Role of fractures in crystalline basement in injection induced seismicity

Crystalline rocks in nature frequently contain pre-existing fractures and being fluid-saturated exhibit complex coupled behavior. For example, induced microseismicity observed during CO2 injection in Illinois Basin is likely to be related to presence of high permeable basement faults that provide fluid overpressure transfer into critically stressed formation. Geomechanical characterization of the Precambrian basement cores from 2100 m depth dictates that for intact rock the overpressure has to be above 10 MPa to induce seismicity, significantly exceeding the values measured during the CO2 storage project. In addition, the permeability of intact material is below nanodarcy and cannot provide pressure transfer into the crystalline basement over the realistic timescales. At the same time, flow tests with fractured rock revealed that presence of fractures provides 3-6 order of magnitude higher permeability, which is very sensitive to changes in effective mean stress and fracture offset. Strength and elastic properties of fractured material are also measured to be significantly lower than those of the intact rock. These features of fractured basement could be the key factor to explain local pore pressure buildup and associated slippage on pre-existing weak planes. Additionally, accurate laboratory measurements of hydromechanical properties of intact and fractured crystalline rock are combined with the numerical modeling to provide the field scale predictions and evaluate the effect of fractures on the stress state changes caused by fluid injection and its implications for induced seismicity.