliper25in.5
UnconfinedCompressive Strength
5,000psi0
Maximum Stress6,000psi0
Overburden Gradient
MD, ft
5,000psi0
Breakout
Kick
Losses
Shear Failure: Echelon
Shear Failure: Knockout
Shear Failure: Breakout Overgauge
Fracture
Fracture
Mud Weight
Safe mud-weight window
sonic data with checkshots to generate syntheticseismograms that are then correlated withpredrilling seismic measurements, providing thedrilling team with a way to locate the drill bitwithin the geophysical environment (above).6
These real-time processes help engineers preparefor pressure changes before drilling into them.
Generating a synthetic seismogram from LWDdata involves combining transit-time (∆t) datawith density measurements, to produce anacoustic impedance (AI) model. This model isconverted into a seismic reflectivity sequence,and then convolved with a selected wavelet toproduce a synthetic seismogram.7 A syntheticseismogram is much more useful when it isdepth-calibrated with either a wireline or while-drilling checkshot or vertical seismic profile(VSP). Although the synthetic seismogram canbe generated at the wellsite, more often, the real-time data are transmitted to an engineeringcenter for processing.
Correlating a synthetic seismogram withsurface seismic traces helps geoscientists andengineers place the borehole trajectory on aseismic section. Calculation of the spatial positionof the borehole relative to seismic markers, orreflectors, allows the drilling team to look aheadto abnormal changes in formation pressure.
Sonic Measurements While DrillingSoon after the introduction of sonic LWDmeasurements in the late 1990s, an operatorexperimented with using sonic LWD measure-ments to improve drilling efficiency in severalmajor operating areas. On an exploration well inthe Gulf of Mexico, USA, in an area known forabnormally pressured formations, sonic anddensity LWD data were transmitted from the rigto the operator’s office. There, geoscientistsgenerated a synthetic seismogram, which wascorrelated to the surface seismic section imagingthe target zone and an overlying overpressuredzone.8 The synthetic seismogram indicated thatthe top of the overpressured zone was 60 ft[18 m] deeper than what the seismic sectionpredicted. This information allowed engineers toplace the casing shoe significantly closer to theoverpressured zone, optimizing casing-settingdepth and improving the safety and drillingefficiency of subsequent borehole sections.
In another early example, BHP (now BHPBilliton) and Schlumberger demonstrated theuse of sonic LWD measurements not only tocalibrate seismic reflections, but also to updatepore-pressure calculations ahead of the bit.9
Several exploration wells offshore WesternAustralia had been abandoned prematurely dueto wellbore-stability problems associated withoverpressured formations.
As the bit approached the predictedoverpressured zone, acoustic velocity acquiredwhile drilling was used to continuously updatethe velocity models derived from existing surfaceseismic and VSP surveys. Simultaneously,engineers at the wellsite used real-time CDRCompensated Dual Resistivity data, sonic LWD,weight-on-bit (WOB), rotary torque and rate-of-penetration (ROP) measurements, in conjunc-tion with the PERT Pressure Evaluation in RealTime program, to monitor changes in porepressure a few meters behind the bit. Thisinformation was used to calibrate the pore-pressure predictions from the seismic andVSP data.
Using multiple techniques for pore-pressureprediction, the operator accurately predictedchanges in formation pressure, identified mini-mum mud-weight requirements and optimizedcasing-setting depth to construct a successfulwell in this hostile environment.
Narrowing the Window of UncertaintyDrilling in technically demanding areas is usuallyassociated with high cost and elevated levels ofrisk and uncertainty.10 Sonic LWD data availablein real time play a key role in reducing cost, riskand uncertainty by updating models createdbefore drilling. However, creating those models
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> Placing a bit on the seismic map using synthetic seismograms. Sonic LWD slowness data are inverted with the densitymeasurement to produce an acoustic-impedance (AI) measurement (process from left to right). The AI is converted tottreflectivity and convolved with a 35-Hz wavelet at each reflector to obtain the synthetic seismogram (right). Geophysicalttanalysis of the seismic data determines the wavelet frequency. With increasing depth, higher frequency seismic signalsare attenuated, so a lower frequency, generally 20 Hz instead of 35 Hz, is used to correlate the sonic LWD data to surfaceseismic measurements. This helps engineers and geoscientists place the bit on the seismic map more accurately.
