Fractured reservoir modeling by discrete fracture network and seismic modeling in the Tarim Basin, China

Petroleum Science, Dec 2011

Fractured reservoirs are an important target for oil and gas exploration in the Tarim Basin and the prediction of this type of reservoir is challenging. Due to the complicated fracture system in the Tarim Basin, the conventional AVO inversion method based on HTI theory to predict fracture development will result in some errors. Thus, an integrated research concept for fractured reservoir prediction is put forward in this paper. Seismic modeling plays a bridging role in this concept, and the establishment of an anisotropic fracture model by Discrete Fracture Network (DFN) is the key part. Because the fracture system in the Tarim Basin shows complex anisotropic characteristics, it is vital to build an effective anisotropic model. Based on geological, well logging and seismic data, an effective anisotropic model of complex fracture systems can be set up with the DFN method. The effective elastic coefficients, and the input data for seismic modeling can be calculated. Then seismic modeling based on this model is performed, and the seismic response characteristics are analyzed. The modeling results can be used in the following AVO inversion for fracture detection.

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Fractured reservoir modeling by discrete fracture network and seismic modeling in the Tarim Basin, China

Pet.Sci. Fractured reservoir modeling by discrete fracture network and seismic modeling in the Tarim Basin, China Sam Zandong Sun 1 2 Zhou Xinyuan 0 1 Yang Haijun 0 1 Wang Yueying 1 2 Wang Di 1 2 Liu Zhishui 1 2 0 Research Institute of Exploration & Development, PetroChina Tarim Oilfield Company , Korla 841000 , China 1 China University of Petroleum (Beijing) and Springer-Verlag Berlin Heidelberg 2011 2 Laboratory for Integration of Geology & Geophysics, China University of Petroleum , Beijing 102249 , China Fractured reservoirs are an important target for oil and gas exploration in the Tarim Basin and the prediction of this type of reservoir is challenging. Due to the complicated fracture system in the Tarim Basin, the conventional AVO inversion method based on HTI theory to predict fracture development will result in some errors. Thus, an integrated research concept for fractured reservoir prediction is put forward in this paper. Seismic modeling plays a bridging role in this concept, and the establishment of an anisotropic fracture model by Discrete Fracture Network (DFN) is the key part. Because the fracture system in the Tarim Basin shows complex anisotropic characteristics, it is vital to build an effective anisotropic model. Based on geological, well logging and seismic data, an effective anisotropic model of complex fracture systems can be set up with the DFN method. The effective elastic coefficients, and the input data for seismic modeling can be calculated. Then seismic modeling based on this model is performed, and the seismic response characteristics are analyzed. The modeling results can be used in the following AVO inversion for fracture detection. Fractured reservoir; Discrete Fracture Network (DFN); equivalent medium; seismic modeling; azimuth-angle gathers - The Tarim Basin is a large-scale superimposed basin. For example, the Tazhong area has undergone multi-stage tectonic movements and crust uplifting. The strata have suffered strong weathering, denudation and corrosion (Wei et al, 2000; Luo et al, 2005) . Therefore, many unconformities are formed. Moreover, the bedrock of the Tazhong carbonate reservoirs is characterized by low porosity and low permeability, and secondary storage space such as fractures, pores and caves are well developed (Yang et al, 2000). As a result, the reservoirs have strong heterogeneity. Carbonate reservoir prediction is a difficult problem, throughout the whole world. However, the Tarim Basin is the energy base in China and exploration is of great importance to the national economy and people’s livelihood. So reservoir prediction in the carbonate rock of the Tarim Basin is more significant than ever. Seismic exploration makes use of the relationship between the propagation characteristics of the seismic wave and the physical properties of the strata. Reservoirs usually show anisotropic characteristics when fractures exist. Therefore, we should investigate the seismic wave field based on the assumption of anisotropy. When we carry out the seismic wave forward