Quantifying Fault Stability in the Fort Worth Basin, Texas

Abstract

From 1970 through 2005 there were 2 Mw ≥2.5 earthquakes in the Ft. Worth Basin (FWB) and from 2006 through mid-2018 there have been 125. Evidence from structural geological analysis, site-specific pore-pressure modeling, temporal and spatial correlation between seismicity and subsurface injection activities, and proximity of seismicity to known or suspected faults, leads to a general scientific consensus that this increase has been induced by increases in pore fluid pressure from waste water injection and from cross-fault pore pressure imbalance due to oil and gas related injection and production. Shallow crustal earthquakes result from the reactivation of and slip on a pre-existing fault or faults. The larger the fault surface area that slips, and the stronger the rock involved, the larger the magnitude of the resulting seismic event. There are two important corollaries of this: (i) earthquake magnitude scales with fault size and rock strength, and (ii) earthquakes can only occur if there is a pre-existing fault appropriately oriented within the ambient stress state for reactivation if sufficient pore fluid pressure increase or imbalance should occur. Here we consider three factors that control the likelihood that a fault will be reactivated by an increase in pore fluid pressure: ambient stress state, orientation and shape of fault(s), and frictional strength of the fault(s). We use newly developed 3D models of pore pressure evolution and fault geometries, together with stress data from a variety of sources, including wells and seismicity, to characterize fault stability within the FWB. Fault stability is assessed on the basis of fault area combined with its proximity, in stress terms, to reactivation, here expressed as pore pressure perturbation required for slip (ΔP_{f crit} ). In a fully 3-dimensional stress state, faults that have large areas with low values of ΔP_{f crit} are less stable than faults that have small areas with low values of ΔP_{f crit} . The stability of fault populations as well as individual faults can be evaluated, and the likely effects of different stress models can be compared.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019