A major research effort in our group addresses the effect of structural discontinuities such as faults and joints on flow of formation fluids in the subsurface. Structural discontinuities usually differ in permeability from the surrounding formation. Opening of fracture or fault surfaces results in increased permeability whereas cementation and mechanical porosity and grain size reduction tend to decrease permeability relative to adjacent undeformed rock. Studies that address these effects are of importance in the petroleum industry, in groundwater resource management, hazardous waste disposal, and potentially for the subsurface sequestration of anthropogenic greenhouse CO2.We address the effects of faults and fractures on subsurface fluid flow using a variety of approaches that include fieldwork, seismic interpretation, petrographic and petrophysical analyses, and numerical flow modeling. Aydin (2000) provides a succinct review of the roles faults and fractures play in hydrocarbon migration and flow. A more general and thorough review of the subject can be found in a publication collectively authored by a committee of the National Research Council (1996). Antonellini et al. (1994a, b, c, 1995, 1999) characterize the permeability structure of deformation bands in sandstone of various formations, sampled from both outcrop and core, providing quantitative estimates of permeability change in sandstone due to faulting. Following up on Antonellini’s work, Matthai et al. (1998a, b) employed a finite element code to model flow in faulted and fracture sandstone reservoirs. Matthai specifically addressed the effect of discontinuities on the hydraulic response around wells due to pumping and water or hydrocarbon extraction.
Taylor et al. (1999, 2000) expanded the modeling approach using mapped diagenetic alteration patterns associated with faults and fractures in sandstone as a constraint for numerical flow simulations. Jourde, Aydin, and Durlofsky (in review) used numerical upscaling methods to model the bulk flow properties of joint-based faults in sandstone.
The potential effect of faults as preferred flow conduits was investigated by Dholakia et al. (1998) in fractured siliceous shale of the Miocene Monterey Formation in southern California. Dholakia’s field observations document the effect of fault slip on creating or enhancing fracture permeability in reservoir units of low matrix permeability. Fault sealing in alternating shale/sandstone sequences was addressed by Koledoye et al. (2000) on a reservoir scale based on 3D seismic interpretation. Koledoye illustrates the importance of differences in mechanical properties between shale and sandstone in fault formation, segmentation, and linkage. He documents that faults form preferentially in sandstone layers resulting in stepped en-echelon arrays. Continued slip along these faults leads to shale smear between fault segments potentially resulting in a fault seal for migrating formation fluids.