Relative permeability and the elastic properties of partially saturated sandstones

Counter-intuitive behaviour in the elastic properties of partially saturated rocks has been observed in laboratory experiments where the rock appears softer when the ratio of stiffer-to-softer fluids residing in its pore space is increased. Such effects can be explained theoretically by an associated dispersive wave propagation mechanism in which fluid flows locally between pores, leading the rock to behave like a viscoelastic solid. The weakening due to the increased volume of stiff fluids can be caused by the lowering of relative permeability which takes place when immiscible fluids are injected into the pore space. This lowering affects the characteristic frequency range where the rock behaves as a viscoelastic solid. In this abstract, we discuss this phenomenon in conjunction with the relevant scales: pore, fracture size, saturation patch and multi fluid Darcy flow taking place between pores, compliant fractures and microcracks.