The inluence o hydraulic racturing on microseismic propagation Xiaolin Zhang 1, Feng Zhang 1, Xiang-Yang Li 1, 1 CPC Geophysical Key Lab, China University o Petroleum, Changping, Beijing, China, 1049 Email: petrozxl@16.com British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK Summary The microseismic method is a crucial technology or locating the racture location in the hydraulic racturing process. Conventionally, the velocity model is usually constructed by well logs, seismic data or calibration shots which ignore the inluence o racturing domain. In this paper, we study the real-time inluence o hydraulic racturing on microseismic propagation. Based on the luid seepage equation, racture mechanics and the critical pressure criterion, we simulate the 3D hydraulic racturing process and obtain the microseismic events and pore pressure distribution. Then the velocity model is constructed with the Coates-Schoenberg method and racture compliances. The 3D ray tracing method is applied to model the microseismic travel time and direction. Simulation results show that, the deviation caused by the racturing domain varies considerably rom dierent receiver locations, the overall deviation increases with racturing progress, and the deviation distribution o travel time and direction are quite similar. Keywords: hydraulic racturing, microseismic events, velocity model, ray tracing 75 th EAGE Conerence & Exhibition incorporating SPE EUROPEC 013 London, UK, 10-13 June 013
Introduction The velocity model o traditional microseismic inversion methods is usually constructed by well logs, seismic data or calibration shots. This static model does not consider the variation o racture and pore pressure in the hydraulic racturing process. In act, the racturing process creates new ractures and alters pore pressure. These change the rock elastic parameter and velocity (Coates et al., 1995; Vlastos et al., 006), and thus change the microseismic travel time and direction. In this paper, we irstly simulate the 3D hydraulic racturing process and obtain the real-time pore pressure and microseismic events distribution, then the Coates-Schoenberg method and racture compliances are applied to calculate the equivalent velocity o racturing domain; inally, 3D ray tracing method is applied to model the microseismic travel time and direction at dierent racturing stages. Besides, we also simulate the microseismic propagation in the pre-racturing velocity model or comparison. This method combines hydraulic racturing simulation and microseismic modelling, considers the inluence o pore pressure and racture, and provides a new way to analyze the inluence o racturing process. Method and Theory In the hydraulic racturing process, some luid lows in the main racture and some leaks to the matrix, the injected luid equals the mass in the racture plus the mass leaks to the matrix (Wangen, 011): d d uur v ρ. V dv + ρ A vd nda = M in dt dt (1) where ρ is the luid density, V is the racture volume, v uur D is the luid low rate, A is the racture area, n v is the low direction, M in is the injection rate. Equation (1) and the luid seepage equation are applied to simulate the pore pressure diusion in the hydraulic racturing process. Shapiro et al. (009) analyzed the microseismic signals o two types: 1. Pore pressure diusion controlled type in which the microseismic distribution has a good agreement with pore pressure.. Hydraulic racturing controlled type in which the microseismic signals correspond to the ast racture extension. In the hydraulic racturing process, the luid in the main racture enhances pore pressure and make the racture extend; the racture extension will produce microseismicity which corresponds to the hydraulic racturing controlled type. The luid which leaks to the matrix will break the stress balance and create cracks or cause cracks to slip; the microseismicity o this type corresponds to pore pressure diusion controlled type. Thereore we divide the microseismic signals into two types in the racturing process: 1. Type F, the microseismicity o this type corresponds to the extension o the main racture.. Type P, the microseismicity o this type