modeling, first, the fracture models should be built according to geological statistics based on the geologic data, seismic data and well logging data. Secondly, through the analysis of the fracture models, the fractured reservoir can be equaled to a specific anisotropic model and the elastic coefficients can be computed. Finally, we can model the seismic wave field. The modeling result can not only help us to study the seismic wave characteristics and provide the guidance to field acquisition, but also can be utilized for AVO inversion to extract fracture information. So the seismic wave modeling is an important part. 2 A research concept for fractured reservoir prediction Fractured reservoirs are an important type of oil and gas reservoir in the Tarim Basin and they show strong anisotropy. When AVO inversion is carried out to predict fracture development, fractures are often viewed as the Horizontal Transverse Isotropy (HTI) medium. However, the real fracture system is rather complicated. So the theoretical inversion results are very likely to contain some errors. In 315 270 225 315 270 225 3.2 Fracture analysis After the geological model is set up, the cores, FMI and fault information from seismic data are needed. According to the geological statistics, the information such as fracture orientation and dip angle are analyzed. The fracture analysis method includes single-well analysis and multi-well integrated analysis. The fracture analysis result of well TZ45 is shown in Fig. 4(a), and the dip direction is recognized to be mainly NESW. Both high angle and low angle fracture exist in well TZ45. The high angle ranges from 65 degrees to 85 degrees, while the low angle ranges from 5 degrees to 20 degrees. An integrated fracture analysis from wells TZ45, TZ86 and TZ88 is shown in Fig. 4(b). Two groups of fractures can be observed in the TZ45 area. One dip direction is NE-SW, with the dip angle ranging from 45 degrees to 85 degrees, and the other is NW-SE, with the dip angle ranging from 10 degrees to 75 degrees. With Bias Without Bias 1:1 Fault set definition Statistic cabibration With Bias 1:1 Fault set definition Statistic cabibration Without Bias (a) Fracture analysis of well TZ45 (b) Integrated fracture analysis (TZ45, TZ86, TZ88) 3.3 Fault analysis According to the seismic interpretation results, two groups of faults are developed in TZ45 area: NE-SW (D1) and NWSE (D2), as shown in Fig. 5. For fault D1, its strike is 204 degrees, and the grid dimension is 1.06. The average length of the fault is 1020 m, and the dip Fischer coefficient is 2.6. For fault D2, its strike is 85 degrees, and the grid dimension is 1.23. The average length of the fault is 1193 m, and the dip Fischer coefficient is 2.9. If D1 and D2 can be assumed to be vertical faults, D2 is cut by D1. 3.4 Discrete fracture network (DFN) modeling Through analyzing the FMI data of the three wells and the fault information, the spatial distribution characteristics can be predicted and the DFN model can be built up. The DFN model of the block fractures around well TZ45 is shown in Fig. 6. The fractures with dip direction of NW-SE are well developed, which are consistent with the FMI interpretation results. Based on the DFN model established, the fracture parameters such as fracture azimuth, strike, orientation and Select orientation: N N N rskl rk sl (4 2 2 ) ( rl sk rs kl )(1 2 where, α, β and μ respectively represents the vp, vs and shear modulus of the background medium. δij is the Kronecker symbol. During the calculation process of C1 and C2, fracture parameters such as fracture density derived through DFN will be used. In this way, elastic parameters can be worked out eventually. 4 Seismic modeling of a fractured reservoir in the Tarim Basin 4.1 Calculation of elastic coefficients By using the real geological, well logging and seismic data in the Tarim Basin, the DFN model is built. The fracture parameters are then derived from the DFN and are listed in Table 1. These parameters will be used to calculate the equivalent elastic coefficients. Based on the fracture parameters obtained from the DFN model analysis, equivalent elastic coefficients are calculated according to the Hudson theory. The elastic coefficients are shown in Eq. (5). From the elastic coefficient matrix we can get the following conclusion: the fractures in this study area are complex and is not the standard HTI or TTI (Tilted Transverse Isotropy) model, and here we define it as a quasi TTI model. On the basis of this elastic coefficient matrix, we conduct the wave field forward modeling using the finite difference method. The model which contains two horizontal layers is designed. The first layer is the quasi TTI medium, and the second layer is isotropic, where the P-wave velocity is 3,000.0 m/s, S-wave velocity is 1,500.0 m/s and the density is 2,000.0 kg/m3. 