Using Seismic Forward Modelling to Assess Fault Stability During Fluid Injection

Wood, Alan; Paton, Douglas A.; Collier, Richard

Secondary hydrocarbon recovery, hydraulic fracturing and CO2 sequestration require that fluids are injected into the subsurface, with a resulting local increase in pore pressure. A major concern is that this increased pore pressure adversely effects fault stability, potentially leading to reactivation and associated seismicity. The stability of a fault in terms of the Mohr-Coulomb failure criteria is dependent on both the physical properties of the fault and its orientation relative to the in-situ stress field, with the orientation commonly being characterised through the use of seismic imaging. Inherent limitations in seismic resolution result in simplification of the identifiable geometries compared to those present in the subsurface. The disparity between realistic and seismically resolvable fault geometries may lead to incorrect estimates of fault stability under specific stress conditions.

Realistic sub-seismic fault geometries, derived from outcrop data, and those geometries which are resolvable in seismic data are compared using three-dimensional seismic forward modelling. The stability of both geometries has been assessed under stress conditions equivalent to those found at depth within parts of the Fort Worth Basin, a productive shale gas province. Fluid injection is simulated by increasing the pore pressure until the failure envelope is reached. The realistic fault geometries fail at approximately half the pore pressure increase of the seismically resolvable fault geometries, suggesting that relying on seismically resolvable geometries may lead to overestimates in fault stability. As a result it is possible that unexpected seismicity may occur during fluid injection into the subsurface.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013