HYDROGEOLOGIC CONTROLS ON PORE-PRESSURE PROPAGATION FROM INJECTION WELLS: IMPLICATIONS FOR INDUCED SEISMICITY

The propagation of pore-pressure from fluid injection wells has the potential to induce seismicity by lowering the effective stresses on preexisting faults. Here we conduct numerical modeling to examine the sensitivity of the various hydrogeologic parameter controls on the propagation of pore-pressure near faults that have the potential to host induced seismicity. The objective is to discern the relative importance of these parameters on the timing, extent, and magnitude of pore-pressure changes. We model pore-pressure changes due to fluid injection in a generic aquitard-aquifer-aquitard system with a permeable vertical fault zone located 1 km from the injection well. We test the sensitivity of pore-pressure to a range of four hydrogeologic and fault parameters: fault zone width, fault zone permeability, the permeability contrast between aquifer and aquitard, and the injection rate. For the base case, we simulated a 3-layer system with fluid injection into the central aquifer (k = 10^-14 m²) confined above and below by less permeable aquitards (k = 10^-16 m²) over 5 years of injection. The vertical fault zone (k = 10^-14 m²) cross cuts the lower aquitard. We observed the largest pore-pressure changes at the base of the fault zone when the permeability of the fault zone was greatest. The magnitude of pore-pressure changes at the base of the fault zone scaled linearly with increases in the fault zone width. Pore-pressure changes also increase linearly with increases in injection rate. When both the aquifer and aquitard permeabilities were low, pore-pressure changes were delayed in reaching the fault zone, but the rate of pore-pressure increase was greater than other scenarios. We conclude that the permeability of the fault zone is the most important hydrogeologic parameter controlling the propagation of pore-pressure. Therefore, permeable faults near injection have the potential to induce seismicity. If the overall permeability of the aquifer-aquitard system is low, it may take years for pore-pressure to propagate from an injection well to nearby faults.