The Geomechanical Study in Pohang Enhanced Geothermal System (EGS) Project in Korea

摘要

The five- year Pohang Enhanced Geothermal System (EGS) project started in Dec 2010 with the goalrnof producing MW scale electricity using a double system of boreholes greater than 4 km deep. Thernproject is a landmark effort by a consortium of industry, research institutes and university with expertisernin geology, hydrogeology, geophysics, geomechanics and plant engineering. Pohang is in southeastrnKorea in the tertiary sedimentary basin under which lays the granitic geothermal reservoir. This paperrnintroduces the geomechanics studies carried out for the project.rnGeomechanics studies for the project include hydraulic stimulation, borehole stability, in situ stressrnmeasurements, reservoir simulation, drilling optimization system, and microseismic monitoring. Thernstudy of hydraulic stimulation includes numerical simulation of hydroshearing and hydrofracturingrnprocess using the boundary and distinct element methods. Laboratory experimentation on the hydraulicrnfracturing process is studied in the biaxial loading system through extensive monitoring of acousticrnemissions. Borehole stability of the site is analyzed consideringthe anisotropy of the reservoir formationrnusing both analytical solution and finite element analysis. In situ stress measurements were conductedrnat around 720 m depth by hydraulic fracturing and observation of borehole breakout and drillingrninduced tensile fractures. Reservoir simulations were carried out in order to optimize the distancernbetween the injection and production wells and it shows that a minimum of 600 m is necessary. Arnsystematic analysis was carried out to optimize time and cost for a drilling process considering variousrninfluencing factors on drilling performance during the full cycle of drilling operations. Stand-alonernsoftware was developed for prediction, real-time assessment, and optimization of drilling performance.rnAn extensive monitoring system ofor microseismicity was designed to locate growth of geothermalrnreservoir and minimize any adverse environmental impacts of the hydraulic stimulation process.