Effect of Pore Microstructure and Fluid Type on the P-wave Velocity of Tight Sandstones

Unlike conventional high-porosity and high-permeability sandstones with relatively simple pore geometry, tight sandstones usually exhibit heterogeneities in the solid frame and the pore structure at different scales. A dual porosity structure was often used to describe the microstructure of the pores in tight sandstones. We measured the P-wave velocities of 9 tight sandstones at different saturation degrees with water and oil under varying confining pressure, using the ultrasonic transmission technique. We analyzed the aspect ratio distribution of the tight sandstone using the velocity-pressure trend data following the method of Deng et al. (2015) . Gassmann fluid substitution and squirt flow mechanisms were both considered in the interpretation of the velocity data in the oil-and water-saturation conditions. The effect of the squirt flow model must consider the effect of the complex pore microstructure represented by a distribution of aspect ratio. Gassmann fluid effect fails to explain the velocity changes as the fluid is replaced. We find that the dual porosity structure of tight sandstones not only affects the velocity of the dry rock, but also determines the velocity variation due to fluid flow mechanisms when the tight sandstone is saturated with different fluids.