Critically Stressed Reservoirs: The Influence of Effective Intermediate Principal Stress in Complex Extensional Fault Regimes

Alan P. Morris¹, David A. Ferrill¹, Darrell W. Sims¹, Danielle Y. Wyrick¹, and Deborah J. Waiting^2
1Department of Earth, Material, and Planetary Sciences, Geosciences and Engineering Division, Southwest Research Institute, San Antonio, TX
²Center for Nuclear Waste Regulatory Analyses, Geosciences and Engineering Division, Southwest Research Institute, San Antonio, TX

Fault and fracture permeability effects are commonly observed in critically stressed hydrocarbon reservoirs, where faults and fractures are poised at the brink of slip or dilation. In normal faulting regimes, the generalized reservoir-scale magnitudes and orientations of the maximum and minimum principal compressive stresses may be known or inferred with some confidence. Uncertainty may increase at the sub-reservoir scale owing in part to spatial variations in structural context, including fault and fracture system geometry and complexity, and temporal changes associated with production. The magnitude of the intermediate principal compressive stress is generally more difficult to constrain than the other principal stresses and is often not considered as important as the maximum or minimum principal stress in controlling fault slip potential. We show that the effective intermediate principal stress exerts a strong influence on the stress state of faults with complex or nonoptimal Andersonian geometry. Under anisotropic stress conditions, faults and fractures with high slip tendency are generally more transmissive. Slip tendency analysis provides a basis for quantifying the effects of uncertainty in the magnitudes and orientations of all principal stresses including the intermediate principal stress, thereby focusing efforts on the most important components of the system and permitting improved risk evaluation when applied in the context of hydrocarbon prospects.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas