Stress-Permeability Characterization and Proposal of Creep Model for Pre-Salt Carbonates Under Lab-Scale Stepwise Hydrostatic Loading and Unloading

ABSTRACT: In an attempt to expand the studies of rock creep deformation from the classic unidimensional creep theory, this study presents results obtained from geomechanical lab-scale tests of pre-salt carbonate cylindrical samples subjected to stepwise hydrostatic loading and unloading ramps, from 30 to 80 MPa and back, followed by short-term 2.5-hour periods of stress hold. All samples were simultaneously subjected to an axial flow of monophasic oil with upstream pressure measurements. By using Darcy's law, it was possible to obtain an estimate of absolute permeability variation as function of measured axial and radial deformation. It was possible to characterize a power fit nondimensional coefficient of the relation between permeability and porosity variations during loading conditions. Such coefficient, despite presenting higher values during stress hold steps (order 10 to 100) than during load ramps (order 0.01 to 0.1), its mean value was observed to be similar to the ones found in the literature (order 1). At unloading stage, most of the samples did not present permeability recovery, indicating a high level of plastic deformation. The authors also propose volumetric creep models for each sample by taking into account their porosity and their variation with each stress step that they were subjected to. 1. INTRODUCTION It is known that reservoir rock permeability varies during the oil and gas exploitation process. This variation occurs due to increase/decrease of fluids pressure and/or rock stress state. In order to better emulate the variation of permeability in geomechanics and production simulators, lab-scale experiments can be performed on reservoir rock samples to obtain relations between permeability and stress states. Petunin et al. (2011) proposed a power relation between the ratios of absolute permeability k and porosity φ to their respective reference state for sandstone, carbonate and fractured rock samples tested with hydrostatic compressions and simultaneous porosity and permeability measurements. Santos et al. (2014) successfully reproduced Petunin's k−φ relation on limestone and dolomite samples by testing them on robust compression test machine and indirectly measuring φ from volumetric deformation εv and estimating k from Darcy's law by imposing axial oil flow on the samples and measuring flow pressure.