Study Of Sandstone Reservoir Porosity Inversion Considering Pore Structure

Rock porosity is the main factor affecting the propagation of seismic waves in rock media.However,many years of academic research have shown that the equivalent elastic properties of rocks with different pore structures and the same porosity vary greatly.Therefore,rock pore structure is also one of the main factors affecting the equivalent elastic properties of saturated rock.In the reservoir prediction process,considering the influence of rock pore structure will improve the prediction accuracy of the reservoir.At present,in petrophysics,there is no specific definition of the physical meaning of rock pore structure.Therefore,in recent years,experts have defined the pore structure according to the specific conditions of the work area.Therefore,this paper defines the pore structure from the aspect of pore shape,and then obtains the porosity inversion formula with pore structure parameters.The porosity inversion formula improves the identification of sandstone reservoirs by introducing geophysical response characteristics of rock pore structures in rocks.Therefore,the inversion theory provides a theoretical basis for the application in sandstone reservoirs.In this paper,the variation of seismic wave propagation velocity in rock is studied by porosity and pore structure.This law proves that both pore structure and porosity are the main factors affecting the propagation velocity of seismic waves.In petrophysics,the Eshelby-Walsh theoretical model compares the pore shape of rock to an ellipsoidal inclusion,defining the pore structure of the rock influenced by Poisson's ratio and flattening.This paper uses this feature to describe the pore structure of sandstone.The forward results of rock pore structure of sandstone reflect that rock pore structure is independent of rock porosity and inversely proportional to pore flatness.Combined with well logging data,this paper analyzes the variation of seismic wave velocity in sandstone reservoirs with three single-phase fluids(gas,oil,water).It can be seen from the forward results of sandstone reservoirs that when the pore structure does not change,the longitudinal wave velocity decreases as the porosity increases.When the sandstone model is full of gas,the seismic wave velocity is greater than the wave velocity when the oil is saturated,and the wave velocity when the oil is saturated is greater than the wave velocity when the water is saturated.It can be seen from the figure that when the porosity is small,the difference of the seismic wave velocity of saturated water,oily and saturated gas is small.As the porosity increases,the difference in seismic wave velocity of saturated,oily,and saturated gas increases.When the pore structure does not change,the longitudinal wave velocity decreases as the porosity increases.When the porosity does not change,the longitudinal wave velocity decreases as the pore structure increases.The equivalent velocity of different saturated fluid rocks changes at the critical point of the pore structure.When the pore structure is smaller than this critical point,the longitudinal wave velocity of different saturated fluid rocks is reversed.When the pore structure is larger than this critical point,different saturated fluid rocks the speed relationship has returned to normal.In this paper,the Gassmann equation is used to replace the actual log data,and the variation law between pore fluid properties is analyzed.The result of fluid replacement reflects that the density of saturated gas is smaller than that of water containing rock in the case of a certain porosity and pore structure.The volumetric modulus of saturated rock is smaller than the value of saturated water,and the curve of saturated oil is in the middle..The equivalent velocity of replacing the oil-filled rock is smaller than that of the saturated gas-bearing rock,and the replacement result is consistent with the forward model of the sandstone reservoir model.By simplifying the Eshelby-Walsh theory and the Gassmann equation,this paper introduces the porosity calculation formula of the pore structure parameters.Taking the seismic data of actual sandstone reservoirs as an example,the porosity inversion formula is used to predict the porosity of sandstone reservoirs.By comparing the inversion curve with the corresponding logging curve,the inversion porosity results are well matched with the logging porosity,and the effectiveness of the method is verified.