Injection-induced Microearthquakes for Reservoir Characterization in Darajat Geothermal Field, Indonesia

摘要

Since 2006, Star Energy Geothermal has continuously monitored microearthquakes (MEQs) associated with production and injection activities in the Darajat field. The 10- to 13-station array has located more than 6,800 events during this period. Experience has shown that most of the induced seismicity is related to the injection of power plant condensate and during the drilling of production make-up wells. The MEQ events around the injectors and the make-up wells are mostly observed at depths extending below the TD of the wells indicating stimulation of the deeper connected fracture system of the reservoir. These MEQ events are interpreted to be induced by thermal contraction and transient pressure increase that occur along the movement paths of the injected condensate. Traditionally, these injection-induced MEQs have been used to interpret flow directions in the reservoir for the injectate and used as "sounding" points for the interpretation of the depth extent of the connected fracture system (i.e., Base of Reservoir or BoR). For these interpretations, the locations of the MEQ's were calculated using a ID velocity model (HYPOPLUS). A recent velocity model improvement was conducted by developing ID and 3D models from tomography inversion. The new velocity model reduced the overall location uncertainties and showed a distinct velocity signature in the reservoir. MEQ events located with the new 3D tomographic velocity model tend to be shallower and more tightly grouped compared to the previous model. Additionally, lower P-wave velocity (Vp) and P- and S-wave ratio (Vp/Vs) are centered over the main production area. The lower Vp extends into the area where productive entries are found in the reservoir, and may be related to the nature of the fracture system in the reservoir and phase changes in the reservoir's fractures and matrix. An attempt to model the 4D Vp/Vs changes and distribution was performed to observe phase changes after a key injection move from central to the northeast corner of the field. Interim results show a systematic small increase in the Vp/Vs ratio that extends southwest from the new injector towards the production area. Further study is underway to confirm the robustness of the inverted results.