SonicslownessDensity
Acousticimpedance Reflectivity Wavelet
Syntheticseismogram
Winter 2005/2006 73
in the first place can be a bottleneck. In 2000,geoscientists began looking at opportunities toincrease the speed and accuracy of while-drillingpore-pressure modeling and prediction.11
In the deepwater Gulf of Mexico (GOM),overpressure causes major drilling hazards.Overpressure is caused by Mississippi Riversedimentation that is rapidly buried comparedwith the time it takes for sediments to expelpore water. This prevents sediments fromcompacting as they are buried and causes thepore fluid to become overpressured. In under-compacted sediments, sediment grain contactsare weak, causing low rock strength and lowacoustic velocities.
Accurate determination of pore pressure is akey requirement to making optimized drillingdecisions in these overpressured environments.Before drilling, pore pressure can be predictedusing seismic velocities—assuming there is aseismic survey available and processed—together with a velocity-to-pore-pressure trans-form calibrated to offset-well data. However, thisprocedure takes considerable time. Syntheticseismograms can be generated quickly, comparedwith the time needed for analyzing seismicvelocities and creating a pore-pressure cube.
As engineers focus on ways to reduce risk anduncertainty, the time required to process andcorrelate seismic and sonic LWD data becomescritical. To speed up this process for prospects inthe northern GOM, Schlumberger geoscientistsdeveloped a pore-pressure cube for the entirearea using data released by the MineralsManagement Service (MMS) (right).12
Checkshot data from the MMS in the Gulf ofMexico were inverted to obtain compressionalvelocity versus depth below the mudline. Thesevelocity functions were then combined withupscaled sonic logs from deepwater wells andkriged to populate a three-dimensional (3D)mechanical earth model (MEM) displaying bothvelocity and levels of expected uncertainty.13
By applying a threshold to the predictedkriging error, maps of undercompaction andoverpressure can be restricted to specific areasof interest. For commercial projects, a confi-dential client subcube may be extracted from thefull GOM pore-pressure cube. Any additionalinformation provided by the operator and dataacquired during the drilling process with sonicLWD and real-time pore-pressure tools is used toupdate the client model, increasing resolution
6. A checkshot is a type of borehole seismic surveydesigned to measure the acoustic traveltime from thesurface to a known depth. Formation velocity ismeasured directly by lowering a geophone to each depthof interest, emitting energy from a source on the surfaceand recording the resulting signal. A checkshot differsfrom a vertical seismic profile in the number and densityof receiver depths recorded; geophone positions may bewidely and irregularly located in the wellbore, whereas avertical seismic profile usually has numerous geophonespositioned at closely and regularly spaced intervals inthe wellbore.
7. A wavelet is a pulse representing a packet of energyfrom the seismic source.
8. Hashem M, Ince D, Hodenfield K and Hsu K: “SeismicTie Using Sonic-While-Drilling Measurements,”Transactions of the SPWLA 40th Annual LoggingSymposium, Oslo, Norway, May 30–June 3, 1999, paper I.m
9. Tcherkashnev S, Rasmus J and Sanders M: “JointApplication of Surface Seismic, VSP and LWD Data forOverpressure Analysis to Optimize Casing Depth,”presented at the EAGE Workshop: Petrophysics MeetsGeophysics, Paris, November 6–8, 2000.
10. Malinverno A, Sayers CM, Woodward MJ andBartman RC: “Integrating Diverse Measurements toPredict Pore Pressure with Uncertainties While Drilling,”paper SPE 90001, presented at the SPE Annual Technical Conference and Exhibition, Houston,September 26–29, 2004.
11. Sayers CM, Johnson GM and Denyer G: “Predrill PorePressure Prediction Using Seismic Data,” paperIADC/SPE 59122, presented at the IADC/SPE DrillingConference, New Orleans, February 23–25, 2000.
12. Sayers CM, den Boer LD, Nagy ZR, Hooyman PJ andWard V: “Regional Trends in Undercompaction andOverpressure in the Gulf of Mexico,” ExpandedAbstracts, 75th SEG Annual Meeting, Houstons(November 6–11, 2005): 1219–1222.
