We conducted a P- and S-wave crosswell survey with Conoco's orbital vibrator source
and three-component receivers at M.LT.'s Michigan Test Site. The receiver and source
wells bracket a known oil reservoir. Applying a nonlinear crosswell traveltime tomography
method, we reconstruct the velocity structures in the oil reservoir using P- and
S-wave data separately. The P-wave tomogram shows a similar image to the one by
Matarese (1993), and it does not present much velocity variations in the oil producing
zone. However, the S-wave tomogram clearly shows the image of the reservoir zone,
which is characterized with low velocities in a pinnacle reef. The S-wave velocities in
the reservoir are about 20% lower than those of the surrounding carbonates.Massachusetts Institute of Technology. Earth Resources Laboratory. Reservoir Delineation
Consortiu

Chauvelier, Chantal

Cox, Dale

Morgan, F. Dale

Queen, John

Toksoz, M. Nafi

Turpening, Roger

Zhang, Jie

DSpace@MIT

SHEAR-WAVE TOMOGRAPHIC IMAGES OF AN OILRESERVOIR AT M.LT.'s MICHIGAN TEST SITEJie Zhang, Roger Turpening, Chantal Chauvelier, F. DaleMorgan, and M. N afi ToksozEarth Resources LaboratoryDepartment of Earth, Atmospheric, and Planetary SciencesMassachusetts Institute of TechnologyCambridge, MA 02139John Queen and Dale CoxConoco, Inc.P.O. Box 1267Ponca City, OK 74603ABSTRACT\Ve conducted a P- and S-wave crosswell survey with Conoco's orbital vibrator sourceand three-component receivers at M.LT.'s Michigan Test Site. The receiver and sourcewells bracket a known oil reservoir. Applying a nonlinear crosswell traveltime tomog-raphy method, we reconstruct the velocity structures in the oil reservoir using P- andS-wave data separately. The P-wave tomogram shows a similar image to the one byMatarese (1993), and it does not present much velocity variations in the oil producingzone. However, the S-wave tomogram clearly shows the image of the reservoir zone,which is characterized with low velocities in a pinnacle reef. The S-wave velocities inthe reservoir are about 20% lower than those of the surrounding carbonates.INTRODUCTIONThe Earth Resources Laboratory (ERL) at M.LT. operates two researeh wells in theMichigan basin, near Traverse City. The two wells-Burch and Stech-bracket a pinna-cle reef, a known oil reservoir, producing through a well owned by Shell Petroleum (seeFigure 1). For the past 19 years, Shell Petroleum has obtained nearly 1 million bblsof oil from the reservoir. Over the years, ERL/MIT has acquired a variety of seismic2-1Zhang et al.data sets over the pinnacle reef, including a 3-D reflection survey, VSP and crosswellexperiments. In 1990 and 1991, BP America Inc. acquired crosswell P-wave data usinga piezoelectric source and hydrophone at M.I.To's Michigan test site (Lee et al., 1992).Applying a nonlinear traveltime tomography method, Matarese (1993) inverted BP'scrosswell data to reconstruct a tomographic image of the reef. All of these previous re-sults, within their limits of resolution, provide us with knowledge of the reef. However,most of them present a view of the carbonate layers (AI Carbonate, Al Evaporite, A2Carbonate, A2 Evaporite) that overlay the reef, but with little or no knowledge of thereef material itself. While this is sufficient for the exploration phase (i.e., finding thereef), it does not help in reserves estimation or reservoir development.In 1995, we conducted the second crosswell experiment at the test site with Conoco'sorbital vibrator source and three-component receivers. The source is sufficiently power-ful for a recording range up to 4,000 ft. Its frequency range is between 70 and 380 Hz.This source can be decomposed into one that is "inline," i.e., in the plane containingthe two boreholes, and another one that is "crossline," i.e., perpendicular to that plane.Therefore, the source can generate both P- and S-wave energy. Figure 2 shows the H-lcomponent recording from an inline source in which P waves dominate, and the