GRC Transactions, Vol. 38, 2014 Hijiori HDR Reservoir Evaluation by Micro-Earthquake Observation Hideshi Kaieda Central Research Institute of Electric Power Industry, Abiko, Chiba, Japan Keywords HDR, Hijiori, hydraulic stimulation, micro-earthquake, reservoir evaluation ABSTRACT Hot Dry Rock geothermal energy development experiments were conducted from 1986 to 2002 at Hijiori in Yamagata prefecture, Japan by the New Energy and Industrial Technology Development Organization (NEDO). In this program two injection wells and two production wells were drilled into granodiorite to extract heat from two artificially created reservoirs at depths of around 1,800 m and 2,200 m where temperatures were measured at 230 degree C and 270 degree C, respectively. The upper reservoir was created from 1,788 m to 1,802 m in the injection well, SKG-2, by injecting a total of 1,080 tons of water. The lower reservoir was produced from 2,151 m to 2,205 m in another injection well, HDR-1, by injecting a total of 2,115 tons of water. These reservoirs were created by injecting water at a high flow rate for a short period of 11 and 12 hours for improving permeability of natural fractures of the rock immediately the injection wells. It was not intended to extend fractures long distance. If the created fractures extend long beyond the injection and production wells area, water recovery during water circulation will be small. The location and size of the two reservoirs were evaluated from micro-earthquake hypocenter distributions. The hypocenters were distributed within a few hundred meters of the injection wells during reservoir creation. This result means that the hydraulic fracturing improved permeability of the rock immediately around the injection wells. However the hypocenters were distributed in a wide area during the water circulation tests. The recovery ratio of produced steam and hot water amount to injected water amount was small of 34.6 % to 54%. This result means that the fractures extended longer and some of the injected water leaked away from the injection and production wells through the fractures. It is needed to plug some fractures to prevent water leaking during water circulation tests. Introduction There are many promising areas for geothermal energy development in Japan. These areas are located around volcanoes where magma chambers exist at shallow depths. If permeability and temperature are high enough, the conventional geothermal development can occur. However, in some areas natural permeabilities of the hot basement rock are too low for conventional geothermal development. In these areas Hot Dry Rock (HDR) or Enhanced Geothermal Systems (EGS) geothermal energy development technologies can be applied. In some cases hot rocks has low permeability but contains many fractures, because fractures of the rocks were partially sealed by carbonate and/or silicate minerals. If hot rocks exist at shallow depth and contain many natural fractures, many flow paths will be created during hydraulic fracturing. It may be effective for extracting heat energy from these rocks to create multiple reservoirs and multiple wells systems. HDR geothermal energy development program was conducted from 1986 to 2002 at Hijiori in Yamagata prefecture, Japan by the New Energy and Industrial Technology Development Organization (NEDO). In this program two injection wells, SKG-2 and HDR-1, were drilled to create reservoirs at different depths of around 1,800 m and 2,200 m where temperatures of 230 degree C and 270 degree C were measured, respectively. The location and size of these two reservoirs were evaluated from micro-earthquake hypocenter distributions. Two production wells, HDR-2 and HDR-3 were drilled based on the hypocenter distributions, and succeeded in penetrating both reservoirs. A long term circulation test (LTCT) was conducted from 2000 to 2002. Water from a river was injected into SKG-2 and HDR-1 and hot water and steam were produced from HDR-2 and HDR-3. A small binary power plant produced electricity of about 50 kw using the steam and hot water produced from HDR-2 and HDR-3 (Tenma et al, 2004). Some of the results of the micro-earthquake analysis and reservoir evaluation obtained through 1995 were described in Sasaki and Kaieda (2000). In this paper the micro-earthquake observations and reservoir evaluation results from the Hijiori HDR experiments, including the LTCT results from 2000 to 2002, are summarized. 295
Hydraulic Operations in the Hijiori HDR Program Artificial Reservoir Creation at Different Depths and Water Circulation The Hijiori HDR site is located at the southern edge of the Hijiori caldera. The logging results from the pre-existing geothermal survey well, SKG-2, indicated the presence of large open fractures in the basement granodiorite dipping steeply to the north and striking east-west. In 1986 an upper reservoir was created in the open-hole section from 1,788 m to 1,802 m of SKG-2 by injecting a total of 1,080 tons of water for 11 hours with a maximum flow rate of 6.2 ton/min and a maximum well-head pressure of 16 MPa. The SKG-2 well-head valve was opened just after injection stopped and hot water flowed back from the fracture to the ground surface. The amount of the discharged water was 380 tons. This test was intended to improve permeability near SKG-2 and not to extend the fractures outward. If the created fractures progressed too much, water loss could occur during the later water circulation operations. Based on the location and size of the reservoir estimated from micro-earthquake hypocenter distributions, production well, HDR-1, was drilled to a depth of 1,805 m to penetrate the reservoir. In 1988 the reservoir was stimulated to improve the hydraulic connection between SKG-2 and HDR-1 by injecting a total of 1,961.1 tons of water into SKG-2 with a maximum flow rate of 6 tons/min and a maximum well-head pressure of 15.5 MPa. In 1989, a one-month circulation test was conducted by injecting a total 45,921 tons of water into SKG-2 and producing steam and hot water from HDR-1 and HDR-2. 