Application of deviation rate in oil and gas reservoir exploration

J Cent. South Univ. Technol. (2008)15: 251-255Do:10.1007/sll771-008-0047-2Application of deviation rate in oil and gas reservoir explorationTANG Jing-tian(汤井田), ZHANG Ji-feng(张继锋), FENG Bing(冯兵), WANG Ye(王烨)(1. School of Info-physics and Geomatics Engineering, Central South University, Changsha 410083, China;2. College of Geology Engineering and Geomatics, Chang'an University, Xi'an 710054, China)Abstract: Three dimensional geophysical models were abstracted and established according to characteristics of oil and gas reservoirThen direct current fields for different models were simulated with finite element software( finite element program generator) byhole-to-surface resistivity method. Numerical solution was compared with analytical one for the homogeneity earth model. And anew parameter of deviation rate was proposed by analyzing different plot curves. The results show that the relative error of solutionfor homogeneity earth model may attain to 0.043%. And deviation rate decreases from 18%to 1%and its anomaly range becomeswide gradually when the depth of oil and gas reservoir increases from 200 to 1 500 m. If resistivity ratio of oil and gas reservoir tosur-rounding rock decreases from 100 to 10 for the resistive oil and gas reservoir, the amplitude attenuation of deviation rate nearlyreaches 8%. When there exists stratum above oil and gas reservoir, and influence of resistive stratum may be eliminated or weakenedand anomaly of oil and gas reservoir can be strengthened.Key words: numerical simulation; deviation rate; oil and gas reservoir; hole-to-surface resistivity method; finite element progranprofiling method to find deep resistive oil and ga1 Introductionreservoir and determined its border. he and LIUemployed hole-to-surface method to transmit the variedIt has been an arduous duty for a long time for squared wave current, referred the case of oil and gaspetroleum prospecting workers to enhance the success reservoir and located its range and border according toratio of drilling hole, speed up the exploration pace and layer resistivity and layer polarization, and achievedreduce the exploration cost. Still, effective method is effective results. WANG et al9-ll implemented waterstarved for solving special modality of the complex oil trough simulation, in which power was supplied throughand gas reservoir, which results in many wells deserted. the steel tube well, and studied the anomaly rule of theSo in this paper deviation rate was proposed and used to ground isoclines, which provided physical base toimprove exploration ability for oil and gas reservoir.determine the distributed scope of different reservoirsBecause oil and gas reservoir is generally buried atMany problems on geophysics have not analyticaldeep place on the subsurface, surface electrical resistivity solutions, so numerical simulation of finite elementmethod does not work well Hole-to-surface method becomes a popular and effective method to solvein which the point source is placed at the near target them[12-14). In this work, direction current fields forregion in a hole can detect anomalous body and different models were simulated by finite elementmaximize signal about inhomogeneous body in the program generator), and a new parameter of deviationvicinity of a borehole, so it is an effective method to rate was proposed by analyzing different plot curvesexplore for deep objects/2-. This method has beenresearched by many scholars. In 1958, Russian 2 Theoryimplemented this method to determine horizontalboundary of the coal bed, and carried out some field test 2. 