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Relative permeability and capillary pressure are the governing parameters that characterize multiphase fluid flow in porous media for diverse natural and industrial applications, including surface water infiltration into the ground, CO2 sequestration, and hydrocarbon enhanced recovery. Although the drastic effects of deformation of porous media on single-phase fluid flow have been well established, the stress dependency of flow in multiphase systems is not yet fully explored. Here, stress-dependent relative permeability and capillary pressure are studied in a water-wet carbonate specimen both analytically using fractal and poroelasticity theory and experimentally on the micro-scale and macro-scales by means of X-ray computed micro-tomography and isothermal isotropic triaxial core flooding cell, respectively. Our core flooding program using water/N2 phases shows a systematic decrease in the irreducible water saturation and gas relative permeability in response to an increase in effective stress. Intuitively, a leftward shift of the intersection point of water/gas relative permeability curves is interpreted as an increased affinity of the rock to the gas phase. Using a micro-scale proxy model, we identify a leftward shift in pore size distribution and closure of micro-channels to be responsible for the abovementioned observations. These findings prove the crucial impact of effective stress-induced pore deformation on multiphase flow properties of rock, which are missing from the current characterizations of multiphase flow mechanisms in porous media.

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In this study, changes in the hydrodynamic properties of Berea sandstone at a constant temperature of 40°C are reported as effective confining stress is increased to 30 MPa. Through a novel consecutive approach, porosity, absolute permeability, drainage relative permeability, and drainage capillary pressure were shown to systematically change with effective stress. The relative permeability measurements were taken using a steady-state method for the N 2 /water fluid pair. A second method, in which a saturated core with the wetting phase was flushed with the non-wetting phase at an increasing flow rate, was used to determine the drainage capillary pressure. Core saturation was determined using a gas separation unit and mass-balance considerations. This study revealed a decrease in porosity and absolute permeability, from 13.16% and 58 mD to 12.24% and 36 mD, respectively, with the increase in effective confining stress. In terms of relative permeability curves, this study showed a systematic decrease in irreducible wetting phase saturation from 0.44 to 0.24 as effective stress increased; this could be interpreted in core-scale as a progressive tendency of the initially water-wet Berea sandstone to the gas phase. The capillary pressure curve also presented an upward shift in response to increased effective stress. These changes in the hydrodynamic rock properties with stress suggest that scaling flow parameters of porous media under effective stress conditions may be required to accurately predict flow behavior under conditions of changing stress.