WELL LOGS AND ROCK PHYSICS IN SEISMIC RESERVOIR CHARACTERIZATION

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1 OTC Paper Number WELL LOGS AND ROCK PHYSICS IN SEISMIC RESERVOIR CHARACTERIZATION Jel Walls, Jack Dvrkin, Matt Carr Rck Slid Images Cpyright 2004, Offshre Technlgy Cnference This paper was prepared fr presentatin at the Offshre Technlgy Cnference held in Hustn, Texas, U.S.A., 3 6 May This paper was selected fr presentatin by an OTC Prgram Cmmittee fllwing review f infrmatin cntained in an abstract submitted by the authr(s). Cntents f the paper, as presented, have nt been reviewed by the Offshre Technlgy Cnference and are subject t crrectin by the authr(s). The material, as presented, des nt necessarily reflect any psitin f the Offshre Technlgy Cnference r its fficers. Electrnic reprductin, distributin, r strage f any part f this paper fr cmmercial purpses withut the written cnsent f the Offshre Technlgy Cnference is prhibited. Permissin t reprduce in print is restricted t an abstract f nt mre than 300 wrds; illustratins may nt be cpied. The abstract must cntain cnspicuus acknwledgment f where and by whm the paper was presented. Abstract Seismic Reservir Characterizatin, als knwn as reservir gephysics, has evlved ver the past several years int a multi-disciplinary, business-critical functin in mst ED&P rganizatins. Sheriff defines reservir gephysics as "The use f gephysical methds t assist in delineating r describing a reservir r mnitring the changes in a reservir as it is prduced." Reservir gephysics is applied acrss a wide spectrum f the ilfield life cycle frm discvery and early develpment t tertiary recvery. One critical part f this prcess is careful analysis and understanding f petrphysical prperties frm well lgs and cre data (seismic petrphysics). The purpse f this paper is t illustrate why seismic petrphysics is s imprtant and t shw hw carefully cnstructed synthetic mdels can help the gescientist interpret acustic and elastic impedance inversin frm seismic data. Intrductin Well lgs are smetimes viewed by gephysicists as "hard data" and nt subjected t the same level f scrutiny as the seismic data. This can be a mistake because well lgs are susceptible t errrs frm a number f surces. In this presentatin we will examine sme f the prcesses and prcedures that allw well lgs t be crrectly used in Seismic Reservir Characterizatin. The basic steps in seismic petrphysics analysis are: Cllect and rganize input data Perfrm gephysical lg interpretatin fr vlume minerals, prsity, and fluids Determine fluid prperties (il API, brine salinity, etc.) and reservir pressuretemperature Perturb reservir prperties using rck physics effective medium mdels (pseud-well mdeling) Cmpute synthetic seismic traces Generate trend curves and crssplts Create graphics and digital utput files. Gephysical Well Lg Analysis Well lg analysis fr gephysics differs in several imprtant ways frm standard lg analysis. In mst cases well lgs are btained fr the purpse f estimating recverable hydrcarbn vlumes. Therefre the zne f interest is mainly the prducing interval(s). Fr gephysics, well lgs frm the basis fr relating seismic prperties t the reservir. While we are still cncerned abut prducing intervals, we als need gd infrmatin abut all f the rck thrugh which the seismic waves have passed. Therefre ur zne f interest is much larger and encmpasses basically everything frm the surface t ttal depth. In all cases the lg data will require sme editing, nrmalizatin, and interpretatin befre they can be used in a reservir study. Several specific analysis steps will be fllwed: De-spike and filter t remve r crrect anmalus data pints Nrmalize lgs frm all f the selected wells t determine the apprpriate ranges and cutffs fr prsity, clay cntent, water resistivity, etc. Cmpute the vlumetric curves such as ttal prsity, Vclay, and Sw

