High‐Resolution Mapping of Yield Curve Shape and Evolution for High‐Porosity Sandstone

Abstract

AbstractUnderstanding the onset and nature of inelastic deformation in porous rock is important for a range of geological and geotechnical problems. In particular for sandstones and siliciclastic sediments, which often act as hydrocarbon reservoirs, inelastic strain can significantly alter the permeability affecting productivity or storativity. The onset of inelastic strain is defined by a yield curve plotted in effective mean stress (P) versus differential stress (Q) space. Yield curves for porous sandstone typically have a broadly elliptical shape, with the low‐pressure side associated with localized brittle faulting (dilation) and the high‐pressure side with distributed ductile deformation (compaction). However, recent works have shown that, for different porous rocks, the curve shape can evolve significantly with the accumulation of inelastic strain. Here yield curve shape and evolution of two high‐porosity sandstones (36–38%) is mapped along different loading paths using a high‐resolution technique on single samples. The data reveal yield curves with a relatively shallow geometry and with a compactive side that is partly comprised of a near‐vertical limb. Yield curve evolution is found to be strongly dependent on the nature of inelastic strain with samples compacted under a deviatoric load (i.e., with a component of shear strain) having peak stress values that are approximately 3 times greater than similar porosity samples compacted under a hydrostatic load (i.e., purely volumetric strain). These results have important implications for predicting how the strength of porous rock evolves along different stress paths, which differ in reservoirs during burial, fluid extraction, or injection.