Variation of the Characteristics of the Shallow Reservoir at the Hijiori Test Site between 90-days Circulation Test and Long-Term Circulation Test Using FEHM Code

Abstract

At the Hijiori Hot Dry Rock (HDR) field in Yamagata prefecture, a deep reservoir and a shallow reservoir have been created by hydraulic stimulation. A Long-Term Circulation Test (LTCT, Term 2 and Term 3) of the Hijiori system was conducted from December 23, 2001 to August 31, 2002. The test used two injection wells SKG-2 and HDR-1, and two production wells HDR-2a and HDR-3. At the beginning of this test, water was injected at a constant flow rate of 8.35 kg/s into both injection wells. After 15 weeks of flow, the injection rate of HDR-1 (completed only in the deep reservoir) was increased to 12.53 kg/s and the injection rate of SKG-2 (completed only in the shallow reservoir) was decreased to 4.17 kg/s, yielding an injection ratio of 3:1. In response to this change in injection distribution, the production flow rate of HDR-2a decreased from 8 to 6kg/s, while the production rate in HDR-3 remained almost constant during the test. Both production wells are open to both reservoirs.
Also, 90 days circulation test (Exp.9102) was carried out in1991. This test was used with the shallow reservoir. Aiming to grasp the variation of the characteristic of the shallow reservoir between Exp.9102 and LTCT, we developed a numerical model of Hijiori reservoir and simulated the test data using the FEHM (Finite Element Heat and Mass transfer) code developed at Los Alamos National Laboratory. The model provided a reasonable match to the observed pressure, temperature and flow data collected during the LTCT and Exp.9102. We compare with the model parameters of Exp.9102 and LTCT. As the results, permeability of the shallow reservoir of LTCT is higher than that of Exp.9102.