5:15 PM - 6:45 PM
[SCG46-P15] Study on focal mechanism and in-situ stress evolution process during the development of Gonghe geothermal field in China
Keywords:Enhanced Geothermal System, Hydraulic Fracturing, Induced Seismicity, Focal Mechanism, In-situ Stress Evolution
Developing and utilizing hot dry rocks is an important part of China's clean energy strategy. The hot dry rock exploration project in Gonghe Basin, Qinghai, China, is the first time China has drilled the hot dry rock with the shallowest burial depth and the highest temperature, achieving a breakthrough in hot dry rock exploration. The Gonghe hot dry rock project in Qinghai consists of five wells, of which No.1 ~ No.3 wells are the fracturing wells in the first stage, which have experienced hydraulic fracturing and cyclic injection-production experiments in 2020 and 2021. In the second stage, No.4 and No.5 fracturing wells and No.3 branch wells were drilled. Using these five wells, the geothermal development project carried out several small-scale hydraulic fracturing and cyclic injection-production modifications in 2022-2023. Finally, the connectivity and injection-production efficiency of fractures in geothermal reservoirs are effectively promoted, and the power generation scale of 300Mw is realized.
In this study, the microseismic monitoring network from 2020 to 2023 was used to record and analyze microseismic events with a magnitude of more than ML0.0 in the whole development process. We found that the rupture types of microseismic events above magnitude 0 were mainly normal fault type and strike-slip type in 2022-2023, in which the normal fault type has a large volume component, while the second type of strike-slip microseismic source has a small volume component, but shows isotropic expansion/compression, or has positive and negative CLVD source attribute components. It can be inferred that these two types of seismic sources are mainly caused by the secondary activation of the hydro-fractures in the previous stage. This is different from the microseismic fracture type that occurred in the hydraulic fracturing reservoir reconstruction stage in 2020~2021 which was mainly thrust type. Although the fracture types corresponding to microseisms have changed obviously in different modification stages, the dominant fracture surface is still in the same direction: NW.
Then, the stress field near the well site in 2023 was obtained using the stress iterative joint inversion method. The results showed that the azimuth of the principal stress axis σ1, σ2 and σ3 (σ1>σ2>σ3) was 322°, 131° and 225° respectively. The corresponding dip angle for the three stress axis was 57°, 33°, and 5°, which is nearly horizontal, indicating that the whole study area is in a tensile stress environment. The r value of the stress shape factor is 0.2, which is small and indicates that the value of σ2 is close to the value of principal compressive stress, and the axis of σ2 is characterized by compressive stress. By comparing and analyzing the stress field characteristics from 2020 to 2023, it is found that the σ1 in 2023 is almost vertical, and the stress property is normal type. The principal compressive stress direction is the SEE direction, which is different from the regional tectonic background stress direction of NEE, and NE, so it has obviously deflected. The principal stress axis in 2023 is close to that in 2022, but it also deflects to a certain extent. Most normal faults in 2023 also deflect in the corresponding direction, which is related to long-term hydraulic fracturing modification.
Based on the inversion results of the microseismic focal mechanism and stress field from 2020 to 2023, we can find that during the development of hot dry rocks, in the early stage of hydraulic fracturing reservoir reconstruction, the induced microseismic events are mainly controlled by the background tectonic stress. However, with the development of the fracture network and the action of thousands of tons of fracturing fluid, the background stress field will shift significantly, which leads to the microseismic events induced in the cyclic injection-production stage being more easily affected by fracture structural characteristics and fluid action.
In this study, the microseismic monitoring network from 2020 to 2023 was used to record and analyze microseismic events with a magnitude of more than ML0.0 in the whole development process. We found that the rupture types of microseismic events above magnitude 0 were mainly normal fault type and strike-slip type in 2022-2023, in which the normal fault type has a large volume component, while the second type of strike-slip microseismic source has a small volume component, but shows isotropic expansion/compression, or has positive and negative CLVD source attribute components. It can be inferred that these two types of seismic sources are mainly caused by the secondary activation of the hydro-fractures in the previous stage. This is different from the microseismic fracture type that occurred in the hydraulic fracturing reservoir reconstruction stage in 2020~2021 which was mainly thrust type. Although the fracture types corresponding to microseisms have changed obviously in different modification stages, the dominant fracture surface is still in the same direction: NW.
Then, the stress field near the well site in 2023 was obtained using the stress iterative joint inversion method. The results showed that the azimuth of the principal stress axis σ1, σ2 and σ3 (σ1>σ2>σ3) was 322°, 131° and 225° respectively. The corresponding dip angle for the three stress axis was 57°, 33°, and 5°, which is nearly horizontal, indicating that the whole study area is in a tensile stress environment. The r value of the stress shape factor is 0.2, which is small and indicates that the value of σ2 is close to the value of principal compressive stress, and the axis of σ2 is characterized by compressive stress. By comparing and analyzing the stress field characteristics from 2020 to 2023, it is found that the σ1 in 2023 is almost vertical, and the stress property is normal type. The principal compressive stress direction is the SEE direction, which is different from the regional tectonic background stress direction of NEE, and NE, so it has obviously deflected. The principal stress axis in 2023 is close to that in 2022, but it also deflects to a certain extent. Most normal faults in 2023 also deflect in the corresponding direction, which is related to long-term hydraulic fracturing modification.
Based on the inversion results of the microseismic focal mechanism and stress field from 2020 to 2023, we can find that during the development of hot dry rocks, in the early stage of hydraulic fracturing reservoir reconstruction, the induced microseismic events are mainly controlled by the background tectonic stress. However, with the development of the fracture network and the action of thousands of tons of fracturing fluid, the background stress field will shift significantly, which leads to the microseismic events induced in the cyclic injection-production stage being more easily affected by fracture structural characteristics and fluid action.