Inversion of Effective Pore Aspect Ratios for Porous Rocks and Its Applications

AbstractBy integrating Gassmann equations with the dry rock frame model, which is called the DEM analytical model, derived from the differential effective medium theory, this paper presents a method for pore structure evaluation and shear wave velocity prediction from the effective pore aspect ratios inverted from P‐wave (and S‐wave) velocity. First, we build a link among P‐wave and S‐wave velocity, density, porosity, saturation and mineral compositions through Gassmann equations and the DEM analytical model. Then, by assuming that the porous rock contains spheroidal pores with a single aspect ratio, we apply nonlinear global optimization algorithm to find the best estimate for the effective pore aspect ratio in terms of minimizing the error between theoretical predictions and experimental measurements. Finally, we estimate the S‐wave velocity from the inverted effective pore aspect ratios. The experimental and well logging examples show that the inverted aspect ratio can be used to accurately indicate the pore structure of the reservoir. For fracture reservoirs, such as granite, its pore aspect ratio is typically less than 0.025, and for pore reservoirs such as sandstone, the pore aspect ratio is usually greater than 0.08. Comparison of the inversion of pore aspect ratio between only from P‐wave and from both P‐wave and S‐wave indicates that the pore aspect ratios from these two methods are almost the same. And, the predicted shear‐velocity only from P‐wave is in good agreement with the measured one. These results show that our method of the shear‐velocity prediction by using the pore aspect ratio inverted from P‐wave is effective.