13. Kriging is a statistical technique used with two-pointstatistical functions that describe the increasingdifference or decreasing correlation between samplevalues as separation between them increases, then todetermine the value of a point in a heterogeneous gridfrom known values nearby.
> Building a three-dimensional (3D) mechanical earth model in the Gulf of Mexico.Seismic, checkshot and sonic data released by the Minerals Management Service(green dots) were gathered from wells in the Gulf of Mexico (top) where pore pressureexceeded 10 lbm/galUS [1,198 kg/m3] and the predicted velocity error was less than ± 1,200 ft/s [± 366 m/s]. The data were then trend-kriged to predict pore pressure, andthen plotted in a 3D model (bottom).
10 11 12 13Pore pressure, lbm/galUS
14 15 16 17
and reducing pore-pressure uncertainty both inthe immediate drilling environment and ahead ofthe bit (above).
Along with improved modeling, technologicaladvances in LWD tools and telemetry systems areyielding more accurate real-time measurementsand in greater quantities. The sonicVISION, newgeneration sonic-while-drilling LWD tool intro-duced in April 2004 has increased confidencein the accuracy of real-time compressional-wave velocities.
Until fairly recently, many believed that itwould be impossible to achieve sonic measure-ments while drilling. Engineers thought that thefast acoustic-signal arrival down the tool collarfrom the transmitter to the receivers woulddominate all the arrivals, making it impossible todiscriminate and record formation arrivals.
With this in mind, the designers of first-generation sonic LWD tools focused on mitigatingdirect collar arrivals. To accomplish this, the
tools were designed around what is referred to asthe hoop-mode frequency range of the collars.This frequency depends on the collar thicknessand diameter, but for most tools, falls in a narrowband between 11 and 13 kHz.
At the hoop-mode frequency, acoustic wavesattempt to expand the collar rather than traveldown to the receiver, thereby attenuating thecollar arrivals at the receivers. By designing thetransmitters to fire within the narrow hoop-modefrequency band and filtering received data to the
74 Oilfield Review
> Reducing uncertainty with pressure data from multiple sources. The degree of uncertainty in a pore-pressure gradient is exemplified by the width andlow resolution of the compressional-velocity (Vp) and pore-pressure gradient curves (1). Velocity data from sonic checkshots are added to the model,somewhat reducing pore-pressure uncertainty (2). Adding mud weights from drilling reports (3) and physical pore-pressure measurements (4) refinesestimates and dramatically improves pore-pressure resolution.
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10 15 2010 15 20P di t lb / lUSPore-pressure gradient lbm/galUSPore pressure gradient, lbm/galUS
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hthpeDeDD
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10 15 2010 15 20P di t lb / lUSPore-pressure gradient lbm/galUSPore pressure gradient, lbm/galUS
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Winter 2005/2006 75
same range, engineers hoped to receive cleanand discernable formation arrivals, free fromdistortion caused by collar arrivals.
This technique proved somewhat satisfactoryfor fast formations where the excitationfrequency falls within the appropriate range.However, for slower formations, larger hole sizesand for lower frequency components of the wavetrain, such as shear and Stoneley waveforms,these first-generation tools did not excite theformation at the optimum frequency and werediscarding data outside of the narrow bandaround the hoop mode (right).
Narrow-band processing also promotedspatial aliasing, a condition in which non-formation arrivals, or processing artifacts,appear within the slowness time coherence(STC) search-band window. Aliasing depends onthe frequency of the transmitted pulse, therecorded waveform frequencies and the inter-receiver spacing. With an almost monofrequencysystem, aliasing was well-developed and led toincorrect picking of events that were notformation arrivals.
Misinterpretation of signal arrivals can alsolimit the usefulness of acoustic data. Previoustools analyzed all acoustic arrivals within a timewindow associated with a depth. So within thisdataset, there could be compressional, shear,mud, collar and aliased arrivals. The tool’sdownhole processors then discriminated thecompressional arrival from other signals basedon the coherency of those events. With compres-sional arrivals being one of the smallest eventsdiscernible in the wave train, their coherency istypically low when compared with other arrivals(below right). Early tools often confused ormisidentified the data, sending incorrect valuesto the surface.
