Inclusion-based rock-physics modelling approach with connected-porosity coefficient estimation for shale reservoirs

Rock-physics modelling provides theoretical basis for predicting elastic and anisotropy parameters from petrophysical properties. However, shale rocks usually develop complex pore structures, wherein isolated and connected pores or cracks may coexist. Conventional methods that assume either isolated or connected pores have limited applicability to shale reservoirs. To this end, this work proposes a shale rock-physics modelling method to address pore complexities. In specific, the proposed method combines inclusion-based and Brown–Korringa models to consider both isolated and connected pores in shales. Connected-porosity coefficient is introduced in the modelling to balance the effects of the two pore types. To better handle pore complexities and improve modelling accuracy, the coefficient and pore aspect ratio are jointly estimated from measured vertical P- and S-wave velocities with a global optimization algorithm. Numerical analysis is performed to analyse the general effects of connectivity and pore geometry on elastic properties of shales. The proposed method is applied to a well data from the Longmaxi shale reservoir in southwest China. The method is also compared with two other methods to show its capability of predicting elastic properties with satisfactory accuracy. The estimated connected-porosity coefficient also facilitates the characterization of velocity anisotropy to some degree.