Equivalent anisotropic permeability of shale rocks: Effect of micro-fractures

Abstract

The extensive opening-mode micro-fractures, including horizontal bedding fractures and vertical structural fractures, are prevalent in shale rocks. Because of a huge difference with many orders of magnitude in permeability from the nearly impermeable shale matrix and those highly conductive micro-fractures, the micro-fractures are believed to contribute substantially to the macro-scale anisotropic flow properties of shale. Nevertheless, up to the present day, there has been no comprehensive research on this topic. Previous works simply employed a fixed pattern of micro-fractures and did their simulations. In this work, the finite difference method is employed to perform steady-state numerical simulation. The impact of micro-fractures on the flow capacity of shale rocks is characterized by the concept of normalized equivalent permeability. The concepts of bedding thickness, filling spacing and filling ratio are utilized to characterize the distribution pattern of horizontal bedding fractures. The length and number of vertical structural fractures are also considered. The validation of several simplistic scenarios against known results guarantees the correctness and accuracy of the numerical code. Comprehensive sensitivity analysis using realistic ranges of parameters based on literature is conducted for the two scenarios: (i) horizontal bedding fractures only, and (ii) horizontal bedding fractures and vertical structural fractures, moreover, both uniform and random distributions are discussed and then compared. Effects of bedding thickness, filling spacing and filling ratio of horizontal bedding fractures, the length and number of vertical structural fractures, the connectivity between horizontal bedding and vertical structural fractures, and the random distribution attribute of fractures on the anisotropic permeability of shale, including horizontal and vertical components, are discussed. This is the very first work that comprehensively investigates the effect of micro-fractures on the anisotropic permeability of shale rocks, which can help improve the understanding of the flow capacity of shale rocks and even the hydraulic properties of similar kinds of fractured material. Also, it is very useful for modeling the fluid flow to enhance oil/gas recovery in shale reservoirs. • Individual and collaborative impact of bedding and structural fractures are explored. • The influence of the length and number of structural fractures are discussed. • Description of bedding fractures by bedding thickness, filling ratio and spacing. • Effects on the horizontal and vertical permeability components are both studied. • The impacts of several realistic randomnesses are thoroughly determined.