H-2component recording from a crossline source which shows SH waves. The receiver is ata depth of 4,936 ft.The design of the optimal acquisition parameters became an important issue prior tothe crosswell experiment in 1995. The goal of the survey was to image the oil producingzone beneath the Al Carbonate. Therefore, we had to generate steep ray paths such thatenergy actually penetrates the producing zone instead of propagating across the regionas head waves in the Al and A2 carbonates. Using the P-wave velocity model fromMatarese (1993), we applied a raytracing method to design optimal source and receivergeometries for a better ray coverage in the target zone (Chauvelier et al., 1996). Thisled to a decision that applies smaller source and receiver spacings near the top andbottom of the survey zone.In this paper, we present a new crosswell tomography method and its applicationto the crosswell data acquired in the 1995 experiment. Both P- and S-wave tomogramsare reconstructed, and the results clearly show the image of the oil reservoir, whichis characterized with low velocities. The S-wave image of the reservoir seems betterresolved, because of larger traveltime delays through the reef.NONLINEAR TRAVELTIME TOMOGRAPHY METHODWe apply a rapid structure-driven wavefront method (Zhang and Toksiiz, 1996) tocalculate P- or S-wave traveltimes for a given velocity structure. The approach appliesgraph theory to expand a wavefront from the source and calculates traveltimes throughthe entire model. It accounts for many wave effects such as diffraction and head waves.Prior to raytracing, we calculate the spatial derivatives of the model and generate sparsetraveltime nodes in the model grid by eliminating the nodes where the derivatives are2-2(((((((S-Wave Tomographic Imagestoo small. This procedure helps reduce computational efforts and increases calculationaccuracy.We solve a regularized nonlinear inverse problem. In particular, we invert traveltimecurves by fitting both traveltimes and their derivatives. Specifically, we minimize thefollowing objective function,<Ji(m) = (1 - w)lld - G(m)11 2 + wllDx(d - G(m))11 2 + .,-IIR(ma - m)1I 2 (1)where d is traveltime data; m is the model parameter, G(m) is the calculated data; D xis a derivative operator; Dx(d) gives traveltime derivatives with respect to distance; Ris a regularization operator (we chose it to be a Laplacian operator); ma is an a priorimodel where we put sonic log constraints; w is a data weighting parameter; and .,- isa trade-off parameter for regularization. To minimize the above objective function, weapply a nonlinear Conjugate Gradient Method.Numerical experiments show that inverting traveltime curves rather than traveltimesgives a more meaningful data fit and maintains a wave resolution as it is implied in thedata (Zhang and Toks6z, 1996).IMAGING THE OIL RESERVOIRTo image the oil reservoir using P- and S-wave data collected from M.LT.'s researchwells, we apply the nonlinear traveltime tomography method to invert P- and S-wavevelocities separately. The purpose of separate inversions is to understand the indepen-dent information of reservoir structure in each type of data. In this study, we used12,400 P- and S-wave first arrival traveltime picks out of a possible 62,000 for the arrayof 200 sources by 155 receivers. Therefore, the current results are preliminary.With a grid spacing of 15 ft, we grid the image area (2,100 ft x 3,015 ft) with141 x 201 cells. In addition to the traveltime data, full waveform sonic logs from receiverand source wells provided slowness information along the borehole trajectories. Weconstruct an a priori model ma with this additional information included, and apply it tothe inversion as described in equation (1). Matarese (1993) inverted P-wave traveltimedata that were acquired during a crosswell experiment in 1993 at the same site andconstructed an image from depth 3,500 ft to 5,500 ft. To make a starting model forP-wave traveltime