Rate type isotach compaction of consolidated sandstone

Laboratory experiments on samples from a consolidated sandstone reservoir are presented that demonstrate rate type compaction behaviour similar to that observed on unconsolidated sands and soils. Such rate type behaviour can have large consequences for reservoir compaction, surface subsidence and induced seismicity resulting from oil and gas production. During traditional single loading rate laboratory experiments, the effect of loading rate on uniaxial compressibility can be ignored to first approximation. In contrast, strongly non-linear compaction occurs when stepwise increases or decreases in loading rate are applied within a loading cycle, during loading after creep or during loading after partial unloading. A modified version of the RTCM rate type compaction model, built on Dieterich’s rate and state friction equation describes the experimental results well. The new isotach version solves a number of limitations of the original model at the cost of an additional parameter that specifies the split between direct (elastic) and secular (creep) strain. The good fits between the updated model and the experimental data suggest that rate and state friction - e.g. on sliding contact surfaces between assemblies of cemented grains or on sliding micro-cracks - causes the observed non-linear compaction. The same rate and state friction phenomena have been linked to the non-linear rate-dependent earthquake generation in volcanism-induced seismicity, suggesting similar rate dependence for the induced seismicity in depleting oil and gas fields