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CHINESE JOURNAL OF GEOPHYSICS Vol.51, No.3, 2008, pp: 639644 DISCRIMINATING GAS AND WATER USING MULTI-ANGLE EXTENDED ELASTIC IMPEDANCE INVERSION IN CARBONATE RESERVOIRS PENG Zhen-Ming 1,2 , LI Ya-Lin 2 , WU Sheng-Hong 2 , HE Zhen-Hua 3 , ZHOU Yong-Jun 2 1 School of Opto-Electronic Information, University of Electronic Science and Technology of China, Chengdu 610054, China 2 Sichuan Geophysical Company, CNPC Sichuan Petroleum, Chengdu 610213, China 3 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China Abstract AVO response for carbonate reservoirs is weaker than the clastic reservoirs. So it is very hard to discriminate fluid in carbonate reservoirs using elastic impedance (EI), even under the optimal incident angle. In this paper, a novel method is proposed for discriminating fluid using multi-angle extended elastic impedance (MEEI) inversion in carbonate reservoirs. We established the fluid-bearing patterns and effective criterions for discriminating gas and water by calculating MEEI of target layers using the known well data. MEEI inversion of real seismic data shows that we can judge whether the reservoir is good or bad or is gas-bearing or water-bearing by observing EEI change trend with various incident angles. The efficiency of the proposed method is validated by the practical application of real seismic data in Sichuan basin. Key words Extended elastic impedance, Multi-angle analysis, Fluid criterions, Fluid discrimination, Carbonate reservoirs 1 INTRODUCTION The concept of elastic impedance (EI) was first introduced by Connolly (1999) [1] , which is suitable for fluid discrimination and lithology prediction for various reservoirs because it includes more abundant information of lithology and fluid than acoustic impedance (AI). Recently, a great quantity of research work for EI has been done by scientists in China and abroad. In order to overcome the drawback of incident angle restriction and the rapid variation of inversion results with incident angle which gives rise to difficulties for interpretation, Whitcombe et al. [2,3] deduced the expression of the extended elastic impedance (EEI) by modifying the original equation of the EI established by Connoly, which makes it more suitable for fluid discrimination and lithology prediction. They calculated the distribution map of lithology impedance and fluid impedance using EEI in an oil field of North Sea and obtained the distinct imaging map of their channel system. Duffaut et al. [4] introduced the “shear wave elastic impedance” (SEI) to solve the problem for lithology prediction with converted waves. Ezequiel [5] presented a formulation of P-to-S converted waves elastic impedance (PSEI) under arbitrary incidence angles and used to distinguish fizz water from commercial gas for predicting area distribution. From 2003, the geophysical circles in China [615] began to pay attention to this subject, presented a series of concepts and theories such as Ray-path elastic impedance (REI), Generalized elastic impedance(GEI), Zoeppritzt elastic impedance(ZEI), etc., and a quantity of research and analysis works have been done in the areas of its practical applications. The authors consider that EEI is effective for lithology discrimination of clastic reservoirs by analyzing and comparing the results of the above methods and computed the real seismic data. However, various case studies show [1417] that AVO response for carbonate reservoirs is weaker than the clastic reservoirs. Difficulties were encountered when we discriminated fluid for carbonate reservoirs under the optimal incident angle according to the literature [3] because all EEI values indicate low value abnormity for gas-bearing or water-bearing reservoirs, E-mail: [email protected]
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Seismic Inversion in Carbonates

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  • CHINESE JOURNAL OF GEOPHYSICS Vol.51, No.3, 2008, pp: 639644

    DISCRIMINATING GAS AND WATER USING MULTI-ANGLEEXTENDED ELASTIC IMPEDANCE INVERSION

    IN CARBONATE RESERVOIRS

    PENG Zhen-Ming1,2, LI Ya-Lin2, WU Sheng-Hong2, HE Zhen-Hua3, ZHOU Yong-Jun2

    1 School of Opto-Electronic Information, University of Electronic Science and Technology of China,

    Chengdu 610054, China

    2 Sichuan Geophysical Company, CNPC Sichuan Petroleum, Chengdu 610213, China

    3 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of

    Technology, Chengdu 610059, China

    Abstract AVO response for carbonate reservoirs is weaker than the clastic reservoirs. So it is very hard to

    discriminate fluid in carbonate reservoirs using elastic impedance (EI), even under the optimal incident angle.

