Effect of effective pressure change on pore structure and elastic wave responses in tight sandstones

Tight sandstone gas reservoirs develop cracks and are subject to high effective stresses. The analysis of seismic wave propagation in media containing pores and cracks as a function of differential pressure is important for seismic interpretation and reservoir characterization. However, the predecessors seldom consider the relationship between the microscopic pore structure of the rock and the fluid flow between pores and cracks. In this paper, ultrasonic wave velocity measurements are performed on tight sandstones from the Sichuan Basin under different differential pressures. Crack parameter inversion based on experimental P- and S-wave velocities is performed, and the crack porosities of tight sandstones under different differential pressures are obtained. The crack porosities and the rock physical parameters are used in the double-porosity media model, and the variation of P-and S-wave velocity dispersion and attenuation under the different differential pressures of the water-saturated tight sandstone samples are modeled. The P-wave velocity dispersion curves predicted by the model agree well with the experimental data. By analyzing crack parameters of tight sandstones, the variations under different effective pressures and porosities are obtained. The 3D fitting results of crack parameters based on the actual rock physics parameters describe the relation between crack structure of tight sandstones and porosity and stress, which can support the prediction of reservoir crack properties in actual seismic exploration.