A new method for determining the equivalent permeability of a cleat dominated coal sample

A simplified flow model, rather than a complex geological model, is usually used for yield prediction and history matching in petroleum industry. The gap between these two models is often bridged in practice by upscaling. Equivalent permeability is an extremely significant parameter for upscaling a coal geological model. However, how to obtain an accurate equivalent permeability is still a scientific challenge because of the unique characteristics of coal samples (e.g. complicated cleat networks and extremely low matrix permeability). In this paper, three existing traditional mean methods, which are widely applied for calculating the equivalent permeability of conventional oil and gas reservoirs, are firstly tested with a random distributed permeability model and compared with finite element method-based numerical analysis to evaluate their potential applicability in various cases. Then, based on the streamline method and the flow-based upscaling technique, a new empirical formula is proposed with statistical algorithm to determine the equivalent permeability of naturally fractured coal samples and verified with numerical analysis of three different coal samples. Finally, the proposed formula is applied to provide the coal permeability of subsections for further finite element analysis of the complicated fluid flow behaviours in a coal block of Guluguba well 2 in Surat basin, Australia. The related application results demonstrated the usefulness of the proposed method.