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

Understanding 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. Plain Language Summary Porous rocks are the geological sponge of the Earth's crust as they store and transmit vast amounts of fluids such as groundwater and hydrocarbons and can also be used for CO2 storage projects. As porous rocks are subject to increased stresses above the rock strength (either tectonic or from pumping fluids in or out), the porosity will change, restricting the amount of storage the rock can provide, as well as the ability for fluid flow through it. As porosity changes, strength also changes. Deformation of porous rock can occur from an increase in pressure, where the stresses are equal in all directions, or from a differential stress where force is applied more strongly in one direction. We show that the change in strength of sandstone as it is deformed