corresponds to the events near the main racture which is controlled by pore pressure diusion by luid leakage. And we take racture mechanics and critical pressure criterion as microseismic judgements o type F and type P (Liu et al., 003; Rothert et al., 003); the mathematical expressions are listed as equation () and equation (3). K( δ, δ, Lp, ) = K. () I 1 3 where KI is the stress intensity actor which correlates to the maximum in situ stress δ 1, the minimum in situ stress δ 3, the racture length L and the pore pressure p, KIC is the critical stress intensity actor. pxt (, ) > C( x). (3) where p is the pore pressure, C is the critical pressure. We assume that there are no ractures in the initial model, and ractures appear ater the microseismicity. According to the Coates-Schoenberg method (Coates et al., 1995), the equivalent elastic matrix o ractured rock can be expressed as ollows: IC 75 th EAGE Conerence & Exhibition incorporating SPE EUROPEC 013 London, UK, 10-13 June 013
( λ + µ )(1 r δ) λ(1 rδ) λ(1 δ ) 0 0 0 λ(1 rδ) ( λ + µ )(1 r δ) λ(1 δ) 0 0 0 λ(1 δ) λ(1 δ) ( λ + µ )(1 δ) 0 0 0 C =, 0 0 0 µ (1 δt ) 0 0 0 0 0 0 µ (1 δt ) 0 0 0 0 0 0 µ where λ, µ are Lame parameters, the equations o δ T, δ, r are: λ r = λ + µ ZT µ / L δt = 1 + Z µ / L T Z ( λ + µ )/ L δt =, 1 + Z ( λ + µ )/ L where Z T are the tangential compliance and normal compliance o racture,1/ L= a/ V, a is the area o racture, V is the grid volume. When pore pressure increases, the eective stress will decrease. The relation between racture compliances and the eective stress can be expressed with exponential decay unction (Vlastos et al., 006): σe / τt ZT = ZT + ( ZT0 ZT ) e (6) σe / τ Z = Z + ( Z Z ) e. 0 where Z T are the compliances or very high eective stress T 0 0 are the compliances or very low eective stress, σ e is the eective stress, τ T and τ are decay constants. (5) (4) In this paper, we irstly simulate the luid seepage and pore pressure distribution with equation (1) and the luid seepage equation, then we simulate microseismic events with equations () and (3). Ater these two steps, equations (4), (5) and (6) are applied to model the velocity o racturing domain and the 3D shooting ray tracing method is used to simulate the microseismic travel time and direction. umerical simulation The racturing model size is 00m 50m 50m, the grid size is 1m 1m 1m, the injection location is (100m, 5m, 5m), and the simulation time interval is 10ms. The maximum in situ stress is along the x direction o value 5Mpa; the minimum in situ stress is along the y direction o value 0Mpa. The 3/ critical stress intensity actor KIC = M m, the initial pore pressure is 19Mpa and the critical pressure C ranges rom 19.05Mpa to Mpa with random distribution (Rothert et al., 003). The 9 1 9 1 matrix Lame parameter λ = 9.31 10 kg m s, µ = 6.144 10 kg m s, the matrix density 3 ρ =.4 g / cm. The receiver locations are R1(10m, 10m, 8m), R(10m, 10m, 16m), R3(10m, 10m, 4m), R4(10m,10m,3m), R5(1m, 0m, 3m), R6(14m, 30m, 3m), R7(16m, 40m, 3m), R8(18m, 50m, 3m). The x and y locations o R1 to R4 are the same, which approximates the vertical well monitor system, the z locations o R5 to R8 are the same which approximates the horizontal well monitor system. In this paper, we analyze the inluence o racturing process to microseismic propagation at times o 60s, 10s, 180s, 40s, 300s, 360s, 40s and 480s. At each time, we choose ive events rom the microseismic events o that time randomly and calculate the travel time and direction at the receiver locations. Besides, we will also calculate microseismic propagation in the pre-racturing velocity model at the same time or comparison. 75 th EAGE Conerence & Exhibition incorporating SPE EUROPEC 013 London, UK, 10-13 June 013