34.6 % of the water was recovered (NEDO, 1990). After HDR-1 was re-drilled to a depth of 2,205 m and was cased to a depth of 2,151 m in 1992, a second (lower) reservoir was created from 2,151 m to 2,205 m in HDR-1 by injecting a total of 2,115 tons of water for 12 hours with a maximum flow rate of 4.3 tons/min and a maximum well-head pressure of 25.5 MPa. The HDR-1 well-head valve was opened just after injection stopped. The amount of the discharged water was 324 tons (NEDO, 1993). Based on the micro-earthquake hypocenter locations observed during hydraulic fracturing, two production wells, HDR-2 and HDR-3 were drilled and succeeded in penetrating both the upper and lower reservoirs. In the 1995 circulation test water was injected into HDR-1 at a flow rate of 60 to 120 tons/hour and steam and hot water were produced from HDR-2 and HDR-3 at a total flow rate of 14 to 15 tons/hour. The total water recover was about 40 % in this circulation test (NEDO, 1996). LTCT was conducted from November 27, 2000 to August 31, 2002 was conducted in three phases. In the first phase from November 27, 2000 to November 15, 2001 water was injected into HDR-1 and steam and hot water were produced from HRD-2 and HDR-3 from the lower reservoir. In the second phase from December 23, 2001 to April 28, 2002 water was injected into SKG-2 and HDR-1, and steam and hot water were produced from HRD-2 and HDR-3 from the lower and upper reservoirs. In the third phase, from June 1, 2002 to August 31, 2002, water was injected and steam and hot water were produced by using the same well configuration system as in the second phase. A small binary power plant was operated to produce electricity of 50 kw using hot water from HRD-2 and HDR-3. A total of 191,409 tons of water was injected into HDR-1 and SKG-2 during the LTCT. Figure 1. Concept of the Hijiori HDR experiments. Table 1. Main hydraulic operations at the Hijiori HDR site. Figure 2. Micro-earthquake observation stations around the Hijiori HDR site. 296
Table 2. List of magnitudes and b-values of the micro-earthquake events observed during the main six hydraulic operations. The total amount of steam and hot water produced from HDR-2 was about 124,765 tons, and from HDR-3 was 43,792 tons, yielding an overall fluid recovery of about 54 % (Tenma et al., 2004). Micro-Earthquake Observation System A shallow borehole seismic network was constructed consisting of ten stations (ST-1 to ST-10) deployed in a circle at a radius of 1.5 to 2 km around the Hijiori HDR site shown in Figure 2. The seismometers were cemented in place at the bottom of 50 to 150 m deep wells to reduce noise levels. Three-component velocity seismometers were used. Signals observed by these seismometers were amplified 100 times at the ground surface of the station and transmitted by wire to a data acquisition room located in the HDR site. The signals were amplified 100 times and recorded in both digital and analog form. The digital recording system used a personal computer and a 16-channel AD converter with a 12-bit resolution. The sampling frequency was 1 khz (Sasaki, 1998). Micro-Earthquake Observation Results (1) Micro-Earthquake Activity Magnitude and b-values of the micro-earthquake events occurring during the main hydraulic operations are summarized in Table 2. From this table, we can see that relatively larger magnitudes and b-values were related to the water circulation tests of 1989, 1995 and 2000-2002, and not during reservoir creation in 1986, 1988 and 1992. This reason is considered that lower pore pressure in fractures resulted in high b-values (Scholz, 1968) because water injection pressure during water circulation was smaller than during reservoir creation, and smaller events occurred during the reservoir creation because smaller area was stimulated during the reservoir creation than during the water circulation. During the LTCT from 2000 to 2002, the flow rate and the well-head pressure of HDR-1, together with the number of microearthquake events observed per day are shown in Figure 3. At the beginning of the LTCT no micro-earthquake events were observed for one and a half months. Micro-earthquake activity then increased suddenly. 62 events were observed in one day (January 11, 2001). A large event with a magnitude of 2.4 was observed within 3 km of HDR-1 on March 29, 2001. After the middle of April, 2001, the number of events decreased to a few events per day as the well-head pressure decreased. Three months after the LTCT terminated, seismic activity increased again and an event with a large magnitude of 2.4 occurred about 2 km west of HDR-1. After this high seismic activity, micro-earthquake activity decreased to background levels. Some people living near the Hijiori site felt ground motion of the large events, but no complain was made. (2) Micro-Earthquake Hypocenter Distributions Arrival times of P-waves were used in a least-squares method to determine the locations of the micro-earthquakes. The initial hypocenter was assumed to be at the bottom of SKG-2 and the initial origin time was determined from the first P arrival time and S - P time at ST-8 by assuming a Poisson s ratio of 0.25. A one-dimensional velocity model was developed using data from well logs from SKG-2. The velocity model used for locating event hypocenters is shown in Table 6. The seismic stations are located at the edge and outside the caldera. The underground structure was considered to be heterogeneous. In order to determine hypocenter Figure 3. Water injection flow rate and wellhead pressure of HDR-1 and number of micro-earthquake events per day during the LTCT. 297
