Slip Tendency Analysis and Risk of Induced Seismicity

English Abstract:

The correlation between unexpected, presumed induced, seismic activity and nearby hydraulic fracturing and its commonly associated water disposal has not gone unnoticed. Elevated pore fluid pressure is usually considered to be the cause of this induced seismicity, triggered by decreased stability (increase in slip tendency) on pre-existing faults and fractures as pore fluid pressure rises. Most active faults experience high slip tendency under present-day stress, and cases of induced seismicity can be explained by increased slip tendency driven by increased pore pressure. Felt earthquakes typically have magnitudes greater than about 2.5, and damaging earthquakes have magnitudes higher than this. Such events can only occur on relatively large slip surfaces, implying that a well-oriented, potentially detectable, fault would need to exist within the rock volume prior to the increase in pore fluid pressure responsible for inducing slip. A well-conducted geological investigation of the proposed site of fluid injection can provide an assessment of existing geologic structures, rock permeability characteristics, and ambient stress states. Combining these into a geologic model and simulating the proposed fluid injection conditions provides a risk evaluation of induced seismicity. Our approach to this simulation uses slip tendency analysis coupled with an anisotropic effective permeability tensor and analytical well hydraulics models provides an alternative to complex numerical models, and provides sufficiently rapid feedback to test a wide range of scenarios. Thus far our results indicate that the most significant variables are the threshold slip tendency at which a fault will slip, and complex permeability anisotropies affecting the pressure distribution within the rock mass.