2 2 [Paper Number 16921] Calibrate the vlumetric curves t cre data if available Crrect snic and density lgs fr mud filtrate invasin if needed Cmpute Vshear n all wells. Missing lg curves can ften be cmputed with a reasnable degree f certainty. There are tw majr ways this is dne. The first is thrugh applicatin f mdern rck physics principles. Fr example, several deterministic methds exist fr btaining density frm snic lgs r snic lgs frm resistivity. The ther apprach is t use neural netwrk technlgy. This is ften required when n direct physical relatinship is available. Well Lg Repair Many, if nt mst, riginal well lgs require editing and crrectin befre they are suitable fr creating synthetic seismgrams. The main reasns are: Wellbre washuts, casing pints, etc Mud filtrate invasin Gaps, r missing data Insufficient lg suites. In these cases, a cmbinatin f theretical, empirical, and heuristic mdels can be applied t attempt t repair the bad r missing data. A cmmn example is the prblem f mud filtrate invasin (Walls, et al., 2001; Vasquez, et al., 2004). Mud filtrate invasin ccurs during drilling with ver-balanced mud weight cnditins. The psitive pressure gradient between the wellbre and the frmatin causes sme f the mud liquids t penetrate int the permeable znes, displacing riginal fluids near the brehle wall. The severity f this cnditin varies greatly depending n permeability, mud weight, mud type, and riginal fluid saturatin. The implicatins fr reservir gephysics are primarily related t the density lg and snic lgs. These tw lgs sample rck prperties clse the the brehle wall. Figure 1 is a schematic diagram shwing apprximate depth f investigatin fr several cmmn lgging tls. Ntice that density and mnple snic are likely sampling the invaded zne. The invaded zne in this example will have higher water saturatin than the un-invaded gas sand reservir. If synthetic seismgrams are made frm the un-crrected snic and density lgs, the results will nt match the seismic data. This cnditin can be easily crrected by perfrming Bit-Gassmann fluid substitutin n the measure lg curves. The saturatin cnditins near the wellbre and in the virgin frmatin can be cmputed frm the shallw and deep resistivity lgs, respectively. Figure 2 shws the riginal and crrected density and snic curves fr a well with water-base mud invasin in gas sand. Figure 3 shws the effect f the crrectin n the synthetic seismgram frm the well. Rck Physics Mdeling and Perturbatins Rck physics mdeling can help us understand the behavir f the reservir and nn-reservir znes and crrect fr sme f the prblems encuntered in well lg data (Avseth, et al., 2001). It is the prcess f finding a rck physics mdel that is cnsistent with the available well and cre data. Fr example, we may find that sme znes in the well are clsely fitted with an uncnslidated sand mdel (Dvrkin and Nur, 1996) while ther znes fllw a cemented sand mdel (Dvrkin, et al., 1994) r elliptical crack mdel (Kuster and Tksz, 1974). These mdels may have adjustable parameters such as pre aspect rati r critical prsity that can be determined empirically frm the lcal data. Similarly sme Vs predictin methds are best calibrated t lcal cnditins if cre Vp and Vs data r diple shear wave lgs are available. Rck physics calibratins can als aid in selecting a fluid mixture mdel such as hmgeneus r patchy distributin (Dvrkin, et al., 1999). Well lg data shuld als be cmpared t available lab data, fr example Han, et al (1986) and t theretical limits such as Vigt (1928) and Reuss (1929) bunds. One purpse f rck physics mdeling is t allw reliable predictin and perturbatin f seismic respnse with changes in reservir cnditins. Fr example, the data in Figure 4 shws P-wave impedance pltted versus ttal prsity fr a well lg frm Alaska. Superimpsed n the data is a set f rck physics mdels with different clay fractins. Figure 5 shws that there is a definite link between clay cntent, water saturatin (Sw), and prsity in the reservir zne. Therefre, if we wish t change prsity, then clay cntent and Sw must als be changed. The rck physics mdel allws fr predictin f seismic prperties away frm the wellbre. Figure 6 shws the results f prsity perturbatin ver the reservir interval in the Alaska well. The gal was t create a pseud-well where the il sand was replaced by wet sand. The riginal reservir sand interval (il filled) has been perturbed by decreasing prsity, increasing Vclay, and increasing water saturatin t 100% as determined by the petrphysical relatins shwn in Figure 5. The resulting acustic impedance curve in Figure 6 shws little change. Hwever, the Pissn s rati fr the perturbed (wet sand) interval increases substantially. Synthetic seismgrams were cmputed fr the riginal (Figure 7) and perturbed well cnditins (Figure 8). A zer phase, 15 hertz Ricker wavelet was used. Frm the synthetic gathers, a stacked trace was cmputed. Acustic and elastic impedance inversin was cmputed n the synthetic traces. Figure 7 shws that fr the