To mitigate these problems, Schlumbergerengineers designed the sonicVISION tool totransmit and receive wide-band acoustic signalsin a frequency range from 3 to 19 kHz, a rangemore likely to generate a measurable responsefrom most formations. Acoustic shear waves aredifficult to acquire with narrow-band toolsbecause they contain lower frequencies thancompressional waves. The sonicVISION toolfrequency is optimized to excite the formationacross a significantly wider frequency band thanthat of previous tools. This allows both shear andcompressional measurements to be routinelymade while drilling in faster formations. Power-output levels were also increased 10-fold to moreeffectively couple the wide-band acoustic energyto the formation.
> Frequency range of the new tool design. The frequency ranges of previousttools were narrowly aligned within the collar attenuation frequency. Newerttools have an expanded frequency range covering a broader spectrum of softand hard formations (yellow bar). Lower frequency arrivals such as Stoneleyand leaky-P (not shown) are now captured.
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itude
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> Acoustic wave train. Once an acoustic signal is transmitted, it travels through the formation, annularfluid, and to some degree the tool, ultimately arriving at the receiver array. Low-amplitude compressionalsignals (red) arrive first, followed in harder rock by the shear arrival. Newer tools take advantage ofslower arrivals such as Rayleigh and Stoneley.
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itude
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The new design also transmits real-timecoherent events, called peaks. The sonicVISIONtool can send up to four peak arrivals uphole at anygiven time, enabling engineers at the surface toaccurately differentiate arrivals rather than relyingon downhole processing. These peaks are thenassembled to form an STC projection log that helpsimprove data accuracy and provides a significantstep forward in data quality control (above).
STC projection logs help engineers accuratelydifferentiate compressional, shear and othermodes in real time. The novel design of thesonicVISION tool now allows engineers to modifylabel limits at the surface thereby improvingextraction of the compressional ∆t and alsoproviding real-time shear data. Accurate discrim-
ination of arrivals improves pore-pressuremeasurement and allows geomechanical inter-pretation based on while-drilling compressional,shear and density data.
Acoustic data can now also be further refinedby acquiring data while the pumps are off.Background noise of the same frequency as sonicmeasurements, generated by drilling andcirculation, can be problematic for makingaccurate acoustic measurements. During adrillpipe connection, the sonicVISION tool canacquire real-time formation velocity measure-ments in a quiet environment, increasingconfidence in the STC projections and potentiallyallowing engineers to observe velocity changescaused by flow-induced stress variance.
To speed data to the surface, Schlumbergerrecently released the TeleScope high-speedtelemetry-while-drilling service. This newmeasurement-while-drilling (MWD) system iscapable of providing enough power to run eight ormore LWD tools while offering up to a fourfoldincrease in data rate over comparable tools. Fieldapplication of these new hardware technologies,in combination with improved pore-pressuremodeling described earlier, promises to enhancedrilling efficiency and reduce geologic and well-construction uncertainty.
Advancements in sonic LWD tool design andtelemetry systems have overcome many of theinadequacies previously inherent in while-drilling sonic measurements. New data proces-sing techniques and improvements in telemetrysystems have minimized earlier limitations,allowing real-time access to while-drilling soniccompressional measurements in almost anydrilling environment.
Seismic, Sonic and Pressure Measurement—Defining the Mud-Weight WindowIn many GOM fields, pore pressure changesrapidly with depth, and tight mud-weightwindows make drilling and completion difficult,or even impossible. One example of an extremelydifficult environment is the Vermillion offshorearea. Here, mud weights often reach 18 lbm/galUS[2,157 kg/m3], the risk of wellbore instability and lost circulation is high, and six or more casing strings are typically required to reachtarget reservoirs.
Today, operators use data retrieved duringdrilling from sonicVISION, StethoScope formation-pressure-while drilling service and other LWD toolsto help improve well-construction efficiency andreduce cost by accurately defining and managingthe effective stress and mud-weight window.