1 Definition of deviation rateresearches and yielded the preliminary results. BEVCFig. I shows the basic principle of hole-to-surfaceand MORRISON made a salt water injection resistivity method. a current source electrode is placed atexperiment by bore-to-surface electrical resistivity the appropriate location that is either under oil and gasmethod. ZHANG et al6- discovered small reservoir in reservoir or above it in a well, while the other is placed atSinkiang using groundborehole trielectrode gradient "infinity". Electrical potential is acquired by electrode Mmethod and proposed a non-boundless trielectrode on the ground when plot lines are at the center of the中国煤化工Foundation item: Projects(2006AA06Z105, 2007AA06Z134)supported by the National二 ogran of ChinaReceived date: 2007-06-25: Accepted date: 2007-07-30CNMHGCorresponding author: ZHANG Ji-feng, Doctoral candidate; Tel: +86-13467538541: E-mtaJ. Cent. South Univ. Technol. (2008)15: 251-2552.2 Weak solution integral form of differentialdirect current fieldIn the rectangular Cartesian coordinate system, thetensional partial differential aquaticdirect current field isat[ a/*a,au),al,aua auwhere a is conductivity, U is electrical potential, I isnents of point A.1)roo, U-0, where r is distance between currentsource point and receiver point. Electrical potential atinfinity "is equal to zeroHole0, where Jn is current densityp is resistivity. The current density at normal componenton the ground is equal to zeroThe interior boundary condition is the naturalFig. I Schematic diagrams of hole-to-surface resistivity methodundary condition, so it is not considered here(M--Electrical electrode of measurement; P.AppareLaplacian equation is applied in the solution domainresistivity; A-Point source):(a)Section view:(b)Plane view except the point source, therefore, Eqn. (2)becomesmouth of a well. This method can detect deep anomaly a ax ol d a dzbody on the subsurface by apparent resistivity anomaliesIn order to analyze the difference of a current sourceAccording to variational principle, Eqn. 3)mayelectrode placed under oil and gas reservoir and above it,become its weak solution integral forma new parameter of deviation rate( @)is defined aad au ad au adUdv=0(4)2.3 Numerical calculation and error analysiswhere Psa and Psu are the apparent resistivities when aA three dimensional computational program wascurrent source electrode is placed above oil and gas developed to compute apparent resistivity with hole-toreservoir and under it, respectively, P is background surface method according to Eqns (2)-(4) by finiteelement program generator( the finite element softwareTheoretically, when a current source electrode is of Flying Arrow Company)SI. By comparing theplaced under oil and gas reservoir, as a result of shielding analytical solution with numerical solution of simplerole of resistive oil and gas reservoir, current density on models, the error of numerical by FEM is analyzedthe surface is smaller than that of background field, andThe finite element solution and the analyticalthe minimum value occurs on apparent resistivity curve;, solution are calculated respectively and compared withon the contrary, when a current source electrode is placed the uniform half space, and the precision conforms toabove oil and gas reservoir, as a result of repulsion role requirement. Fig.2 shows section plot under theof resistive oil and gas reservoir, current density on the homogeneous earth, when the current source electrode issurface is larger than that of background field and the placed at depth of 600 m in the subsurface. In order tomaximum value occurs on apparent resistivity curve.us, for existence of oil and gas中国煤化工(1600 is selected,ate may increase anomaly amplitude of oil and gas wiCN MHG computation, Itsreservoir; for none of oil and gas reservoir, deviation rate averashould approach to zeroerror is 0.18%J Cent. South Univ Technol. (2008)15: 251-255Fig 4 Finite element model of borehole located at center of oiland gas reservoir(WL=Wg=100 m, d=10 m, A=200 m or 600 mAnalytical solutionFinite elemor1500m)1200-800-40004008001200Distaoil and gas reservoir or under it Hexahedron is selectedFig2 Electrical potential profile of current source electrode and the grid size is 50x 50X5 in 10 m range over theplaced at subsurfaceobject body and50×50×50 in other regions.It can be seen from Fig.5 that the apparent3 Simulations and deviation rate analysisresistivity responses show great differences, and theirshape takes on symmetrical distribution in the3.1 Geophysical model of oil and gas reservoirbackground field approximately. When current sourceOil and gas reservoir is the basic unit of the oil and