The Permeability of Faulted Sandstone Subjected to Variable Stress-states and Stress-paths
Texas A&M University, Department of Geology and Geophysics, Center for Tectonophysics, College Station, TX, USA
The permeability structure of faults can alter fluid pathways within oil and gas reservoirs. A fault can be either a conduit or barrier to flow and predicting the permeability structure of faulted rock relies on assumptions about the relationship between permeability and in-situ stresses. The stress history of the rock is not generally incorporated in flow predictions. The results of out experiments constrain the dependence of permeability on stress-state and stress-path and will enable better modeling and prediction of the permeability of faulted sedimentary rocks.
Most traditional triaxial deformation experiments are conducted by holding confining pressure constant and increasing the differential stress until failure. An end-member stress-path, important in active tectonic and basin settings, is that of increasing fluid pressure, which causes decreasing mean stress, constant differential stress, and eventually failure. I used a standard stress-path to induce faulting in porous (~7%), low-permeability (10-3 mD) arkosic sandstone, which created a uniform starting permeability and fault structure between samples. Once faulted, each sample underwent constant mean- or differential- stress loading away from and re-approaching the failure envelope and permeability was measured at discrete stress-states using the transient pulse-decay method. Fault permeability was measured and mapped in stress space relative to the empirical failure model. Preliminary results show that, at the scale of our sample, average mean stress and not proximity to failure is the first order control on rock permeability.
AAPG Search and Discovery Article #90183©2013 AAPG Foundation 2013 Grants-in-Aid Projects