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Impact of Varied Approaches for Fault Slip Potential Analysis in the Fort Worth Basin, Texas

Abstract

Fault slip potential (FSP) analysis is increasingly used by operators and regulators to assess the probability that existing faults will be critically stressed following pore pressure increases associated with saltwater disposal (SWD). This type of screening-level probabilistic analysis is beneficial because it accounts for practical uncertainties in fault orientation and properties; stress orientation and magnitudes; and initial and perturbed pore pressure. Despite its simplicity, FSP presents users with many possible analysis approaches. Conclusions drawn from FSP analysis will vary depending on whether the selected approach is regional versus local, screening versus comprehensive, deterministic versus probabilistic, detailed environmental inputs versus idealized, etc.

To illustrate the effect of differing approaches, we apply the freely-available FSP tool to assess the hazard of fault reactivation in the Fort Worth Basin (FWB) where an increase in seismicity rate beginning in 2008 has been linked to SWD. A new 3D interpretation of the basin has identified 251 basement-rooted faults, a newly-refined in situ stress model presents better geomechanical constraints, and a new basin-wide hydrogeologic model provides estimates of pore pressure evolution in the SWD interval.

First, we consider a case of no increase in pore pressure, but include the effects of native variation in pore fluid density and stress in each of four subareas. This approach yields an average basin-wide FSP of 11%. We next test a pore pressure increase of 1 MPa yielding an average FSP of 28%. This approach allows for intuitive assessment of an absolute pore pressure increase, but it yields relatively high FSP in areas of shallower basement, such as toward the Llano uplift to the southwest. For a test that is independent of depth, we consider a basin-wide pressure gradient of 9.8 MPa/km initial followed by a modest 0.4 MPa/km increase. With this approach, we obtain a lower average FSP of 26%. Finally, we illustrate the effects of parameter uncertainties. For faults that have recently produced earthquakes, we employ deterministic fault dip which results in significant local increases in FSP due to the decrease in the occurrence of less-optimal faults in the stochastic population. Including spatiotemporal pore pressure data from the hydrogeologic model results in a more heterogeneous distribution of FSP that is better associated with earthquake occurrence.

Using insights from our various analysis approaches, we provide context-specific recommendations for FSP use.