Borehole Acoustics and Loggingand

Reservoir DelineationConsortia

Annual Report

1998

Earth Resources LaboratoryDepartment of Earth, Atmospheric, and Planetary Sciences

Massachusetts Institute of TechnologyCambridge, MA 02139

We would like to express our sincere appreciation to Sue Thrbak and Lori Weldon,without whose work the publication of this annual report would not be possible.

Copyright © 1998 Massachusetts Institute of TechnologyEarth Resources Laboratory

Copying is permitted only for internal purposes of the sponsors of theM.LT. Borehole Acoustics and Logging and Reservoir Delineation Consortia

This report was typeset at ERL in Computer Modern Roman using 'I'gX.

Borehole Acoustics and Loggingand Reservoir Delineation

Consortia

Annual Report

1998

Principal Investigator M. N. Toksiiz

Contributors A. Al-DajaniT. AlkhalifahH. J. AlshammeryC. 1. BurchD. R. BurnsM. G. ImhofM. L. KrasovecO. V. MikhailovF. D. MorganJ. F. OlsonJ. H. QueenP. M. ReppertW. L. RodiF. ShenJ. SierraM. N. ToksiizZ. Zhu

Report Editors D. R. BurnsE. A. HendersonK. Jesdale

Table of Contents

1. EXECUTIVE SUMMARY-CHARACTERIZATION OFRESERVOIR FLUID FLOW PROPERTIESby Daniel R. Burns and M. Nafi ToksozIntroduction. . . . . . . . . . . . . . . 1-1Fluid Flow Induced by Seismic Waves 1-2Fluid Flow Directionality . 1-3Scattering and Scale Effects 1-4Work in Progress . . . . . . 1-4

2. NEAR-SURFACE SCATTERING FROM HIGH VELOCITYCARBONATES IN WEST TEXASby Matthias G. Imhof, M. Nafi Toksoz, Charles I. Burch, and JohnH. QueenAbstract . . . . . . . 2-1Introduction. . . . . 2-1West Texas Dataset 2-3Irregular Topography. 2-6Near-Subsurface Heterogeneities 2-7Discussion and Conclusions 2-8Acknowledgments. . . . . . . . . 2-9References. . . . . . . . . . . . . 2-10Appendix A: Boundary Element Method. 2-12Appendix B: Elastic Multipole Expansions. 2-13Appendix C: Asymptotic Elastic Free Surface 2-15Figures 2-21

3. SCALE AND FREQUENCY DEPENDENCE OF REFLECTIONAND TRANSMISSION COEFFICIENTSby Matthias G. ImhofAbstract. . . 3-1Introduction . 3-1M~~d.. 3~

Example. . 3-4Conclusions 3-5

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Acknowledgments. 3-6References . 3-7Figures 3-8

4. SENSITIVITY ANALYSIS OF AMPLITUDE VARIATION WITHOFFSET (AVO) IN FRACTURED MEDIAby Mary L. Krasovec, William L. Rodi, and M. Nafi ToksiizAbstract . . . . . . . 4-1Introduction. . . . . . . 4-1The Forward Model .. 4-2Results and Discussion. 4-4Conclusions . . . . 4-5Acknowledgments. 4-5References . 4-6Figures 4-7

5. SCATTERING CHARACTERISTICS IN HETEROGENEOUSFRACTURED RESERVOIRS FROM WAVEFORM ESTIMATIONby Feng Shen and M. Nafi ToksiizAbstract . . . . . . . . . . . . . . . . . 5-1Introduction. . . . . . . . . . . . . . . 5-2Signal Parameter Estimator Function 5-4Conclusions . . . . 5-10Acknowledgments. . . . . . . . . . . . 5-11References. . . . . . . . . . . . . . . . 5-12Appendix: Signal Parameter Estimation 5-14Figures 5-17

6. SHEAR WAVE BIREFRINGENCE IN REVERSE VSP: ANAPPROACH TO 3-D SURFACE P TO S CONVERTED WAVESby Jesus Sierra and John H. QueenAbstract .Introduction. . . . . . . . . . . . . . . .Propagator Matrix Method . . . . . . .Interpretation of the Propagator MatrixSynthetic Data and Application.P to S Converted WavesConclusions . . . .Acknowledgments. . ..References. . . . . . . .Appendix: Simulated AnnealingFigures .

