Analyses of the Hijiori Long Term Circulation Test

Analyses have been completed to evaluate different operating strategies for the Hijiori Long Term Circulation Test (LTCT). To ensure that the Geocrack2D model used to represent the LTCT was valid, 1991-1996 testing at Hijiori was first simulated. After reasonable matches were obtained with the existing data, the model was used to predict behavior during the two year long LTCT. Five cases were analyzed: (1) a nominal case with injection flow of 16 kg/s, (2) a case with half the nominal flow, (3) a case with double the nominal flow, (4) a case where HDR-2a was blocked at the lower fracture (selected to block the observed cold flow at the lower fracture in HDR-2a), and (5) a case with enhanced connectivity between the lower and upper fracture. In all cases, injection was in HDR-1, with production from HDR-2a and HDR-3. For the nominal case, the HDR-2a mixed production temperature is predicted to reduce from 240 C to 180 C while the HDR-3 temperature remains approximately constant at 240 C. The highest production temperatures occur for Case 4, when the lower fracture is blocked in HDR-2a, with the final production temperature after two years being 214 C. BACKGROUND A Long Term Circulation Test is planned for the Hijiori Geothermal Reservoir. The test is expected to run for two years. To prepare for the test, analyses have been performed to evaluate different operating strategies for the test. Since short term tests were performed in 1991, 1995, and 1996, a large amount of data has been gathered from the reservoir. This data and the configuration of the reservoir (well locations and depth) constrain the possible operating regimes. The data also provide benchmarks with which to validate models of the reservoir. Reservoir Geometry The Hijiori reservoir is located on the southern boundary of the Hijiori caldera. The reservoir wells intersect two major fractures at depths of about 1800 and 2200 m. The fractures are part of the ring structure around the caldera and strike east-west and dip steeply to the north, at an angle of about 70 degrees from the vertical. A schematic of these fractures is shown in Figure 1. Figure 1: Schematic of the Hijiori reservoir Three wells intersect the fractures, as shown in Figure 2. HDR-2a and HDR-3 are production wells that are open (not cased) below about 1500 m. HDR1 is the injection well and is presently cased to a depth of about 2151 m. At the lower fracture, the separation distance is about 80 m between HDR-1 and HDR-2a and about 130 m between HDR-1 and HDR-3. As evidenced by test data, the upper and lower fractures are the dominant flow paths in the reservoir. Both HDR-2a and HDR-3 connect to the upper fracture, while at the lower fracture HDR-1 and HDR-2a have strong connections (the HDR-3 lower fracture connection is not as direct). -2400 -2300 -2200 -2100 -2000 -1900 -1800 -1700 -1600 -1500 -400 -300 -200 -100 0 100 200 300 400 HDR-2a HDR-1