Compaction Localization: Effects on Permeability and Reservoir Mechanics
David J. Holcomb1, William A. Olsson1, John W. Rudnicki2
1Sandia National Laboratories, Albuquerque, NM
2Dept. of Civil and Envir. Eng., Northwestern University, Evanston, IL
Recent field and laboratory studies in porous sandstones have reported localized compaction under conditions for which uniform compaction is usually assumed. Simultaneous measurements of stress, strain, acoustic emission locations and permeability during experiments on a sandstone revealed up to a two-order-of-magnitude decrease in permeability in the compacted zone. Correlation of local strain measurements and acoustic emission locations made on the same specimen show that the compaction process proceeds as a propagating front approximately 20 mm thick. The inhomogeneous nature of the observed compaction and its temporal evolution suggest locally increased pore pressures and spatial changes in the effective permeability. In addition, the results suggest the possibility that interaction between compaction-induced fluid pressure and compaction localization should lead to a phenomenon analogous to dilatancy hardening, impeding the propagation of compaction bands. An analysis of conditions for the inception of these bands, similar to that used for the inception of shear bands, shows that they can form for stress states on a yield surface “cap”, as is often used to model porous materials. A specific calculation for axisymmetric compression of a material with an elliptical shaped cap reveals that compaction bands are the only allowed mode of localized deformation when lateral confining stress exceeds a critical value that depends on the parameters of the ellipse. The critical slope of the hydrostatic stress vs. inelastic volume strain curve for compaction band formation increases (favoring band formation) with distance from the hydrostatic axis for stress states on the cap, reaching zero at the transition to shear banding (equality in preceding inequality) for normality and a positive value for non-normality. The precise conditions depend in detail on the evolution with inelastic strain and on the nature of the intersection of the cap with the shear yield surface. The occurrence of localized compaction in reservoirs will alter the flow characteristics by comparison with those expected for more uniform compaction and significantly affect both economic and operational aspects of reservoir management. The localized form of these features makes detection by surface geophysical or borehole measurements difficult and, consequently, understanding the conditions for their formation is essential.
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