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7. REFLECTION MOVEOUT INVERSION FOR HORIZONTALTRANSVERSE ISOTROPY: ACCURACY AND LIMITATIONby AbdulFattah AI-Dajani and Tariq AlkhalifahAbstract. . . . . . . . . . . . . . . 7-1Introduction. . . . . . . . . . . . . 7-2Reflection Moveout in HTI Media. 7-3The Inverse Problem. . . . . . . . 7-4Error Analysis 7-5The Inverse Problem in Layered Media. 7-12Discussion and Conclusions 7-16Acknowledgments. . . . . . . . . . . 7-18References. . . . . . . . . . . . . . . 7-19Appendix: Sign of HTI Parameters. 7-20Figures 7-22

8. BOREHOLE ELECTROSEISMIC MEASUREMENTS IN DOLOMITE:IDENTIFYING FRACTURES AND PERMEABLE ZONESby Oleg V. Mikhailov and M. Nafi ToksozAbstract. . . . . . . . . . . 8-1Introduction. . . . . . . . . 8-1The Physical Phenomenon. 8-2The Theoretical Model. 8-2Field Experiments 8-3mcldD~a. ~3

Discussion . . . . . 8-6Conclusions . . . . 8-6Acknowledgments. 8-6References. 8-7Figures 8-8

9. ELECTROSEISMIC LOGGING FOR THE DETECTION ANDCHARACTERIZATION OF PERMEABLE ZONES:FIELD MEASUREMENTS AND THEORYby Oleg V. Mikhailov, Daniel R. Burns, and M. Nafi ToksozAbstract. . . . . . . . . . . . . . . . . . . . . . . . . . 9-1Introduction. . . . . . . . . . . . . . . . . . . . . . . . 9-1Electrical Field Induced by a Borehole Stoneley Wave 9-2Electroseismic Logging Technique . . . . . . . . . . . . 9-3Field Experiment . . . . . . . . . . . . . . . . . . . . . 9-5Preliminary Analysis of the Stoneley-Wave-Induced Electrical Signals 9-8Theoretical Model for the Stoneley-Wave-Induced Electrical Potential 9-10Comparison of the Field Data and Theory: Amplitude-Versus-Frequency

Dependence of the Stoneley-Wave-Induced Electrical Potential 9-15

vii

Discussion . . . . . 9-15Conclusions . . . . 9-17Acknowledgments. 9-17References . . . . . 9~19Appendix A. Analysis of the Mysterious Electrical Signal M-M 9-21Appendix B. Relationships Bet\veen the Amplitude ofthe Electrical Potential

Oscillation and the Amplitudes of the Signals Measured Using the4-Electrode and the 2-Electrode Arrays 9-22

Figures 9-24

lO.SEISMOELECTRIC LABORATORY MEASUREMENTS INA BOREHOLEby Zhenya Zhu and M. N afi ToksozAbstract. . . . . . . . . . . . . . . . 10-1Introduction. . . . . . . . . . . . . . . 10-1Borehole Models and Measurements . 10-2Results in Fractured Borehole Models 10-3Results in Sandwiched Borehole Models 10-5Conclusions . . . . 10-6Acknowledgments. 10-6References. 10-7Figures 10-8

l1.LABORATORY STUDY OF FREQUENCY DEPENDENTSTREAMING POTENTIALSby Philip M. Reppert and F. Dale MorganAbstract. . . 11-1Introduction. . . . . . . 11-1Theory. . . . . . . . . . 11-2Experimental Approach 11-4Discussion and Applications. 11-5Conclusions . . . . 11-6Acknowledgments. 11-6References . 11-7Figures 11-8

12.INTERVAL ATTENUATION ESTIMATIONby Hafiz J. Aishammery and M. Nafi ToksozAbstract. . . . . 12-1Introduction. '. . . . . . 12-1Definition of Q . . . . . 12-2Spectral Ratio :rvIethod . 12-3Q Estimation Accuracy 12-4

V III

Water Tank ExperimentConclusions . . . .Acknowledgments.References .Figures .

13.TOWARD THE SIMULATION OF ATTENUATION:OSCILLATORY FLOW IN POROUS ROCKby John F. OlsonAbstract .Introduction .Simulation Method .Preliminary ResultsConclusions ....Acknowledgments.References .Figures .

14.DISPERSION ANALYSIS OF CROSS-DIPOLE DATAby Xiaojun Huang, Daniel R. Burns, and M. Nafi ToksozAbstract .Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Flexural Dispersion and Formation Anisotropy . . . . . . . . . .A New Rotation Method for Mismatched Sources and Receivers.Application to Four-Component Cross-Dipole Data.Effects of Rotation on Dispersion Analysis .Conclusions . . . .Acknowledgments. . . -.'References .Figures .

