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Sandia National Laboratories, PO Box 5800, Albuquerque, NM 87185-0750, USA
1 Present address: Hughes Christensen, The Woodlands, TX 77380, USA
2 Present address: BP America, Houston, TX 77079, USA (e-mail: Joanne.Fredrich{at}bp.com)
Recent field, laboratory, and theoretical studies suggest that under certain stress conditions, compaction of porous rock may be accommodated by narrow zones of localized compressive deformation oriented perpendicular to the maximum compressive stress. Triaxial compression experiments were performed on Castlegate sandstone, an analogue reservoir sandstone, that included acoustic emission detection and location. Initially, acoustic emissions were concentrated in horizontal bands that initiated at the sample ends (perpendicular to the maximum compressive stress) but, with continued loading, progressed axially towards the sample centre. High-resolution field-emission SEM was performed to elucidate the micromechanics of compaction. The microscopy revealed that compaction of this weakly cemented sandstone proceeded in two phases: an initial stage of porosity decrease accomplished by breakage of grain contacts and grain rotation, and a second stage of further porosity reduction accommodated by intense grain breakage and rotation. Quantitative stereological measurements corroborated the decrease in the intergrain spacing and the increase in grain boundary contacts that the microstructural observations suggest occurred during the first stage of compaction. The microstructural data show that a five-fold increase in the surface area per unit volume resulted from the extensive microfracturing that occurred during the second stage of compaction.