    In this paper, a novel method is proposed for discriminating fluid using multi-angle extended elastic impedance

    (MEEI) inversion in carbonate reservoirs. We established the fluid-bearing patterns and effective criterions for

    discriminating gas and water by calculating MEEI of target layers using the known well data. MEEI inversion of

    real seismic data shows that we can judge whether the reservoir is good or bad or is gas-bearing or water-bearing

    by observing EEI change trend with various incident angles. The efficiency of the proposed method is validated

    by the practical application of real seismic data in Sichuan basin.

    Key words Extended elastic impedance, Multi-angle analysis, Fluid criterions, Fluid discrimination, Carbonate

    reservoirs

    1 INTRODUCTION

    The concept of elastic impedance (EI) was first introduced by Connolly (1999)[1], which is suitable for fluiddiscrimination and lithology prediction for various reservoirs because it includes more abundant informationof lithology and fluid than acoustic impedance (AI). Recently, a great quantity of research work for EI hasbeen done by scientists in China and abroad. In order to overcome the drawback of incident angle restrictionand the rapid variation of inversion results with incident angle which gives rise to difficulties for interpretation,Whitcombe et al.[2,3] deduced the expression of the extended elastic impedance (EEI) by modifying the originalequation of the EI established by Connoly, which makes it more suitable for fluid discrimination and lithologyprediction. They calculated the distribution map of lithology impedance and fluid impedance using EEI in anoil field of North Sea and obtained the distinct imaging map of their channel system. Duffaut et al.[4] introducedthe shear wave elastic impedance (SEI) to solve the problem for lithology prediction with converted waves.Ezequiel[5] presented a formulation of P-to-S converted waves elastic impedance (PSEI) under arbitrary incidenceangles and used to distinguish fizz water from commercial gas for predicting area distribution. From 2003, thegeophysical circles in China[615] began to pay attention to this subject, presented a series of concepts andtheories such as Ray-path elastic impedance (REI), Generalized elastic impedance(GEI), Zoeppritzt elasticimpedance(ZEI), etc., and a quantity of research and analysis works have been done in the areas of its practicalapplications.

    The authors consider that EEI is effective for lithology discrimination of clastic reservoirs by analyzing andcomparing the results of the above methods and computed the real seismic data. However, various case studiesshow[1417] that AVO response for carbonate reservoirs is weaker than the clastic reservoirs. Difficulties wereencountered when we discriminated fluid for carbonate reservoirs under the optimal incident angle according tothe literature [3] because all EEI values indicate low value abnormity for gas-bearing or water-bearing reservoirs,

    E-mail: [email protected]

  • 640 Chinese J. Geophys. Vol.51, No.3

    even under the optimal incident angle. We can effectively predict reservoirs, but it is beyond its capabilities touse the above-mentioned method for evaluating the reservoir quality and discriminating gas- or water-content.On the basis of the EEI theory, we computed the multi-angle extended elastic impedance (MEEI) of targetlayers and observed its varying trend with various incident angles. In our experiments, we found that the MEEIresponses indicate the significant difference for gas-bearing or water-bearing reservoirs by combining loggingdata (P- wave, S-wave and density) of drilled wells with the results of well log interpretation. It is provedthat using MEEI inversion to discriminate fluid for carbonate reservoirs is effective and feasible and effectivecriterion for discriminating gas and water was established in this paper. The efficiency of the proposed methodsis validated by the application of real data in Sichuan basin.

    2 THE DEFINITION OF ELASTIC IMPEDANCE AND EXTENDED ELASTIC IMPEDANCE

    Elastic impedance (EI) is established on the basis of simplified equation of Aki-Richards[18]. If just considerthe incident angles less than 30, then

    R() = A+B sin2 , (1)

    where A =12

    (vpvp

    +

    ), B =

    vp2vp

    4(vsvp

    )2(vsvs

    )2

    (vsvp

    )2(

    ), and vp,vp, vs,vs, , donate

    the average value and difference of compressional (P-wave) velocity, shear wave velocity and density in adjacentformation, respectively.