electrode is placed above oil and gas reservoir, thegas accumulated in the earths crust, and it has theapparent curve gets the maximum value because of theuniform pressure system and the oil-water interfacerepulsion role of resistiMost of the petroleum hydrocarbons moving are trapped electric current; when current source electrode is placedby nonporous rocks or other barriers that would not under resistive oil and gas reservoir, the apparent curveallow it to migrate further, thus oil and gas reservoir isformed. Most reservoirs contain oil, gas and waterhas the minimum value. Peak value is reduced withGravity acts on the fluids to try to separate them in the increasing buried depth of oil and broadening gasreservoir according to their density, with gas being onreservoir and anomaly range. The shape of deviation ratethen oil and then water. In general, these interfaces curve is similar to the apparent curve when currentare often approximate to the level and horizontal source electrode is placed above oil and gas reservoirprojection lines over oil and gas interface, and oil and see Fig 6), and the amplitude is greater and is almostwater interface are often parallel with contour line of equal to sum of the amplitude of the apparent curvestructure approximately, as shown in Fig 3. This model above and under oil and gas reservoir, the resolution isthe lamellose body in the higher obviously compared with Fig. 5homogeneous semi-infinite space, as shown in Fig 41.121-h2003一h1500m8883Natural gasPetroleumFig-3 Schematic diagram for oil and gas reservoir-1200-800-4003.2 Deviation rate analysis for results of numericalDistance/msimulationFig-5 Apparent resistivity response when current3. 2.1 Model of borehole encountering center of oil and electre中国煤化工 agas reservoirThe resistivities of the oil and gas reservoir andCN MH Gs of deviation ratesurrounding rock are 500 and 80 Q2 m, respectively. curve may be influenced by the depth of oil and gasCurrent source electrode is at the depth of 5 m above the reservoir. When the depth of oil and gas reservoir is 200J. Cent. South Univ. Technol. (2005251-255u(resistivity ratio of oil and gas reservoir to surroundingh=200msmall gradually. When u is larger than 1.00, deviation3-h=1500mrate changes obviously, and if resistivity ratio of oil andgas reservoir to surrounding rock decreases from 100 to10 for resistive oil and gas reservoir, amplitudeattenuation of deviation rate is nearly 8%. When u issmaller than 1.00, the two deviation rate curves aresensitive to resistive anomalous body, but not sensitive toconductive one3.2.3 Model with one resistive layer200-800-40004008001200Fig9 shows a finite element model with one layer.Distance/mThere exists a depth of 100 m with layer under whichfig 6 Deviation rate curves of oil and gas reservoir withthere is a anomaly body in uniform half space. Thedifferent depthsresistivities for layer, anomaly body and uniform halfm,deviation rate reaches 18%, and when the buried space is 200, 500 and 80 Q2 m, respectively. Mesh anddepth is 1 500 m, the maximum value is only about Iother parameters are similar to previous one. As shownin Fig. 10, the deviation rate changes very small whenFor the latter, the curve changes extremely mild mainly there exists a resistive layer above oil and gas reservoirbecause the object body is relatively small. In numericalsimulation process, in order to enhance computationAlthough the resistivity of a layer is relatively large, thedeviation rate is almost not disturbed by virtue of itsspeed and reduce computation time, the object bodyshould be small, and grid cutting should be sparscharacteristic. The results shows that the deviation raterelatively.can weaken resistive layer above anomaly body, so3.2.2 Model of different ratio between oil and gas strong anomaly may be obtained. It is very important forreservoir and surrounding rockus to explore deep object body. And it should be notedModel was built and meshed with hexahedrons(see2000mGroundFrom Fig 8. it can be seen that with the decrease ofLayerGroundFig7 Schematic diagram of model under different resistivity Fig 9 Schematic diagram for one resistive medium layer modelratios