High Resolution Delay Time and Shear Wave Splitting Tomography for Reservoir Monitoring of The " RR " Geothermal Field, West Java, Indonesia Using Micro-earthquake Data
Conference Paper · March 2015
DOI: 10.13140/RG.2.1.1899.4088
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High Resolution Delay Time and Shear Wave Splitting Tomography for Reservoir Monitoring of The “RR” Geothermal Field, West Java, Indonesia Using Micro-earthquake Data
Rexha Verdhora Ry1, Tania Meidiana1, and Andri Dian Nugraha2
1Master Program of Geophysical Engineering, Faculty of Mining and Petroleum Engineering, Institut Teknologi Bandung, Jalan Ganesha No.10, Bandung 40132, Indonesia
Email: [email protected]
2Global Geophysical Group, Faculty of Mining and Petroleum Engineering, Institut Teknologi Bandung, Jalan Ganesha No.10, Bandung 40132, Indonesia
Email: [email protected]
ABSTRACT
Intensive geothermal exploitation at the “RR”
geothermal field in West Java, Indonesia, induces micro-earthquakes which are monitored by a local seismometer network. Using this network, tomographic inversions were conducted for the three-dimensional Vp, Vs, and Vp/Vs structures of the reservoir for January 2007 to December 2009.
We used dense parameterization blocks to image velocity structure of reservoir with high resolution.
We relocated hypocenters location and updated one- dimensional initial velocity models using Velest method. Then, we conducted seismic tomographic inversions using delay time tomography. We also conducted shear wave splitting (SWS) tomography to image subsurface anisotropy distribution. Our tomographic inversion results indicate the presence of low Vp, low Vs, and low Vp/Vs at depths of about 1 – 3 km below MSL. This features were interpreted as steam-saturated rock in the reservoir area of The
“RR” geothermal field. The existences of the reservoir area are supported by the data of well- trajectory. Futhermore, SWS tomography results image high anisotropy located at elevations of about -0.5 to -3 km and can be related to the fractures caused by injection well in the geothermal field. The extensive low Vp/Vs anomaly that occupies the reservoir is attributed to depletion of pore liquid water in the reservoir and replacement with steam.
Continuous monitoring of Vp, Vs, and Vp/Vs is an effective geothermal reservoir characterization and depletion monitoring tool and can potentially provide information in parts of the reservoir which have not been drilled.
Keywords: MEQ, velest, delay time tomography, shear wave splitting, reservoir monitoring.
INTRODUCTION
Micro-earthquakes (MEQ), also known as micro- seismic, may be utilized in the exploration, production, and monitoring phases of the development of a geothermal field. This technique maps active fault failure on shear zones, as well as fluid compression. Intensive geothermal exploitation
at the “RR” geothermal field in West Java, Indonesia, induces micro-earthquakes which are monitored by a local seismometer network. Using this network, tomographic inversions were conducted for the three-dimensional Vp, Vs, and Vp/Vs structure of the reservoir. These velocity structures can be used to analyze the presence of water or stream in reservoir (Ry & Nugraha, 2014).
We used micro-earthquakes data occurred during three years from the period of January 2007 to December 2009. Only data recorded by at least 3 stations were used. The data are composed of 2427 micro-earthquake events which consists of 11318 P wave and S wave phases. The parameterization model blocks used had dimensions of 1 x 1 x 0.25 km3, so it can image velocity structure of reservoir with high resolution.
First, hypocenters and one-dimensional velocity model were updated using Velest method in purpose to estimate best location of sources and best initial velocity models. Using the results, then seismic tomographic inversions were conducted using delay time tomography, based on the differences of observed and calculated travel times. The travel times on the three-dimensional velocity model were calculated using ray tracing pseudo-bending method. Norm and gradient damping were added to constrain blocks without ray and to produce smooth solution. The ray tracing and inversion algorithm used was developed in Matlab environment.
Using velocity structure of S wave phase (Vs), we conducted shear wave splitting tomography to image anisotropy distribution (Meidiana & Nugraha, 2014). We used cross-correlation method to analyze shear wave splitting (SWS) parameter. In conjunction with Vp, Vs, and Vp/Vs, three- dimensional shear wave splitting method is expected to analyze the structure and direction of the fracture.
METHODOLOGY
Updated Hypocenter and 1-D Velocity Model The precise 1-D velocity model is a requirement to determine hypocenter location and as an initial
velocity model for seismic tomography (Kissling et al., 1994). One of the method to determine 1-D velocity model is coupled velocity-hypocenter method using Velest program (Kissling, 1995). This method performs Joint Hypocenter Determination (JHD) to solve the coupled hypocenter-velocity model problem for micro-earthquakes. Using this method, we relocated hypocenters and updated initial one-dimensional velocity model.
Delay Time Tomography
The pseudo-bending method was used in this seismic tomography study as ray tracing method to determine possible ray path in 3-D velocity model and calculate synthetic travel time from hypocenter to receiver. Pseudo-bending (Um & Thurber, 1987) is an approach in minimization of travel time based on Fermat’s Principle by giving small perturbations gradually on ray paths. Ray tracing algorithm was developed by Syahputra et al. (2012).
