Growth Process of the Reservoir in the Yunomori HDR/HWR Test Site -An Approach Based on a Model of Reservoir Growth in the Shear Mode

Abstract

HDR/HWR reservoirs consist of multiple cracks which are either natural or artificial. In this paper, we examine the growth process and change in morphology of such reservoirs during hydraulic stimulation. To this end, we first construct a model based on shear dilation due to self-propping induced by frictional shear slip along preexisting planes of weakness. Then, we apply the model to the reservoir created by hydraulic stimulation performed through well TG-2 at the Yunomori test site of the HWR project of the New Energy and Industrial Development Organization (NEDO), Japan. In the application, we employ the data estimated by spinner logging and FMI (Formation Micro Imager) survey. Through spinner logging, four zones of well TG-2 have been identified to accept the injected fluid. The orientation of the cracks in each of the four zones has been identified through the FMI survey. Roughly speaking, two sets of cracks are predominant in the test site, i.e., almost horizontal cracks and almost vertical cracks which are striking in the northwest direction. We also employed the stress field estimated by the ASR (anelastic strain recovery) method. It is revealed that the morphology and growth process of the reservoir at the test site is dependent on the injection flow rate during the hydraulic stimulation. For a small injection flow rate, the vertical cracks of all the four zones and horizontal cracks in the shallower two zones are active. For a large injection flow rate, all cracks of the four zones become active to accept the injected fluid. Regarding the contribution of the four zones to accept the injected fluid, the shallower two zones play a major role for a small injection rate. For example, all the injected fluid flows into the shallower two zones for the injection flow rate that is less than 50 h m3. However, for the injection flow rate of 500 h m3, 30% of the injected fluid flows into the shallower two zones while the remaining 70% flows into the deeper two zones. The transition takes place at around the injection flow rate of 200-300 h m3. The morphology of the reservoir estimated by the present model fairly well agrees with the estimation obtained by using the location of AE (acoustic emission) events observed during the hydraulic stimulation.