Sonic LWD, real-time formation pressure andother while-drilling tools were successfully usedto reduce risk and operational uncertainty whiledrilling a well in Vermillion Block 338 during2005. In this well, which was owned by Helis Oil &Gas LLC and operated by PPI TechnologyServices, engineers planned and executed anaggressive drilling program. This programextended both the 9 ⁄5⁄⁄ -in. intermediate casing and7-in. liner strings to sufficient depths to eliminatea string of casing common to wells in the area, inthis case, a 5-in. casing string. These efforts notonly reduced well cost, but more importantly,eliminated the difficulties associated withslimhole drilling and the completion limitationsinherent in small production casing.
76 Oilfield Review
> Compressional and shear peaks available in real time. Because of improvements in downhole tooland telemetry systems, slowness time coherence peaks can now be sent to the surface forevaluation and labeling while drilling (Track 2). Previously, the semblance projection was availableonly by processing tool memory after the tool was pulled from the borehole (Track 1). The semblanceprojection based on the real-time peaks (Track 3) is consistent with the memory-mode data. Stationmeasurements of compressional ∆t (white circles) acquired during quiet periods, such as whenpumps were off during pipe connections, also confirm the accuracy of the real-time data. ThesonicVISION system has the unique capability to modify label limits (Track 2) at surface for betterextraction of compressional data, and for the first time, real-time shear data. These improvements inreal-time quality control make the compressional input, used for pore-pressure calculation ofminimum mud weight, more robust. Combining the real-time compressional and shear data alsoenables geomechanical calculations of the maximum mud-weight window.
X,300
Depth,ft
pMin MaxAmplitude
Slowness ProjectionRecorded Mode
0 1Slowness Projection
∆t Compressional fromReceiver Array ∆t Compressional from Receiver Array
µs/ft40 240µs/ft
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∆t Shear from Receiver Array
µs/ft40 240µs/ft
µs/ft40 240µs/ft
µs/ft40 240µs/ft
∆t Peak 1 Compressional Computed Uphole, Real Time
µs/ft40 240µs/ft
∆t Shear from Receiver Array
Maximum and Minimum ∆t CompressionalLabel Limits
Maximum and Minimum ∆t Shear Label Limits
Slowness Time Coherence Peaks
40 240 µs/ftµs/ft 240µs/ft
Slowness ProjectionReal Time
0 1Slowness Projection
pMin MaxAmplitude
Winter 2005/2006 77
Accurate prediction of geologic target depthand pore pressure is essential to the success ofaggressive drilling plans. Helis and PPI engineersbased their initial well design on mud weightsfrom wells in the area. They next approachedSchlumberger to refine these predictions usingthe GOM 3D mechanical earth model, to befurther refined with while-drilling sonic data.
While-drilling data were transmitted bysatellite to a remote operations and collaboration
center where the wellbore hydrodynamics andgeomechanical earth models were updated inreal time using data from the rig (above). Toaccount for variations in lithology and sediment-compaction rates, the nonlinear normal compac-tion transform established during predrillingplanning was validated and recalibrated whiledrilling using sonicVISION data and directpressure measurements from the FPWDFormation Pressure While Drilling tool.
Correlating data acquired from thesonicVISION and FPWD tools significantlyincreased confidence in the real-time pore-pressure prediction model. These measurementsallowed predrilling uncertainties associated withthe velocity-to-pore-pressure transform to beproperly defined while drilling. The calibratedtransform was then applied to revise and update
> Telemetry to engineering centers. The wellsite engineer collects drilling, mud and sonic LWD data,tthen transmits this information to the engineering center where a team of experts analyzes andprocesses the data. Once the results are returned to the wellsite, initial pore-pressure predictions(A) are updated with pore-pressure estimations (B), ultimately reducing the cone of uncertainty(C) and providing more accurate predictions of pore pressure ahead of the bit (D).
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the predrilling pore-pressure model, both behindand ahead of the bit (right).
The results, including the mud-weightrecommendations, were then conveyed to the rig,and action was taken to ensure that the surfacemud weight, the equivalent circulating density(ECD) and the equivalent static density (ESD)were kept within the limits of the mud-weight window.
