Reservoir Evaluation by Tracer Tests during a Long Term Circulation Test at the Hijiori HDR Test Field, Yamagata, Japan

Abstract

A long-term circulation test (LTCT) was conducted between 27 November 2000 and 31 August 2002 at a Hot Dry Rock (HDR) test site in Hijiori, Japan. The LTCT wasseparated into two stages. In the first stage, fluid was injected into only a lower reservoir. In the second stage, a dual circulation test was carried out during which fluid was injected into both an upper and a lower reservoir. During the LTCT, we carried out tracer tests every one or two months for monitoring flow in the reservoir. In these tests, tracer reagents including sodium fluorescein were pumped into the lower and upper reservoirs. Fluids from two production wells were sampled and analyzed to obtain tracer response curves. A fiber-optic fluorimeter was also used to obtain real time fluorescein concentration. After fluid analysis, we obtained the tracer response curves and corrected the curves to analyze the change of real reservoir volume with circulation progress. In the first stage of the LTCT, the breakthrough and mode volume of HDR-3 tended to increase. But the volume of HDR-2a tended to decrease and the volume rapidly changed between the 2nd and 3rd tracer tests. And fluid geochemistry and well temperature of HDR-2a rapidly changed between the 3rd and 4th tracer tests. These changes were influenced by an anhydrite scaling process. Firstly, the anhydrite around the injection well dissolved and then precipitated mainly in the lower reservoir and the flow path volume became larger. In the case of HDR-3, this process continued during the circulation. But in the case of HDR-2a, thermal breakthrough occurred in the flow path due to cool injection water. After that, the dissolved anhydrite precipitated in the production well and the flow path volume became smaller. As the fluid pressure from the lower reservoir became higher, the fluid flow from the upper reservoir to the production well stopped and thefluid geochemistry and wellhead temperature rapidly changed. In the dual circulation test, the breakthrough and mode volumes of HDR-2a and HDR-3 via SKG-2 tended to decrease during four months of circulation. In the upper reservoir, thermal breakthrough occurred like in the lower reservoir from HDR-1 to HDR-2a in the first stage. After one month of circulation interruption, thereservoir temperature became higher and the breakthrough and mode volumes increased due to anhydrite precipitation.