Borehole fluid injections are frequently used for diverse geo-technical and geo-energy applications. As such hydraulic stimulations modify the pore-fluid pressure level, they are typically accompanied by the occurrence of microearthquakes. Following the seismicity-based reservoir characterization approach, the spatio-temporal evolution of such earthquakes can be evaluated to quantify in-situ fluid transport properties of the rock, assuming that they are constant with time and pressure. However, field and laboratory experiments demonstrate that permeability can be significantly pressure-dependent.
The main objective of this thesis is to further investigate the phenomenon of fluid injection induced earthquakes taking into account pressure-dependent hydraulic transport properties. In particular, a power-law as well as an exponential-dependent diffusivity model are considered in detail. They both lead to non-linear diffusion equations which are solved numerically to compute synthetic seismicity for the injection and post-injection phase.
The analysis of their spatio-temporal characteristics shows that the triggering front concept still holds for the case of a pressure-dependent hydraulic transport. However, instead of providing an in-situ diffusivity estimate, the triggering front is found to hydraulically characterize the medium not before but after stimulation including hydraulic fracturing of the rock. Additionally, synthetic seismicity also demonstrates that the triggering front signature depends on the diffusivity model in use. For the power-law diffusivity model, this signature can yet change from a square root into a cubic root of time dependency. This behaviour is observed and verified for hydraulic fracturing induced seismicity from the Barnett Shale.
For the post-injection phase, the spatio-temporal analysis of induced seismicity shows that the back front provides an estimate for the minimum principal component of the permeability tensor. This is demonstrated not only from synthetic seismicity but also from seismicity recorded during the two geothermal reservoir stimulations of Ogachi and Fenton Hill. Additionally, it is found that the linear diffusion back front is only applicable for a weak non-linear fluid-rock interaction. For such situations, which seem to correspond to Ogachi and Fenton Hill, a cubic root of time-dependent power-law function is then a good approximation of the exact back front. For a strong non-linear fluid-rock interaction like the hydraulic fracturing, the back front is found to deviate from its linear diffusion signature. This is observed from synthetic seismicity, which shows similar spatio-temporal characteristics as the ones observed from Horn River Basin hydraulic fracturing induced seismicity.
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