A Tool to Assist Pressure Management by Detecting and Localizing Low Permeability Faults

Abstract:

Fluid injection and its subsequent induced seismicity have significantly increased recently. Injection of fluids due to wastewater disposal, geothermal energy, seasonal natural gas storage and geologic carbon storage causes pressure buildup, which reduces the effective stresses. This reduction brings the stress state closer to failure conditions, which may yield shear slip of pre-existing fractures or faults. Shear slip induces seismic events, which in some cases are felt by the local population. Felt induced seismicity negatively affects public acceptance and may lead to the closure of injection projects. To avoid inducing felt seismic events, a good pressure management is crucial. We propose a methodology to identify and locate undetected low permeability faults using diagnostic plots. This method is useful to assist decision making to adopt the proper mitigation measures to keep overpressure below the maximum sustainable injection pressure when a low permeability fault is causing an additional overpressure within the injection formation. Diagnostic tools allow a rapid identification of the divergence between the pressure measurements and the expected overpressure evolution in a homogeneous aquifer. The divergence time is an indicator of the presence of a low permeability fault and can be used to determine its position. We formulate the problem in its dimensionless form, so it can be generalized to all injection sites. We apply our methodology to water and CO₂ injection through a horizontal well in a confined aquifer that has a fault parallel to the well. Nevertheless, the methodology can be extended to other geometrical configurations and geological settings.