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EXECUTIVE SUMMARY-CHARACTERIZATION OFRESERVOIR FLUID FLOW PROPERTIES

Daniel R. Burns and M. Nafi Toksoz

Earth Resources LaboratoryDepartment of Earth, Atmospheric, and Planetary Sciences

Massachusetts Institute of TechnologyCambridge, MA 02139

INTRODUCTION

The characterization of subsurface fluid flow is the key to exploration, production, andmanagement of oil, gas, geothermal, and groundwater reservoirs. From a research per­spective the question then becomes how can we measure or estimate the spatial vari­ations in physical properties which control the flow of these fluids? Ideally we wouldlike to be able to remotely identify fluid type and provide accurate estimates of in-situporosity and permeability values in three dimensions. In some situations this may bepossible, but in others such a goal may not be attainable. If accurate estimates of thesephysical properties cannot be achieved, there may still be important and useful relatedinformation available. For example, being able to provide accurate estimates of thedirection of maximum fluid flow may be a very important piece of information for fielddevelopment and drilling decisions. The Earth Resources Laboratory has focused ondeveloping methods for using seismic waves to estimate flnid flow properties. Seismicwaves provide higher resolution data than potential field measurements, can be polar­ized for anisotropy measurements, can be used in surface and borehole applications.and ERL has significant experience in the physics and modeling of wave propagation illcomplex media.

Our report this year provides results in three specific areas related t.o our overallgoal of fluid flow characterization. The first. involves research into methods which utilizet.he actual motion, induced by seismic waves, of fluids ill porous rock. The passing of aseismic wave t.hrough a rock cont.aining a viscous fluid result.s in relative motion betweent.he fluid and the solid matrix. This mot.ion dissipates seismic energy resulting in theatt.enuat.ion of the passing wave. The motion of the fluid, which contains ions, alsocreates an elect.rical field which can be measured. We present result.s in both of theseareas in this report. A second research area is the measurement of flow direct.ionality.

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Burns

The presence of open fractures with a preferred orientation will control the directionof fluid flow in a reservoir. Such fractures will also have a large effect on the elasticproperties of the reservoir rock. The resulting elastic anisotropy may be observable inboth surface seismic data as well as borehole acoustic logs. We present several papers onthe effects of anisotropy on surface seismic data and AVO signatures, as well as on dipolesonic logs. Finally, because fluid flow is only one of the many factors which effect seismicwaves, we must continue to improve our understanding of wave propagation in complexmedia. We present two such papers which look at scattering and scale dependent issues.The way in which property variations at different spatial scales affect seismic waves isan important research area that we will continue to investigate in the coming year.

The following sections provide a more detailed summary of the results in each ofthese three broad areas.

FLUID FLOW INDUCED BY SEISMIC WAVES

A seismic wave impinging on a fluid-filled porous rock results in relative motion betweenthe viscous fluid and the solid matrix. This motion may be bulk flow or local '~quirt'

flow. Although there is evidence of both types of mechanisms from laboratory studiesof rock samples, the way that fluid moves in a porous rock when a seismic wave passesis not well understood. Regardless of the mechanism, such relative motion results inattenuation of the seismic wave, with the frequency dependence of that attenuationbeing related to the type of fluid flow mechanism at work. Olson presents a numericalmodeling approach that allows us to visualize such fluid motion. His results provide anexciting glimpse into the complexity of fluid flow in rocks, and will help us to understandthe dominant attenuation mechanisms in different rocks.

The use of surface seismic data to estimate fluid flow properties based on attenua­tion measurements is much more difficult. In order to use reflection data to estimatevariations in reservoir attenuation, we must be able to isolate any amplitude changesto a specific interval of interest. Alshammery and Toksoz present results from a lab­oratory study in which they estimate the quality factor of several materials based ona comparison of the reflections from the top and base of a slab suspended in a watertank. The Q estimates they obtain are in good agreement with the actual values, butthey point out that overburden attenuation may degrade the estimates as a function ofsource receiver offset due to different travel path lengths.