    Connolly[1] constructed an equation for calculating reflectivity similar to acoustic impedance as follows

    R() =EIi+1 EIiEIi+1 + EIi

    12EIEI

    12ln(EI), (2)

    where EI is called as elastic impedance. Comparing Eq.(1) and (2) and by simple computation, EI expressionscan be written as follows

    EI() = v(1+sin2 )

    p v(8K sin2 )s

    (14K sin2 ), (3)

    where vp is P-wave velocity, vs is S-wave velocity, is density and K =

    [(vsivpi

    )2+(vsi+1vpi+1

    )2]/2 is the average

    value of the square of the ratio of P to S velocity. is the incident angle of P-wave. The main problems incalculating EI using Eq.(3) is that its value dramatically decreases with incident angle, which probably losesome information for analyzing elastic impedance at near and far angles, for example, the difference of fluid orlithology change.

    Therefore, in order to eliminate the effects, Whitcombe et al.[3] deduced a new formula of EI by normalizingthe original EI equation, as follows

    EI() = 00

    [(

    0

    )a(

    0

    )b(

    0

    )c], (4)

    where 0, 0, 0 are constant coefficient, which donate average value of vp, vs, in reservoir intervals, respectively., , are vpi , vsi , i and at different depth, respectively, and

    a = 1 + sin2 , b = 8K sin2 , c = 1 4K sin2 . (5)Although Eq.(4) can remove the effect of dramatic EI decrease with incident angle and keep the same

    orders of magnitude with post-stack acoustic impedance, the reflectivity R() are probably larger than 1 con-flicting with the real seismic recorder. Thus, Whitcombe[3] further modified the EI equation, and replaced sin2 by tan ( is theoretical incident angle being expanded, which changes between 90 and 90), scaled the re-flectivity R() by multiplication by cos. The so-called extended elastic impedance (EEI) can be formulatedas

    EEI() = 00

    [(

    0

    )p(

    0

    )q (

    0

    )r], (6)

  • Peng Z M et al.: Discriminating Gas and Water Using Multi-Angle Extended Elastic Impedance Inversion 641where

    p = cos+ sin, q = 8K sin, r = cos 4K sin. (7)

    EEI value calculated by Eq.(6) not only eliminates the effect of EI dramatic decrease with incident angle,but also controls the reflectivity within [1,1] and is more suitable to real seismic recorder and beneficial todirectly detect lithology and fluid.

    3 MEEI WELL MODEL AND FLUID CRITERION FOR CARBONATE RESERVOIRS

    Fluid detection using EEI needs to build the fluid discrimination model according to the prior information.In general, the relationship between EI and water saturation at fixed incident angle is adopted and then theinterpretation scheme is determined. However, these methods have two disadvantages: one is that it is difficultto predefine a reasonable incident angle; the other is that the multi-angle information of pre-stack seismic datais lost under the fixed incident angle. In the study we found that the EEI inversion results of target layers showlow value abnormity under the gas-bearing or water-bearing conditions in fixed incident angle (for example,16), and it is hard to discriminate the gas and water in reservoir. Some literatures also introduce the approachto partition gas and water boundary by EI inversion for the limited partial stack seismic section at near andfar incident angle and the cross-plot analyses are done for inversion results and data samples are artificiallyclassified for gas or water. However, this method is probably affected by various artificial factors, because thesedata are not completely divided, and cause the large interpretation error.

    This paper presents a new model for discriminating fluid using multi-angle extended elastic impedance,(MEEI), in which the change trend of EEI with incident angles is observed continuously by calculating a seriesof EEI within the efficient range of incident angles for reservoir zone. Then the corresponding fluid criterionsare determined by prior information of fluid-bearing (for example, log interpretation results). The analyses oflogging data (P-wave velocity, S-wave velocity and density) from several wells show that the proposed method canconstruct an efficient fluid-bearing pattern for carbonate reservoirs. Fig. 1 shows EEI varying with the incidentangle (theoretical angle ) of gas-bearing and water-bearing reservoirs of well X6. This well was interpreted asgas-bearing layer at 43844386 m and 44714480 m depth and as water-bearing layer at 44884511 m. Wecan see from the figure that EEI value increases with the incident angle decreasing for water-bearing reservoirsand EEI value decreases with the incident angle increasing for water-bearing reservoirs, while it changes slowlyfor gas-water-bearing. To further verify the regularity and obtain a consistent conclusion, the same analysesare also done for the other wells in this area. Fig. 2 shows that EEI values for gas-bearing and water-bearinglayers of well X4 and X5 are consistent with Fig. 1. The EEI change trend of gas-bearing layer in well X1 isalso basically consistent.

