Importance of Fabric on the Production of Gas Shales

Abstract

Abstract The heterogeneity and complexity of gas shales cause substantial and often inexplicable variability in the production histories of gas wells. A major factor contributing to this variability is the microfabric of the matrix and the fracture network of the reservoir. It is widely postulated, although not proven, that the gas production from shales is controlled principally by Darcy flow through the fracture system and the matrix is considered important principally for gas storage. In order to gain insight and test the relative importance of fracture spacing and matrix diffusion/flow on the production of gas shales, we have developed a 2-dimensional numerical simulation model, which considers the flow of gas through both the shale matrices and the fractures for varying fabrics utilising experimental data obtained from a variety of important gas shales. The results of initial, constant parameter, numerical simulations showed that for a wide range of relative fracture permeability, matrix permeability/diffusivity and fracture spacing, the productivity of a gas shale reservoir is dependent on matrix diffusion rates. The diffusion rates and stress dependent fracture permeability data when integrated into the numerical simulator can be tested against measured production histories leading to more accurate production forecasts of new reservoirs and optimisation of field design. Introduction The production histories of gas shale reservoirs vary substantially and often inexplicably even between wells within a field with similar reservoir permeability and gas content (i.e. Luffel et al., 1993; Hill and Nelson, 2000; Bustin, 2005). One important factor, that is not generally considered, but may attribute to this variability is the fabric of the rock, which controls the fracture porosity and permeability for gas and water flow as well as the efficiency of gas diffusion/flow out of the matrix. In order to test the relative importance of shale fabric on the performance of shale gas wells, we have developed a 2-dimensional (D) numerical simulation model which uses both experimental and field data as model input parameters. Understanding the complex interaction between the heterogeneity of gas shale properties and their producability will allow more accurate predictions of the production history, which is critical to economic evaluations and in the design of development programs.