4.2 Acquisition system During the process of wavefield modeling, the acquisition system is shown in Fig. 9. The circle point in the center represents the reflection point, and shots and receivers are located on the intersections between circles and radial lines. Concentric circles with equal distances represent different offsets, while the circle point represents shot and receiver. 4.3 Wavefield characteristics Using the parameters given in 4.1, staggered-grid highorder finite difference forward modeling is conducted. The characteristics of the azimuthal gathers in different offsets are analyzed, the wavefield features are provided in Fig. 10 and Fig. 11. From Fig. 10 and Fig. 11, we can get the following conclusions. The wavefield features are rather complex for this quasi TTI anisotropic medium. For data with the same offset, the traveling time of reflection wave changes versus azimuth. The time difference between the minimum and the maximum traveling time of the P-P wave increases as the offset rises. Shear wave splitting exists when the shear wave propagates through the quasi TTI medium (as shown by the reflected shear wave PS1, PS2 and the direct shear wave S1, S2). From what we have mentioned above, we find that when the data processing is conducted, as the velocity in different azimuths varies, data processing toward different azimuths is necessary. The wavefield obtained through the forward modeling provides the input data for the following AVO inversion method, which acts as an important bridge in the fracture prediction flow shown in Fig. 1. 5 Seismic modeling of a theoretical fracture model In order to further analyze the seismic responses of the fractured reservoir, a theoretical fracture model is designed, which contains two layers. The first layer is an isotropic medium, and the second layer is a monoclinic anisotropic medium. The model parameters and fracture parameters are shown in Table 2 and Table 3. Based on this model, seismic modeling is performed using the staggered-grid high-order finite difference method ( Cerjan et al, 1985 ; Wang et al, 2005 ; t, s t, s 1.0 1.0 0.001 5.1 Wavefield of a monoclinic medium We select the parameters in model 1 (Table 2). Fig. 12 shows the angle gathers with different offsets of the model. From 0º to 180º, the reflection travel-time curves show negative sine characteristics. The longest travel-time of the PP wave appears at about 55º and the shortest appears at about 145º. With the offset increasing, the difference between the shortest and the longest increases. At about 55º, the PS wave travels at the slow S wave velocity and there is only PS2 whose energy is very strong. At about 145º, the PS wave travels at the fast S wave velocity and there is only PS1 whose energy is also very strong. And at other azimuth angles, PS1 and PS2 exist at the same time, but their energies are weak. So converted waves are sensitive to fractures and it is feasible to use the converted waves to detect the fractures. Moreover, the azimuth must be considered during processing the azimuthS1 S2 PS2 PS1 S 1 S 2 P PP P P P angle gathers. 5.2 Influence of fluids We still select the parameters in model 1 (Table 2). For analyzing the fluid influence, we respectively use oil and water to fill the fractures. Fig. 12(b) and Fig. 13 show that: from 0º to 180º, when fractures are filled with oil or water, the PP reflection travel-time curves also have negative sine characteristics. Regardless of whether the fractures are filled with oil or water, the PP, PS1 and PS2 show similar characteristics. When fractures are filled with oil or water, the difference between the longest and shortest travel time is smaller than that when fractures are filled with gas. In summary, when the fractures are filled with gas, the model displays obvious azimuth-anisotropy, while when filled with oil or water, the azimuth-anisotropy is weak. 5.3 Influence of base-rock For studying the base-rock influence, the model is changed from model 1 to model 2 (Table 4). The base-rock of the fractured reservoir is changed from clastic rock to carbonate rock, and the fracture parameters are kept the same (Table 3). Fig. 14 shows the modeling results