The initial requirements for setting the 95⁄5⁄⁄ -in.casing were constrained by a 13-lbm/galUS[1,558-kg/m3] fracture point derived fromprevious experiences in the field. However, thecalculated fracture gradient derived from while-drilling formation velocity and density measure-ments indicated that the rock strength wassubstantially higher, and capable of accom-modating a heavier drilling fluid.
The mud weight was increased to13 lbm/galUS based on the real-time pore-pressure analysis as drilling approached 6,800 ft[2,072 m]. Using real-time sonic LWD data,while-drilling pressure measurements andadvanced data processing techniques, geo-scientists at the remote collaborative centerestablished a safe mud-weight range that allowedthe driller to reach a depth of 8,187 ft [2,495 m]before running 95⁄5⁄⁄ -in. casing; at casing depth, theECD was within 0.1 lbm/galUS [11.98 kg/m3] ofthe calculated fracture gradient.
Once the 95⁄5⁄⁄ -in. casing was set, drillingresumed with an 8 ⁄1⁄⁄ -in. bit. At 9,500 ft [2,896 m],the annular pressure exceeded the fracturegradient, and circulation was lost. Time-lapseresistivity analysis indicated two zones near theprevious casing shoe where the formation hadprobably been damaged.
On further evaluation, engineers believedthat the cost of remedial squeeze operationsoutweighed the risk of drilling ahead with tighthydraulic control and a maximum mud weight of17.5 lbm/galUS [2,097 kg/m3]. Carefully moni-toring and maintaining the annular pressureswithin an accurately calibrated hydraulic-pressure envelope allowed the operator to com-plete the well at 12,507 ft [3,812 m] in the targetreservoir without an additional string of casing.
The combined efforts of Schlumberger, PPIand Helis engineers eliminated the preplanned5-in. casing string and avoided the difficultiesassociated with slimhole drilling and completion.Sonic LWD, while-drilling pressure measurementsand careful hydrodynamic monitoring using theAPWD Annular Pressure While Drilling toolsucceeded in identifying pore-pressure changesand fracture points, and allowed drilling toproceed within the constraints of a narrow mud-weight envelope.
Engineers significantly reduced the uncer-tainty associated with pressure-predictionmodels by updating the predrilling velocity-to-pore-pressure transform using sonic LWD dataand measuring true formation pressure. Thecritical 95⁄5⁄⁄ -in. casing depth was pushed 1,187 ft[362 m] deeper than planned, eliminating anentire casing section and reducing well cost bymore than US$ 1.7 million.
A Sound Future for While-Drilling Acoustic ToolsA new generation of sonic LWD tools is helpingdrillers, engineers and geoscientists make manydecisions that facilitate safe and cost-effectivewell construction. By supplying timely infor-mation on formation velocity, while-drillingacoustic tools have proved to be a valuable assetto the well-engineering team.
Today’s sonic LWD systems are providingaccurate acoustic data that in turn, are beingprocessed in real time to reliably determine porepressure and the geophysical limits of formationsbeing drilled. When combined with seismic andother real-time data, this information helpsgeoscientists see ahead of the bit to the nextgeologic horizon and beyond. Defining the mud-weight window while drilling enables engineersto deviate from predrilling casing designs,pushing casing seats to greater depths andsignificantly reducing well cost.
Much like the development of sonar early in the 20th century, advances in modelingsoftware, acoustic tool design and decision-processing utilities are helping engineers see the unseen and make sound drillingdecisions, reducing cost and increasing well-construction efficiency. —DW
78 Oilfield Review
> Drilling in a narrow mud-weight window. The top of the pore-pressure rampis confirmed at around 6,800 ft by the sonic (red) and formation pressure while-drilling measurements (green diamonds). Between 7,000 and 8,000 ft[2,134 and 2,438 m], a significant divergence between the predrilling model(green curve) and actual pore pressure represents an example of theimportance of using real-time measurements to update the predrilling model.As drilling progressed below 9,000 ft [2,743 m], accurate pore-pressureprediction, pressure measurements and hydraulic modeling allowed thedrilling team to maintain the mud weight (black curve), equivalent static (bluediamond) and equivalent circulating (purple curve) densities within a narrowwindow just below the real-time fracture gradient (gold curve).
