GEOLOGIC CONTROLS ON SUBSURFACE MECHANICAL AND HYDROLOGIC PROPERTIES: IMPLICATIONS FOR SEAL FAILURE AND INDUCED SEISMICITY

Deep injection of large volumes of disposal fluids and CO₂ injection schemes focuses on a suite of rocks that have been overlooked, and place attention on the integration of hydrogeology, geomechanics, and subsurface structural geology as never before. Uncertainty regarding the strength and mode of failure of reservoirs and the potential for seismicity and seal failure is significantly reduced by combining geologic analyses, rock mechanics, and hydromechanical modeling. W argue that such systems are tractable, and Hintzman et al. (2012) suggest maintaining a fluid volume balance (injection=withdrawal) is one of the most effective means of reducing the risk of induced seismicity. Basal reservoir injection promotes downward propagation of elevated fluid pressures into the crystalline basement where large, damaging earthquakes occur. Experimental analyses of fine-grained clastic rocks, representative of unconventional reservoir and seals, reveal that siltstones exhibit a range of tensile and uniaxial compressive strengths. Coupled with variation in coefficients of friction this results in a range of failure envelopes that predict mode I, hybrid shear, and shear failure within a stratigraphic sequence, confirmed by field observation. Analyses of sedimentary interfacial seals reveal a range of structural and sedimentary features that affect caprock integrity, provide short circuits for fluid flow (seal-bypass) and influencing the propagation of fractures across interfaces. Coupled numerical hydrogeomechanical models incorporate experimental and field derived fluid flow properties and structure of faulted sedimentary-basement sequences will hind cast induced seismic events may provide insights to the factors controlling induced seismicity, and provide information on the permeability structure of the crystalline basement and potential for seal failure. The “frac gradient” analysis of the response of the subsurface around the few high P and Q deep injectors does not capture this behavior. Integrated analyses of the pressure changes up to 10-20 km from a wellbore together with sedimentological and structural heterogeneity in the subsurface provides a more a robust method to enhance insights into seismic and environmental safety considerations and addresses economic and societal demands.