of model 2 with different fluids in fractures. Comparing with Fig. 12(b), Fig. 13 and Fig. 14, we find that when the fractures are filled with the same fluid, but the base rock is different, the difference between the shortest and the longest reflection travel-time decreases from clastic base-rock to carbonate base-rock. Therefore it is easier to distinguish gas from oil and water in clastic rock than in carbonate rock. 6 The reflection coefficients characteristics of a fractured reservoir Spherical divergence exists when performing seismic modeling, so it is not suitable for amplitude analysis. To do this more easily, the Zoeppritz equation is applied in this paper to analyze the reflection coefficient characteristics of a t, s 0.0 0.5 1.0 1.5 0.0 0.5 1.0 1.5 t, s t, s 0.0 0.5 1.0 1.5 0.0 0.5 1.0 1.5 P PP PS2 P PP PS2 S PS1 S PS1 (a) Angle gathers of x and z components at offset 1200m t, s (b) Azimuth-angle gathers x and z components with fractures filled with water fractured reservoir. Reflection and transmission are produced when a seismic wave arrives at the elastic interface. The seismic wave is considered as a plane wave. According to Huygen’s principle, the incidence, reflection, and transmission waves should satisfy Snell’s Law as well as the displacement continuity and the stress continuity on the boundary (Wu, 2006) . Then, the Zoeppritz equation which describes the reflection coefficient and transmission coefficient under anisotropic condition can be obtained. For example, the Zoeppritz equation of a compression wave under anisotropic conditions is shown as follows: 6.1 Characteristics of reflection coefficients with incident angles We select the model 1 (Table 2) and calculate the reflection coefficients (Fig. 15). From Fig. 15, the following conclusions can be derived. For a certain azimuth, the reflection coefficient changes with the incident angle (denoted by the slowness), and if the azimuth angle changes, the reflection coefficient also changes. The lines on the 30º and 120º are respectively perpendicular and parallel to the bisecting line of the included angle between the two groups of fractures, and the P-wave velocity is smallest on 30º and is biggest on 120º, so the reflection coefficient is different obviously from the slowness of 0.25 s/m to the slowness of 0.35 s/m. The lines on the 90º and the 150º are respectively parallel to the fracture strikes, and the corresponding reflection coefficients are different because the fracture intensities are not the same for fractures in these two directions. Fig. 15(e) displays the differences of the PP reflection coefficients on different azimuth angles and the differences on middle offsets are more (a) Azimuth-angle gathers x and z components with fractures filled with gas 0 40 80 120 160 0 40 80 120 160 t, s t, s t, s Azimuth angle, ° 120 160 0 40 80 0 40 80 (c) Azimuth-angle gathers x and z components with fractures filled with water obvious than those on near offsets. 6.2 Characteristics of reflection coefficients with azimuth angles Fig. 16 shows the reflection coefficients changing with the azimuth angles for the model 1 (Table 2). The reflection coefficient changes with the azimuth angle for a certain fixed incident angle. Slowness of 0.198 s/m, 0.286 s/m, 0.358 s/m and 0.418 s/m are selected as examples, the PP reflection coefficient changes gently at 0.198 s/m, 0.358 s/m and 0.418 s/m, while the PP reflection coefficient changes dramatically at 0.286 s/m. The azimuth AVO characteristics exist on the top interface of the fractured reservoirs, and we might be able to use this characteristic to invert fracture development. PP PSV PSH SVP SVSV SVSH SHP SHSV SHSH 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 Slowness, s/m ( ( 1.0 0.9 0.8 0.7 e0.6 d u it0.5 l p 0.4 m A0.3 0.2 0.1 0.0 For analyzing the influence of fluid properties on reflection coefficients, the fractures of the model 1 (Table 2) are filled respectively with oil and water. At the 30º azimuth, selecting the slowness area from 0.05 s/m to 0.25 s/m, when the fractures are filled with gas (Fig. 15(b)), the PP reflection coefficient decreases apparently almost to 0.06. However, when the fractures are filled with oil or water (Fig. 17(a) and Fig. 17(b)), the PP reflection coefficient decreases apparently almost to 0.09. At the slowness from 0.20 s/m to 0.30 s/m, the PP reflection coefficient when filled with gas is smaller than that when filled with oil or water. The law is the same at the 120º azimuth. For example, the PP reflection coefficient is 0.2 at 0.3 s/m when filled with gas, while the PP reflection coefficient is 0.2 at 0.28 s/m when filled with oil or water. From Fig. 17(a) and Fig. 17(b), we find that when the azimuth is 30º or 120º, the PP reflection coefficient when filled with oil is similar with that when filled with water. Therefore, it is very difficult to distinguish oil from water from the reflection coefficient. 