of oil and gas reservoir to surrounding rockWith oneresistive layeWithout中国煤化工1200-800-40004008001200CNMHG 800 1200Distance/mFig 8 Deviation rate curves at different resistivity ratios of oil Fig 10 Deviation rate curves for model with and without oneand gas reservoir to surrounding rocklayer of resistive mediumJ Cent. South Univ. Technol. (2008)15: 251-255that deviation rate is invalid for conductive layer becausePAUL B, Crosswell electromagnetic and seismic imaging: Anof its suction for currentexamination of coincident surveys at a steam flood project4 Conclusions[6] ZHANG Tian-iun, ZHANG Bai-lin, LI Nie. Small reservoirdiscovery using groundborehole trielectrode gradient method J]. OilGeophysical Prospecting, 1997, 32(4): 520-531 (in Chinese1)The amplitude of anomaly is increased greatly by [7 ZHANG Tian-lun, ZHANG Bai-lin. Looking for residual oils withhe deviation rate in hole-to-surface resistivity method. Ifnon-boundless trielectrode section method using the drilling casingthe current source electrode is placed above resistive oilas electrodes[J] Journal of Southwest Petroleum Institute, 1999,and gas reservoir and under it, respectively21(1): 29-34. (in Chinese)amplitude of anomaly may be increased obviously by[8] HE Zhan-xiang. LIU Xue-jun. High-power surface-boreholedeviation rate for resistive oil and gas reservoir, even tolectrical method in predicting reservoir boundary and itsapplication[J]. Petroleum Exploration and Development, 2004, 31(5):about twice74-76.(in Chinese)2)Deviation rate is very sensitive to resistive object [91 WANG Zhi-gang. HE Zhan-xiang WEI Wen-bo, DENG Ming 3-1body. When resistivity ratio of oil and gas reservoir tophysical model experiments of well-to-ground electrical survey(Jsurrounding rock is larger than 1.00 and it is changed byOil Geophysical Prospecting, 2005, 40(5): 594-597(in Chinese1/10, the deviation rate changes sharply and amplitude (10) WANG Zhi-gang. HE Zhan-xiang WEf Wem-bo, Study on sorchange of deviation rate nearly approaches 8%.problems upon 3D modeling of dc borehole-ground method[J].3)When there exists a resistive layer for homo-Computing Technique for Geophysical and Geochemical Exploration,2006,28(4}322-377.( in Chinese)genous half space, resistive layer above anomaly body WANG Zhi-gang. HE Zhan-xiang. HE Yu. Research ofcan be weakened and relatively strong anomaly can bethree-dimensional modeling and anomalous rule on borehole-groundobtained by using deviation rate. So it is feasiblecertain geology and geophysics condition andd can(2):87-92.(in Chinese)provide theoretical instruction in the actual work.[12] DAI Qian-wei, FENG De-shan, HE Ji-shan. Finite difference timedomain method forward simulation of complex geoelectricity groundReferencesenetrating radar model[]. Journal of Central South University ofchnology,2005,12(4):478482[13] ZHOU Guo-feng, LI Xiao-yan, SHI Yao-wu, XU Bin-shi1] BEVC D, MORRISON H F Borehole-to-surface electrical resistivitymonitoring of a salt water injection experiment[ ]. Geophysics, 1991Three-dimensional finite element analysis of critical pre-twist strainangle for torsional axis[J]. Jourmal of Central South University of56(6:769-77[2] YANG F W, WARD S H. On sensitivity of surface-to-borebole[14] TANG Jian-guo, ZHANG Xin-ming, CHEN Zhi-yong, DENGresistivity measurements to the attitude and the depth to center of athree-dimensional spheroid). Geophysics, 1985. 50(7): 1173-1178Yun-lai. Finite element simulation of influences of grain interaction3 POIRMEUR C, VSSEUR G Three-dimensional modeling of aon rolling textures of fce metals(). Jourmal of Central Southfield survey[J]. Geophysics, 1988, 53(1): 85-10[15] WAN Shui, NIELSEN M P Generating finite element program of[4] WANG Zhi-gang, HE Zhan-xiang, LIU Hai-ying Three-dimensionalavier-stokes equation by means of FEPG [J]. Joumal of Shipinversionof borehole-surface electrical data based on quasi-Mechanics,2007,l1(3)328-332analytical approximation). Applied Geophysics, 2006, 3(3)[6]XU Shi-zhe. Finite element method in geophysics [M]. Beijing:Science Press, 1994: 159-160. (in Chinese)[5] MICHAEL H G PAUL M, JOONGMOO B, JOHN W, LARRY C,(Edited by CheN Weiping中国煤化工CNMHG