For resolve tomography inversion, iterative damped least square was implemented to minimize the differences between observed and calculated travel times. We also added norm and gradient damping to constrain blocks without ray and to produce smooth solution model, respectively (Widiyantoro et al., 2000).
Shear Wave Splitting Tomography
The method of shear wave splitting (SWS) can determine the direction of micro-cracks by polarization of S wave (Liu et al., 2005). The wave of S phase propagating perpendicular to the fracture will arrive later while the other which is parallel to the fracture will arrive sooner in the seismograph (Gao & Crampin, 2008). In this study, we used rotation correlation method (Zhang et al., 2007b) to
get the shear wave splitting parameters. There are two parameters of shear wave splitting: delay time of shear wave splitting and polarization direction.
While Cross-Correlation Coefficient (CCC) is maximum, the corresponding value of delay time and azimuth are chosen as the polarization direction and delay time shear wave splitting.
Anisotropy percentage in this study is obtained from tomographic inversion equation completion:
[ ] = [ ][cos ][∆ ] (1)
where dtSWS is delay time of shear wave splitting, α is polarization direction, A is Kernell matrix from ray tracing on Vs, and ΔU is slowness deviation obtained from tomographic inversion. After that, we can get K from equation (Zhang et al., 2007a):
= ∆ = (2)
In the following equation, we used K x 100, known as the anisotropy percentage, to characterize the anisotropy structure (Zhang et al., 2007a).
Therefore, we can directly solve equation using one step iteration and non-negative least square (Kim et al., 2006).
RESULT AND DISCUSSION
The range of hypocenters depth were between elevations of 0.5 to -7 km where the deep micro- earthquakes occurred at depths of 5 ─ 7 km below mean sea level (or elevations -5 to -7 km) and shallow micro-earthquakes occurred at elevations -1 to 0.55 km (shown by Figure 1). The distribution of this micro-earthquake events is well correlated to distribution of injection well and reservoir area.
Figure 1. The map of the distributions of the relocated micro-earthquake events this study (green dots), well (black lines), and monitoring stations (black reverse triangles).
Figure 2 shows updated one-dimensional velocity models. These velocity models of P and S wave will be used in tomographic inversions as an initial model. There is an indication that low velocity zone is located at depths of 2 to 2.5 km below mean sea level. The interpretation will be followed next.
Checkerboard Resolution Test (CRT) is a forward modeling method that aims to test the reliability of the inversion technique used in tomographic inversion and to see the resolution throughout the model space. The resolution test will be performed on the same data configuration of the hypocenter and station. Model anomalies obtained by multiplying the +10% and -10% of the initial velocity model (1-D velocity model). Figure 3 shows the resolution of our tomographic inversion.
Figure 2. 1-D velocity model updated using Velest.
Figure 3. Three-dimensional structure of CRT using iterative damped least square. The
positive anomalies are shown by blue color and negative anomalies are shown by red color.
Figure 4 shows the result of delay time tomographic inversion in vertical cross section. The zones of low Vp/Vs ratio (around 1.6) can be interpreted to be associated with steam-saturated rock (Takei, 2002;
Wang et al., 1990). These zones can be identified as the reservoir of “RR” geothermal field. The reservoir zones are located at 12 - 18 km WE, 8 - 12 km NS, and depth of 1 - 3 km below MSL. The existences of the reservoir area are supported by the data of well-trajectory, where the zones of high Vp/Vs are around the injection wells and the zones of low Vp/Vs are around the production wells.
Our interested area are shown in slice E–E’ (or N-S 10 km) and F–F’ (or N-S 11 km). The micro- earthquake events occurred intensively below injection well. Around this area, tomogram of velocity structure present the zones of low Vp, intermediate Vs, and low Vp/Vs. Then, we used the results of shear wave splitting tomography to observe the fracture in this area. The structure of anisotropic percentage in this interested area are shown in Figure 5.
The anisotropy percentage varies from 0 to 1.8 %.
High anisotropy located at elevations of about -0.5 to -3 km can be related to the fractures caused by injection well in the geothermal field. The activity of injection well induces micro-earthquake and produces new fracture zones. This phenomenon may indicate the depletion of pore fluid and affect the phase system of reservoir.
Previous studies stated that the reservoir in the “RR”
geothermal field is the dominance of the water reservoir (Stimac et al., 2008). It becomes interesting when tomographic imaging identify the zones in reservoir area to be associated with steam- saturated rock. Gunasekara et al. (2003) mention that the exploitation and production of the geothermal field can cause changes in the reservoir phase system. The progressive depletion of pore fluid causes the replacement of pore fluid with vapor. In addition, the pressure drop in the reservoir causes a decrease in the boiling point, resulting in boiling and vapor phase is formed. However, whether changes occur in the phase system of “RR”
geothermal field reservoir, it could not be concluded yet.
