Surface subsidence from conventional geothermal operations has been observed and studied for many decades. Surface subsidence can induce a devastating effect in the structure and safety of facilities, such as buildings, pipelines, and other infrastructure. It can also interrupt the balance ecosystems nearby. Frequent monitoring of the surface level and according management of the geothermal operations can reduce the risk of subsidence.
In this study, we investigated the surface deformation due to a geothermal field operation located in New Zealand. Surface features including lakes and rivers were located at the ground surface above the geothermal field. After several years of operation, surface subsidence was measured at the field. This was the primary motivation to analyze the impact of development plans on ground level changes. Since produced water from the geothermal operation was re-injected into the reservoir with cold temperature, both pressure and temperature impacts were studied to evaluate the surface subsidence effect.
An integrated 3D geomechanical model was developed based on the geological, fluid, and heat flow models available. Pressure and temperature changes, due to different production and injection scenarios, were transferred to the 3D geomechanical model to estimate surface deformation. A history match of the geomechanical model was first performed against the historical subsidence survey data. Rock mechanical properties play an important role in such studies. These properties are usually estimated from available log data, well test data, and lab test data. The geomechanical model was calibrated with the observed historical survey data and then applied to simulate future scenarios to predict surface subsidence and provide a guideline to optimize field development.
Analysis & Conclusion
After finding a good history match for the subsidence model compared with the historical surface level measurements, several scenarios were simulated to predict future subsidence and the anticipated level of surface tilt (differential movement). The results helped to evaluate the risk of surface deformation, whether any surface structures or features are likely to be damaged due to ongoing geothermal operations (e.g. whether potential higher water level may develop in a lagoon/lake where the outlet of the lake will experience less subsidence than the overall area of the lake). An operation guideline such as production/ injection rate and location was recommended based on the simulation results.