    The calculation of real data from several wells shows that MEEI analysis is effective for discriminating gasand water in carbonate reservoirs and its response is obvious. Comparing with acoustic impedance, EEI also

  • 642 Chinese J. Geophys. Vol.51, No.3

    shows low value for gas-bearing or water-bearing reservoirs. However, MEEI for the different fluid in reservoirinterval show the different responses as follows: (1) EEI increases with the increase of incident angle for water-bearing case; (2) EEI decreases with increasing incident angle for gas-bearing case; (3) EEI changes slowly withthe increase of incident angle for gas-water-bearing case.

    In practice application, gas-bearing cases of a few wells show inconsistent result with the above fluidcriterion, in which EEI increases with incident angle increasing. It is caused by absence of S-wave velocity inthis well and using Castagna equation to fit it. Because the relationship between P-wave and S-wave velocityis not linear, it causes the uncertainty of calculating results. Generally, according to Castagna equation, P-wave and S-wave velocity at most layers are correlative. However, practice shows that those zones with poorcorrelation between vp and vs are just the places for favorable reservoir bodies. Therefore, the results and theiraccuracy will be affected due to the absence of S-wave velocity in fluid discrimination using MEEI inversion.

    For the sake of analyzing problem, the label of the abscissa is the theoretical angles (not the actual incidentangle) in Fig. 1 and Fig. 2, which change within 0 90. The relationship between the actual incident angleand the theoretical angles is monotonic increasing or decreasing, and we aim at observing the EEI change trendwith incident angle for gar-bearing or water-bearing reservoirs rather than numerical value of actual incidentangles. It will not affect the result of analyzing problem, although the theoretical angles are not converted tothe actual incident angles.

    4 CASE STUDY AND RESULTS ANALYSIS

    MEEI analyses are conducted for logging data and the gas and water criterions are established by forwardcalculation based on the above approaches, then the real seismic data can be inversed for angle trace gatherswith pre-stack NMO and pre-stack time migration. In the study area, MEEI inversion was tested for two 2-Dseismic line across gas well and water well, respectively.

    Figure 3 and 4 are the results of MEEI inversion and fluid detection for real 2-D line across typical gaswell and water well, respectively. Fig. 3a, 3b, and 3c are the EEI inversion sections at gas-bearing reservoirintervals of inline 450 (across well X4) with various actual incident angles (5, 13, and 21), where navy bluecolor donates the low EEI value. We can see from the regions marked as red ellipses in the figures that thecolor of inversion results within gas-bearing reservoir interval deepens gradually with incident angles increasing,indicating that EEI values decrease with incident angles increasing. The EEI inversion results of reservoirintervals of inline 557 (across well X6, interpreted as water-bearing layer in logging ) under various actualincident angles (5, 15 and 25) are shown in Fig. 4a, 4b, and 4c. Similarly, we can see from the regions markedas purple ellipses in the figures that the color of inversion results within water-bearing reservoir interval lightensgradually with incident angles increasing, indicating that EEI values increase with incident angles increasing.

    The inversion results of real seismic data are all consistent with fluid discrimination criterion establishedin this paper. Moreover, model-based inversion of MEEI was realized for the 3-D working area of 100 m2.Consistency verification has been done by using 3 wells of the 6 wells drilled in the area as constraining conditionsand taking the others as the unknown wells, the resultant consistency ratio is 100%. The test results show thatthe proposed method based on MEEI for discriminating fluid is efficient and feasible.

    5 CONCLUSIONS

    The fluid discrimination criterions are established by analyzing logging data at reservoir intervals andforward calculation in this paper. The test results for real 3-D seismic data show that it is an effective approachto discriminating gas and water in carbonate reservoirs using MEEI inversion and is better than EEI inversionunder the fixed incident angle or EEI cross-plot analysis between near and far angles. Although the presentmethod is used to detect fluid in carbonate reservoirs, it also has referential significance to lithology identificationin sand-shale formation or clastic reservoirs.

  • Peng Z M et al.: Discriminating Gas and Water Using Multi-Angle Extended Elastic Impedance Inversion 643

    The actual incident angles is (a) 5, (b) 13, (c) 21. The actual incident angles is (a) 5, (b) 15, (c) 25.

    ACKNOWLEDGMENTS

    Authors would like to express their gratitude to Sichuan Geophysical Company of CNPC Sichuan Petroleumfor permitting to publish this project. This work has been supported by the Aeronautic Science Foundation ofChina (20060112116), Research Fund for the Doctoral Program of Higher Education of China (20070614016)and National Natural Science Foundation of China (60572092).

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