Figure 1, let, shows the pore pressure distribution at 60s. Figure 1, right, shows the microseismicity at 60s in which the red points represent the main racture (type F), and the black, green and blue points represent the microseismic events o type P at dierent stages. Figure, let, shows the 3D P- wave velocity model or 60s, Figure, right, shows the x-y velocity proile at the middle depth o model (z=5m). Figure 1: The pore pressure and microseismic distribution at 60s. Let) 3D pore pressure distribution. Right) 3D microseismic distribution. Figure : The P-wave velocity model or 60s. Let) 3D P-wave velocity model. Right) X-y velocity proile (z=5m). Figure 3: The travel time and direction deviation between racturing velocity model and preracturing velocity model. Let) Travel time deviation. Right) Travel direction deviation. Here, we only study the inluence on microseismic P-wave propagation; the inluence on S-wave propagation can be calculated in the same way. Figure 3, let, shows the travel time deviation between racturing velocity model and pre-racturing velocity model. Figure 3, right, shows the travel direction deviation. The deviation is the average o ive dierent microseismic events which have the same 75 th EAGE Conerence & Exhibition incorporating SPE EUROPEC 013 London, UK, 10-13 June 013
receiver location and occurrence time. As shown in Figure 3, the deviation o dierent receivers at the same time is quite dierent, the overall deviation increases with hydraulic racturing progress. Except or the lower right corner in Figure 3, the deviation in travel time and direction are very similar. The deviation in Figure 3 is the average value, the maximum time deviation is 0.65ms (approximately m in length), and the maximum direction deviation is 5 degrees. As to the speciic microseismic event, the maximum time deviation is 1.03ms (approximately 3.1m in length) and the maximum direction deviation is 10.64 degrees. ote that, the receivers in the model are only about 90m rom the injection point and we only consider one racture extension in the intact model. In act, the real hydraulic racturing process will create several ractures at the same time, and the receivers are very ar rom the injection point. Moreover, the microseismic propagation will also be inluenced by natural ractures. Considering these actors, the deviation in the real racturing process may be much higher compared to the simulation result. Conclusions In this paper, we simulate the 3D hydraulic racturing process, calculate the real-time velocity model o racturing domain and analyze the inluence o racturing process on microseismic propagation. Through the above theory and simulation, we can draw the ollowing conclusions: 1. In the hydraulic racturing process, the main racture controls the pore pressure and microseismic distribution.. In the simulation, the deviation o dierent receivers caused by the racturing domain is quite dierent, the overall deviation increases with racturing progress and the deviation distribution o travel time and direction are similar. In the real racturing process, we can calculate the inluence on dierent receiver locations with this method. The simulation result can be used to design the acquisition geometry and estimate the deviation o received data. Acknowledgements This research is supported by CPC Geophysical Key Lab, China University o Petroleum, Beijing, and by ational atural Science Fund Projects (o.u1680). We thank Dr. Shuangquan Chen or his instructions and suggestions. Reerences Coates R T and Schoenberg M, 1995. Finite-dierence modeling o aults and ractures, Geophysics, 60(5), 1514-156. Elma Rothert and Serge A. Shapiro, 003. Microseismic monitoring o borehole luid injections: Data modeling and inversion or hydraulic properties o rocks, Geophysics, 68(), 685 689. Liu J.J., Feng X.T. and Fei G.H., 003. Study on Mathematical Model o Three Dimensional Hydraulic racturing, Chinese Journal o Rock Mechanics and Engineering, (1), 04-046. Magnus Wangen, 011. Finite element modeling o hydraulic racturing on a reservoir scale in D, Journal o Petroleum Science and Engineering, 77, 74-85. S. A. Shapiro and C. Dinske, 009. Fluid-induced seismicity: Pressure diusion and hydraulic racturing, Geophysical prospecting, 57, 301-310. S. Vlastos, E. Liu, I.G.Main, M. Schoenberg, C.arteau, X.Y. Li and B. Maillot, 006. Dual simulations o luid low and seismic wave propagation in a ractured network: eects o pore pressure on seismic signature, Geophys.J.Int, 166, 85-838. 75 th EAGE Conerence & Exhibition incorporating SPE EUROPEC 013 London, UK, 10-13 June 013