2 3 4 5 6 7 8
-8 -7 -6 -5 -4 -3 -2 -1 0 1
Seimic Velocity (km/s)
Depth (km)
Vp (updated) Vs (updated) Vp Vs
5 10 15 20 5
10 -6 15
-5 -4 -3 -2 -1 0 1 2
North - South (km) West - East (km)
Velocity Perturbation
Depth (km)
% V
-15 -10 -5 0 5 10 15
Figure 4. Tomogram of vertical cross-section (North-South) for: (a) Vp, (b) Vs, and (c) Vp/Vs ratio at N-S 9, 10, 11, and 12 km, and well-trajectory (production: black lines, injection: red lines). Only area with good resolution are shown. Positive anomalies are shown by blue color and negative anomalies are shown by red color.
a) b) c)
Figure 5. Tomogram of vertical cross-section (N-S) for anisotropy percentage at N-S 10 and 11 km, and well-trajectory (production:
black lines, injection: yellow lines). Dash black lines show area of good resolution.
CONCLUSION
The inversion results indicate the presence of low Vp/Vs at around 1.6 in the reservoir area at elevations of -1 to -3 km (MSL = 0 km), interpreted as steam-saturated rock in the reservoir area of
“RR” geothermal field. The existences of the reservoir area are supported by the data of well- trajectory, where the zones of high Vp/Vs are around the injection wells and the zones of low Vp/Vs are around the production wells.
High anisotropy located at the interested area may indicate new fracture zone and the depletion of pore fluid. When compared with other studies in this field, it is possible that the reservoir’s phase system has changed from water-saturated to steam- saturated. However, this interpretation still needs to be compared with other geophysical and geological studies in the study area. We hope that continuous monitoring of Vp, Vs, and Vp/Vs is an effective geothermal reservoir characterization and depletion monitoring tool and can potentially provide information in parts of the reservoir which have not been drilled.
REFERENCES
Gao, Y. and Crampin S. (2008), “Shear Wave Splitting and Earthquake Forecasting”, Terra Nova, 440-448.
Gunasekara, R. C., Foulger, G. R., and Julian, B. R.
(2003), “Reservoir depletion at The Geysers geothermal area, California, shown by four- dimensional seismic tomography”, J. Geophys.
Res., Vol. 109, No. B3, 2134.
Kim, D., Sra, S., and Dhillon, I. S. (2006), “A New Project Quasi-Newton Approach for the Nonnegative Least Square Problem”, Technical Report, The University of Texas.
Kissling, E., W.L. Ellsworth, D. Eberhart-Phillips, and U. Kradolfer, 1994, J. Geophys. Res. 99, 19635-19646.
Kissling, E., 1995, Program VELEST USER’S GUIDE – Short Introduction, Zurich: ETH Zuerich, pp. 1-31.
Lou, M., and Rial, J. A. (1997), “Characterization of Geothermal Reservoir Crack patterns Using Shear Wave Splitting”, Geophys., 62.
Meidiana, T. and Nugraha, A. D. (2014), “Cross- Correlation Method in the Shear Wave Splitting Tomography using Micro-earthquake Data”, PIT HAGI 39.
Ry, R. V. and Nugraha, A. D. (2014), “Improve Micro-Earthquake Hypocenter using Simulated Annealing and Travel Time Tomography Inversion in Geothermal Exploration”, 3rd ITB Geothermal Workshop 2014.
Stimac, J., Nordquist, G., Suminar, A., and Azwar, L. S. (2008), “An Overview of The Awibengkok Geothermal System, Indonesia”, Geothermics 37, Elsevier.
Syahputra, A., Nugraha, A.D., and Fathkhan (2012),
“Development of 3D Seismic Tomography Software: Application to Synthetic Data for Geothermal Exploration”, 1st ITB Geothermal Workshop 2012.
Takei, Y. (2002), “Effect of pore geometry on VP/VS: From equilibrium geometry to crack”, J.
Geophys. Res., Vol. 107, No. B2, 2043.
Um, J. and Thurber, C. (1987), “A fast algorithm for two-point seismic ray tracing”, Bull. seism. Soc.
Am., 77, pp. 972-986.
Wang, Z., Batze, M. L., and Nur, A. M. (1990),
“Effect of different pore fluids on seismic velocities in rock”, Can. J. Explor. Geophys., Vol. 26 NOS. 1 & 2, pp. 104-112.
Widiyantoro, S., Gorbatoc, A., Kernnett, B. L. N., and Fukao, Y. (2000), “Improving Global Shear Wave Traveltime Tomography Using Three Dimensional Ray Tracing and Iterative Inversion”, Geophys.J.Int., 141, pp 747-758.
Zhang, H., Liu, Y., Thurber, C., and Roecker, S.
(2007a), “Three-dimensional shear wave spliiting tomography in the Parkfield, California, Region”, Geophysical Research Letters, 34.
Zhang, H., Liu, Y., and Thurber, C. (2007b),
“Systematic Temporal and Spatial Seismic Anisotropy Study around Parkfield, California”, University of Wisconsin-Madison.
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