inversion, we extend his image on the basis of well logs to a rangeof depths from 3,285 ft to 6,300 ft, and replace the image at depths from 4,500 ft to5,500 ft with a constant velocity. The starting model for S-wave inversion is created byscaling the initial model for P-wave inversion with a factor of 1/1.73.Figure 3 shows the inversion results. Both P- and S-wave tomograms clearly showa low-velocity zone in the reef. This low-velocity image did not appear in the previoustomogram constructed by Matarese (1993). In particular, the S-wave velocities in thezone are about 20% lower than those of the surrounding carbonates. We found thislow-velocity image area well corresponds to a porous and permeable reservoir that wasinferred from Shell's log data. The porosity in the reservoir is about 10%. Currentlythis reservoir zone is mostly saturated with water, and oil production is in the late stage.2-3Zhang et al.CONCLUSIONSThe crosswell experiment in 1995 at M.LT. 's Michigan test site successfully providedP- and S-wave data for high-resolution tomographic imaging..We owe this success toConoco's orbital vibrator source and three-component receivers, and also to the optimalsurvey design and new tomography approach. The tomographic images gave us a newlook and new understanding in the reservoir.ACKNOWLEDGMENTSThis work was supported by the Reservoir Delineation Consortium at the MassachusettsInstitute of Technology.2-4(((((((S-Wave Tomographic ImagesREFERENCESChauvelier, C., R. Turpening, and J. Queen, 1996, 1995 Data acquisition program atthe Michigan Reservoir Delineation research facility, M.LT. Earth Resources Labo-ratory Borehole Acoustics Logging and Reservoir Delineation Consortia 1996 AnnualReport.Lee, J., J. Matarese, M.N. Toksiiz, and W. Turpening, 1992, Nonlinear traveltime tomog-raphy applied to MIT's Michigan test site, Geotomography, Proc. 2nd SEGljSEGInternat. Symp. on Geotomography, 457-472.Matarese, J., 1993, Nonlinear traveltime tomography, Ph.D. Thesis, Massachusetts In-stitute of Technology.Zhang, J. and M.N. Toksiiz, 1996, Nonlinear refraction traveltime tomography, ExpandedAbstracts, 66th Ann. Internat. SEG Mtg., 2011-2014.2-5Zhang et at.Michigan Test Site Geology(vp = 22,500 ftlsVp = 14.900 ftIsVp = 23,000 ftlsVp = 14,900 ftls20001500offset (tt)1000500I I •MITlBurch 1-20B • Shell MIT/Stech 1-21A(receiver well) • (source well)••F Salt I • Vp = 14,400 €Us•••StrinJJcr in F Salt • Vn = 18 300 fUsF Salt II • Vo = 14.400 €Us. E Unit Shale • Vp = 15,500 ftls•C Shale • Vn = 14.000 ftls•..... Stringer in F Salt • ....... Vp = 17,000 ftls•••B Salt • Vp = 14,900 fUs•••-•A2 Carbonate •• -......;::::::A2 Salt ••-Al Carbo Brown Nia~aran • Vp = 21,300ftfs•Al Salt (ReservOir) •-"•Vp _ 21,500 ftls-Gray Niagaran Vp =19.000 ftls.".. Schoolcraft Carbonate ~ Vo,... 22.000 ftlsBurnt Bluff Carbonate Vp = 21,500 ftIsoW01oo~ooo01ooo~01oo0101ooFigure 1: Geology of the ERL/MIT Michigan test site for the 3500-5500 ft depthinterval.2-6S-Wave Tomographic Imagesinline source, H-1 component crossline source, H-2 component3303.J::15.Q)o4936627950 100Time (ms)200 250330349366279100 200Time (ms)300Figure 2: Crosswell data recorded at a receiver at a depth of 4936 ft. The H-l componentfrom the inline source mainly shows P waves, and the H-2 component from thecrossline source shows SH waves.2-7(Zhang et al.S-wave TomogramOffset (tt)-.150 ·..·.1000 . 1950328560006300P-wave TomogramOffset (tt)-150 1000 1950328560006300",4000 4000~ ~:= :=~ ~..c: ..c:- -0. 0.Ql 5000 Ql 5000C C~!1i·l·12500 20000 23000P-wave Velocity (ftIs)--'-'!!Ii'i'!1('/j_ \'117600 9000 11 000 13000S-wave Velocity (ftIs)(Figure 3: P- and S-wave velocity reconstruction using traveltime data and slowness logs.2-8

Shear-Wave Tomographic Images Of An Oil Reservoir At MIT's Michigan Test Site

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Shear-Wave Tomographic Images Of An Oil Reservoir At MIT's Michigan Test Site

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