3 [Paper Number 16921] 3 riginal well cnditins, there are P-wave impedance and Pissn s rati anmalies in the inverted data. Figure 8 shws the same results fr the perturbed reservir cnditins, where prsity is less and Vclay is greater than riginal. In this case the P-wave impedance anmaly is abut the same as riginal, but the Pissn s rati anmaly is much smaller. The reflectivity versus ffset was cmputed fr riginal and perturbed reservir cnditins (Figure 9). Fr riginal cnditins, the amplitude changes frm psitive t negative (phase reversal) at abut 20 degrees ffset. In the perturbed well, the amplitude crsses zer at abut 40 degrees. These mdels allw us t make a much imprved interpretatin f the acustic and elastic impedance inversin. Fr example, we can say with certainty that acustic impedance inversin alne will nt be enugh t discriminate il frm wet sand. Hwever, negative seismic Pissn s rati anmalies will be indicative f il saturatin, while the wet sand will have almst n Pissn s rati anmaly. Effects f Prductin Histry In time-lapse r 4D seismic prjects, the bjective is t infer fluid prductin frm tw r mre seismic surveys recrded at different times in the reservirs prductin life cycle. Figure 10 illustrates that multiple wells lgs and seismic surveys may have all been recrded at different times. Therefre, in rder t get the best well t seismic tie, sme wells may need t be mved frward in time (prductin histry) and thers may need t be mved backward in time, depending n when they were drilled in relatin t when each seismic survey was sht. Rck physics mdeling allws us t make these time shifts by changing saturatin, pre pressure, and even prsity in the key reservir intervals. The resulting changes in Vp, Vs, and density can then be used t created synthetic seismgrams that crrespnd t each seismic survey. Further, seismic differences can be cmputed t allw us t make quantitative predictins f changes in the reservir (Figure 11). Even when there is nly ne seismic survey, wells lgs may need crrecting fr prductin effects. Referring t Figure 10, cnsider the situatin where Wells 1 and 2 were drilled prir t the recrding f seismic survey ne, and well 3 was drilled after survey ne was recrded. If all f these wells penetrate the prducing znes, then changes in pressure and saturatin will have ccurred during the intervening time. Left uncrrected these changes may cause well t seismic miss-ties. imprved well-t-seismic ties, imprved calibratin f seismic attributes t reservir prperties, and mre reliable mdels f seismic respnse due t reservir changes (vertically laterally, and temprally). These mdels can imprve interpretatin f 3D seismic data, especially acustic and elastic impedance inversin. This imprved interpretatin can reduce drilling risk, enhance field prductivity, and ultimately increase asset value. References Sherriff, R., 2000; Encyclpedic Dictinary f Applied Gephysics Avseth, P., Mukerji, T., Jrstad, A., Mavk, G. and Veggeland, T., 2001; Seismic reservir mapping frm 3- D AVO in a Nrth Sea turbidite system: Gephysics, Sc. f Expl. Gephys., 66, Dvrkin, J., and Nur, A., 1996; Elasticity f high-prsity sandstnes: Thery fr tw Nrth Sea datasets, Gephysics, 61, Dvrkin, J., Nur, A., and Yin, H., 1994; Effective prperties f cemented granular materials, Mechanics f Materials, 18, Dvrkin, J; D. Ms, J. Packwd, A. Nur, 1999; Identifying patchy saturatin frm well lgs, Gephysics,Vlume 64, Issue 6, pp Kuster, G.T. and Tksz, M.N., 1974, Velcity and attenuatin f seismic waves in tw-phase media: Part 1. Theretical frmulatins, Gephysics, 39, Walls Jel D, and M. B. Carr, 2001; The Use f Fluid Substitutin Mdeling fr Crrectin f Mud Filtrate Invasin in Sandstne Reservirs, 71st Annual Meeting f Sciety f Explratin Gephysicists, San Antni, TX. Han, D.-H., 1986, Effects f prsity and clay cntent n wave velcities in sandstnes: Gephysics, 51, Reuss, A., 1929, Berechnung der fliessgrense vn mishkristallen: Zeitschrift fur Angewandte Mathematik und Mechanik, 9, Vasquez, G.F., L. Dilln, C. Varela, G. Net, R. Vells, and C. Nunes, 2004; Elastic lg editing and alternative invasin crrectin methds, The Leading Edge, Vl 23, N. 1, pp Vigt, W., 1928, Lehrbuck der Kristallphysik, Teubner, Leipzig. Summary The primary benefits f seismic petrphysics are

4 4 [Paper Number 16921] Uncrrected SEISMIC SYNTHETIC OFFSET SYNTHETIC Crrected SEISMIC SYNTHETIC OFFSET SYNTHETIC Figure 1: As a result f water-base mud invasin, the lgs used t make synthetic seismgrams (snic and density) may be seeing wetter rck than the seismic wave. Figure 3: Original (left) and crrected (right) synthetic seismgrams in a well with water-base mud invasin in a gas sand. First grup f traces are stacked seismic near the wellbre. Secnd grup is stacked synthetic traces. Third grup is synthetic gather. Figure 2: Original and crrected snic and density lgs in a well with water-base mud invasin in a gas sand. Figure 4: Predicted and measured P-wave impedance versus prsity.

5 [Paper Number 16921] 5 Figure 5: Relatinship between water saturatin, Vclay, and ttal prsity fr pay sand interval. Figure 6: Results f prsity and Vclay perturbatin ver the reservir sand interval (black is riginal data, red is perturbed data). Gather 0 t 60 degree Stack trace (red) Well lg P imp. Inverted P imp. Well lg PR Inverted PR Figure 7: Original reservir cnditins: Synthetic traces, well lg impedance, inverted impedance (frm stacked trace), well lg Pissn s rati, and inverted Pissn s rati (frm gather).

6 6 [Paper Number 16921] Gather 0 t 60 degree Stack trace (red) Well lg P imp. Inverted P imp. Well lg PR Inverted PR Figure 8: Perturbed reservir cnditins: Synthetic traces, well lg impedance, inverted impedance (frm stacked trace), well lg Pissn s rati, and inverted Pissn s rati (frm gather). X Seismic 2 Well 2 Y Well 1 Seismic 1 Well 3 Figure 9: Reflectivity versus angle f incidence fr riginal well (1) and perturbed reservir cnditins (2). Time (prductin) Figure 10: Schematic diagram shwing differences in recrding time and prductin histry between three wells and tw seismic surveys.

7 [Paper Number 16921] 7 Figure 11: Changes in density (rh), Vp, P-wave impedance, and synthetic seismgrams caused by reservir depletin.