Table 3. Velocity model of the Hijiori site (Sasaki, 1998). Depth (km) Vp (km/s) Vs (Km/s) 0-0.11 2.1 1.24-0.25 2.5 1.48-0.45.1 1.83-0.65 3.6 2.13-0.85 4.0 2.36-1.46 4.7 2.78-2.00 5.2 3.07-8 5.4 3.19 locations with high accuracy, calibration tests with dynamite blasts were conducted. Three blasts with 1 kg of explosives were conducted at a depth of 1795 m in SKG-2. The average of the P-wave travel time residual measured from the blasts was obtained for each seismic station and was used for a P-wave station correction. Using these correction values, the velocity model and P-wave arrival times at each station during the calibration tests, the blasting position was determined within 10 m of the measured location by well logging (Sasaki, 1998). The determined micro-earthquake hypocenter locations of the five main hydraulic operations are shown in Figure 4. The number of located micro-earthquake events was 29, 63, 270, 127, 337 and 307 during the 1986, 1988, 1989, 1992, 1995 and 2000-2002 hydraulic operations, respectively. Micro-earthquake hypocenters observed during the 1986 creation of the upper reservoir are distributed in a small area within a few hundred meters around the injection wells. This distribution means that only a small area near SKG-2 was stimulated. However during the water circulation test in 1989, water recovery was small (34.6 %) and the microearthquake hypocenters were distributed more than 1 km east of SKG-2. During the 1992 creation of the lower reservoir microearthquake hypocenters were distributed within 500 m of HDR-1. The hypocenters are distributed over a wide area during the 1995 circulation test. During the LTCT micro-earthquake hypocenters are distributed over wider than before. There is seismically quiet area around HDR-1 in the LTCT hypocenter distributions shown Figure 4. Observed AE hypocenter distribution of the main six hydraulic operations. 298 2001-2002 in Figure 4. This area is seismically active during the lower reservoir creation and short-term circulation in 1992 and 1995. This result means that during the reservoir creation and short-term circulation operations fractures were stimulated near the injection wells but during the long-term circulation tests (LTCT) fractures extended away and the injected water leaked from the reservoirs. Therefore the water recovery was small during the long-term circulation tests. In order to improve water recovery, some fractures extended longer must be plugged.
Conclusions In Japan, there are many promising areas for geothermal energy development where we can access hot, low permeability rocks at shallow depths. These rocks contain many poorly connected natural fractures. These fractures are partially sealed by carbonate minerals. Multiple reservoirs and multiple wells systems are thought to be necessary to obtain large water recovery during water circulation operations between injection and production wells through the artificially created reservoirs. Hot Dry Rock geothermal energy development experiments were conducted at Hijiori in Yamagata prefecture, Japan by NEDO. In this program two injection wells and two production wells were drilled to extract heat from two artificially created reservoirs at depths of around 1,800 m and 2,200 m. The purpose of these reservoir creation operations was to improve permeability around the injection and production wells, not extend fractures outward. If created fractures extend long beyond the injection and production wells area, some of water will flow away from the injection and production wells area and water recovery during water circulation will be small. The location and size of the two reservoirs created during the experiments was evaluated from micro-earthquake hypocenter distributions. The event hypocenters were distributed in a small area around the injection wells during reservoir creation. This result indicates that the hydraulic fracturing improved permeabilities around the injection wells. However the hypocenters were distributed wide in area during the water circulation tests in which the water recovery of amount of produced steam and hot water to injected water was small of 34 % to 54%. This result means that the fractures extended outward and some of the injected water leaked away from the injection and production wells. These fractures must be plugged to inhibit water loss. References NEDO, 1990, Summary of HDR Technology Development Program in FY 1989, New Energy and Industrial Technology Development Organization (NEDO) NEDO, 1993, Summary of HDR Technology Development Program in FY 1992, New Energy and Industrial Technology Development Organization (NEDO) NEDO, 1996, Summary of HDR Technology Development Program in FY 1995, New Energy and Industrial Technology Development Organization (NEDO) Sasaki, S., 1998, Characteristics of microseismic events induced during hydraulic fracturing experiments at the Hijiori hot dry rock geothermal energy site, Yamagata, Japan, Tectonophysics, 289, 127-139. Sasaki, S. and Kaieda, H., 2000, Determination of stress state at the Hijiori HDR site from focal mechanisms, Proceedings of World Geothermal Congress 2000, 3859-3864. Scholz, C. H., 1968, Microfracturing and the inelastic deformation of rock in compression, J. Geophys. Res., Vol. 73, 1417-1432. Tenma, N., Yamaguchi, T., Okabe, T., and Zyvoloski, G., 2004, Estimation of the characteristics of the Hijiori reservoir at the HDR test site during a long-term circulation test, term 2 and term 3, Geothermal Resources Council Transactions, Vol. 28, 245-249. 299
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