Experimental And Theoretical Study Of Partially Saturated Fluid Tight Sandstones

Unlike conventional high-porosity and high-permeability sandstone reservoirs with relatively simple pore structures,the characteristics of tight sandstone reservoirs,including rock heterogeneity,microscopic pore structure and pore fluid distribution,are quite complex,and the main influencing factors of elastic parameters unclear.These characteristics make conventional rock physical models unable to accurately simulate the experimental results of tight sandstones,and cause difficulties in seismic prediction of tight sandstone reservoirs.In this paper,a combination of petrophysical experiments and theory is used to study the petrophysical properties of tight sandstone reservoirs,which is of great significance for the fine division of tight sandstone reservoirs.First,the established ultrasonic experiment platform was used to measure the ultrasonic P-and S-wave velocities of tight sandstone samples under different effective pressures,different fluid types and saturation conditions.Through experiments,it is found that for tight sandstones,the microscopic pore structure is the main controlling factor for the velocities change.With the increase of pressure,the soft pores with small pore aspect ratio are gradually closed,and the influence caused by the pore structure is reduced.The P-wave velocity changes with the saturation as a step-like change.The P-wave velocity first increases steadily,then increases sharply,and then increases steadily.The S-wave velocity also increases with the increase in saturation,which is not consistent with the conclusion of Gassmann theory.Low frequency experimental results show that as the pressure increases,the velocity dispersion in the seismic frequency band gradually decreases.Secondly,a double-scale rock physical model was established based on the results of the partially saturated tight sandstones measured by the experiment and the dual pore structure of the tight sandstone.The model considered the continuous pore aspect ratio spectrum of the tight sandstone.The Brie model was used to simulate the fluid distribution in the soft pores of tight sandstone,so that the squirt flow model could simulate the fluid flow effect in partially saturated samples.The double-scale rock physical model has a good fitting effect on tight sandstones.The simulation results show that the velocities change of tight sandstone is caused by the interaction of local flow and patch saturation.As the pressure increases,the squirt flow effect weakens.Patch saturation is the main controlling factor,in which the shear wave velocity with the increase of saturation is mainly affected by the squirt flow.In the seismic frequency band,the change of elastic parameters of tight sandstone saturated with fluid is mainly controlled by the pore structure and fluid effect.