When a seismic wave induces fluid flow, a streaming electrical current is produced.This current is caused by the adsorption of charged ions to the surface of the solid grains,and the resulting charge excess in the pore fluid. The relative motion of this fluid createsan electrical field which can be measured. This electroseismic effect provides anotherway to measure remotely the movement of fluids in a reservoir. Mikhailov et al. presentfield observations of this phenomenon in a borehole logging mode. They show that theeffect can be measured in the field in sedimentary rocks (dolomite), and that the effectcan be related to the interconnected porosity in the formation. They also find that by

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Executive Summary

making the measurement over a range of frequencies, it may be possible to estimatethe permeability of the formation. Zhu and Toksoz likewise study the electroseismiclogging technique in laboratory scale models. Their results indicate that this effect canbe used to identify open fractures, and that the amplitude of the signal is related to thefracture aperture. Finally, Reppert and Morgan provide laboratory measurements of thestreaming potential caused by fluid motion through porous materials. They measuredthe effect for a capillary tube, a porous filter, and a sample of Boise Sandstone over afrequency range which covered the critical frequency for each material. Their estimatesfor the average pore diameter based on the critical frequency are in good agreementwith the actual values, and support the idea that the measurement of electroseismicdata over a range of frequencies may provide a means of estimating permeability.

FLUID FLOW DIRECTIONALITY

Although we are most interested in obtaining direct indicators of in-situ permeabilityvariations, information about the direction of fluid flow can be critically important forreservoir development decisions. The presence of aligned fractures results in elasticanisotropy which can be characterized by seismic measurements. AVO measurementsas a function of azimuth can provide information about the orientation of fractures andthe fracture density which is related to the fluid volume and permeability. Krasovecet al. studied the sensitivity of AVO parameters to fracture density, pore fluid, andlithology variations. They found that the 'c' parameter in a three-term Shuey-typeparameterization of the AVO behavior is particularly sensitive to fracture density varia­tions for vertical fractures. Shen et al., using 3-D finite difference models and field data,found that the spectral characteristics of reflections from the top and base of a fracturedreservoir contain information about fracture density and the correlation scale lengthsof the fracture distribution. They used a high resolution spectral analysis technique tostudy variations in the spectral signatures of reflections as a function of offset. Theirresults indicate that scattering from fractures aligned normal to the propagation pathresults in more energy at higher frequencies as a function of offset than would be seenfor data acquired in areas without fractures or along the fracture strike.

Al-Dajani and Alkhalifah investigate the effect of an HTI medium on the normalmoveout of seismic reflections as a function of azimuth. If measurements are availablefor several source-receiver azimuths, the anisotropy parameter and symmetry axis di­rection can be obtained. Sierra and Queen present a method for estimating shear wavebirefringence from 3-D surface P-S converted wave data based on an equivalent reverse­VSP formulation. They show that it is possible, using a range of frequencies and offsets,to obtain information about 'subsurface anisotropy from converted wave data.

In-situ anisotropy directions, due to aligned fractures, stress, or intrinsic materialproperties, can also be obtained from cross-dipole sonic logs. Huang et al. presenta frequency domain rotation method that handles situations in which the two dipolesources are not matched. This method can improve the accuracy of the rotation of

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the data into fast and slow shear wave directions. Accurate rotation is also critical foranalyzing differences in the dispersion characteristics of the fast and slow flexural modes.Differences between these dispersion curves may indicate variations in the propagationdirections as a function of distance from the borehole.

SCATTERING AND SCALE EFFECTS

The characterization of fluid flow effects in reservoirs takes place within the context ofcomplex subsurface geological conditions which scatter seismic energy. These scatteringeffects can provide us with information about the nature of small-scale heterogeneitieswhich may impact reservoir production. However, if we are interested in the analysis ofsubtle fluid flow effects in seismic data, such scattering may be an unwelcome compli­cation. Imhof et al. studied the effects of near-surface scattering on seismic reflectiondata. They found that for data acquired in West Texas, scattering was dominated bythe presence of vugs and cavities in the near surface limestones. These heterogeneitiesset up a waveguide that trapped energy near the surface. Irregular topography andheterogeneous weathering layers did not appear to be important factors for this data.

Imhof, in a second paper, studied the effect of different spatial scale heterogeneitieson the reflection and transmission of seismic waves. Using wavelet transforms to filterdifferent spatial scales from a sonic log, he calculated reflection and transmission coef­ficients as a function of frequency. The results indicate that transmission coefficientsare generally independent of frequency and scale, while reflection coefficients are verysensitive to even the small-scale perturbations of slowness.

WORK IN PROGRESS

A number of other related projects are ongoing at ERL, including: geostatistics, fastwave propagation modeling and imaging algorithms (phase-screen type propagators;variable grid finite difference methods), hydrofracture imaging and monitoring, VSPwhile drilling, and a field acquisition research program that includes reverse VSP usinga downhole vibrator source and surface 3-D seismic using random receiver layouts.

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