Dynamic effect in capillary pressure during displacement process in ultra-low permeability sandstone reservoirs

The relationship between the capillary pressure and saturation is vital to describe and simulate the multiphase flow. This relationship is also fundamental to understand characteristics of oil and gas reservoirs and make the reservoir development plan. Traditionally, the capillary pressure is measured under the equilibrium process; however, such equilibrium is hard to establish when the multiphase flow is expected in low permeability porous media, and the capillary pressure is dynamic. This work conducts specially designed dynamic displacement experiments to examine the dynamic effect in capillary pressure in ultra-low permeability sandstone oil reservoirs. The dynamic capillary pressure, the dynamic relative permeabilities, the dynamic coefficient and the time derivative of water saturation are obtained. Results show that the dynamic effect in capillary pressure of ultra-low permeability sandstone cannot be neglected during the reservoir evaluation and development process. Difference between the dynamic and the steady capillary pressures becomes more significant for less permeable porous media, with a higher dynamic coefficient and a stronger dynamic effect. Wettability advancement has been triggered during the dynamic displacement process, the water-wet rock seems to turn from water-wet before the displacement to be oil-wet during the displacement process. The difference between the dynamic and the steady relative permeabilities becomes obvious, and the dynamic effect of capillary pressure cannot be neglected when the permeability reaches ultra-low. The dynamic coefficient can reflect the uniform of the displacement.