Geomechanical characterization for the CO_2 injection test site, offshore Pohang Basin, SE Korea

摘要

We are conducting a small-scale CO_2 injection demonstration project in offshore Pohang Basin, SE Korea to test various techniques related to CO_2 sequestration, which include reservoir characterization and modelling, drilling and completion, and monitoring systems. The target brine aquifer for CO_2 storage is 60 m thick sandstone/conglomerate formations at a depth range between 770 and 830 mbsf (meter below seafloor), which were verified by a seismic survey and a cored borehole (980 m deep). A number of steeply dipping and NE-striking faults cross the target aquifer. To analyze potential risk of shear activation along the faults, we characterize in situ stress state at the site. Borehole image logs showed borehole breakouts along the whole logged section to -700 mbsf, which consistently indicate an average maximum horizontal principal stress (SHmax) direction of NW-SE. A leak-off test conducted at the bottom of a casing shoe (700 mbsf) yielded the magnitude of the minimum horizontal principal stress (Shmin) lower than the vertical stress (S_v). For the given Shmin and S_v conditions, we used the logged breakout widths and laboratory determined rock compressive strength to constrain possible SHmax magnitudes that could create the observed breakouts. We utilized our estimated stress conditions to analyze slip tendency of the faults. Additional information obtained from LOT is cap rock permeability, which suggests the formation above the storage reservoir is suitable for a leakage barrier. All regional scale faults turn out to have relatively low slip tendency under the given stress condition, suggesting a relatively low risk of triggering shear activation of faults during CO_2 injection.