6.4 Influence of base-rocks For analyzing the influence of the base-rocks, we select the model 2 (Table 4) with carbonate rock as base-rock. As shown in Fig. 18, the curves are different when filled with gas compared to filled with oil or water at 30º and 120º. So we can distinguish gas from oil and water, but oil and water are almost indistinguishable from the PP reflection coefficient. 0 40 80 PP PSV PSH SVP SVSV SVSH SHP SHSV SHSH PP PSV PP PSV PSH SVP SVSV SVSH SHP SHSV SHSH PP PSV PSH SVP SVSV SVSH SHP SHSV SHSH PP PSV PP PSV PSH SVP SVSV SVSH SHP SHSV SHSH (a) Reflection coefficients at 30º and 120 º azimuth with fractures filled with oil 1.0 0.9 0.8 0.7 e0.6 d itu0.5 l p0.4 m A0.3 0.2 0.1 0.0 0.00 1.0 0.9 0.8 0.7 e0.6 d itu0.5 l p0.4 m A0.3 0.2 0.1 0.0 1.0 0.9 0.8 0.7 ed0.6 u lit0.5 p m0.4 A 0.3 0.2 0.1 0.00.00 1.0 0.9 0.8 0.7 e0.6 d itu0.5 l p0.4 m A0.3 0.2 0.1 0.0 0.00 7 Conclusions 1) The geological, seismic and well log data respectively describe the fractures at different scales, and an integration of the three can be used to build up the fracture models by the DFN method and the fracture parameters can be calculated. The equivalent medium theory uses the fracture parameters to compute the elastic coefficients of the anisotropic medium, which is an important bridge to connect the microscopic fractures and macroscopic reservoir modeling. 2) Using the real geological, seismic and well longing data, the DFN model is built to calculate the equivalent elastic coefficient. The wavefield forward modeling is conducted using the staggered-grid high-order finite difference method. The modeling result shows that for data with the same offset, the traveling time of the reflection wave changes with the azimuth. Additionally, the time difference between the minimum and the maximum traveling time of the P-P wave increases as the offset rises. Shear wave splitting exists when a shear wave propagates through the quasi TTI medium. The wavefield through the forward modeling not only helps us to understand the seismic response of the fractured reservoirs, but also provides the input data for the study of the fracture inversion method. 3) From an analysis of the monoclinic anisotropy, we learn some characteristics of the reflection coefficients, the slowness and the azimuth angles. The PP reflection coefficients in middle offsets are sensitive to the fractures, so the middle offsets are better than the others when inverting the fractures. The seismic data are more sensitive to gas than oil or water, so gas can be distinguished from oil and water, while oil can not be distinguished from water. When the baserocks are different, it is much easier to distinguish the fluids in the clastic rock than in carbonate rock. Azimuth anisotropy can be used in detecting fractured reservoirs. Theoretically, the acquisition system for anisotropy detection should be fullazimuth and have equal fold. Data processing toward different azimuth is necessary as the velocities in different azimuth are not the same. Acknowledgements The work is co-supported by the National Basic Research Program of China (Grant No.2011CB201103) and the National Science and Technology Major Project (Grant No.2011ZX05004003). The authors would like to thank the Laboratory for Integration of Geology and Geophysics (LIGG) at China University of Petroleum for the permission to publish this work and the Tarim Oilfield Co., PetroChina for their help in providing field data. Also, Shell Co. is thanked for the financial assistant to the first author. Cerjan C , Kosloff D , Kosloff R , et al. A nonreflecting boundary condition for discrete acoustic and elastic wave equations . Geophysics . 1985 . 50 ( 4 ): 705 - 708 Crampin S. Effective anisotropic elastic constants for wave propagation through cracked solids . 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Sam Zandong Sun, Xinyuan Zhou, Haijun Yang, Yueying Wang, Di Wang, Zhishui Liu. Fractured reservoir modeling by discrete fracture network and seismic modeling in the Tarim Basin, China, Petroleum Science, 2011, 433-445, DOI: 10.1007/s12182-011-0161-x