A novel model to determine gas content in naturally fractured shale

Abstract

A shale reservoir’s gas content is an indispensable, crucial parameter used in exploration, formation evaluation, block selection and exploitation. Canister gas desorption measurements are used to quantify lost gas volumes that are key to determine the gas content of the reservoir Usually, a lost gas volume is evaluated using the United States Bureau of Mines (USBM) direct method and/or a polynomial curve-fitting method. These methods extrapolate desorption data to time zero in the curve of a cumulative desorption volume versus the square root of time. Lost gas time is the time spent lifting the core out of a hole, handling and placing it inside a canister for evaluation., For cores with densely developed natural fractures, neither the USBM method nor the polynomial curve-fitting method can accurately estimate the lost gas volume if the lifting, handling and canister storage takes a long time. The lost gas period is a non-isothermal gas desorption process; the surrounding temperature keeps declining. Experiments were conducted to determine the methane desorption volumes of a Duvernay shale sample under various pressures and temperatures. Using Boyle’s law and the multilayer adsorption model, a novel mathematical model was built to calculate lost gas volume, including free gas and desorption gas. Because gas desorption is an endothermic procedure, equations for the inhaled energy were developed to overcome the van der Waals force between the molecules, deorbit the vibration equilibrium and the migration of molecules into the air. Finally, a shale sample with dense natural fractures from the Duvernay formation of Alberta, Canada, was used to validate the mathematical model. For comparison, the results from the USBM method, polynomial curve-fitting method and model’s calculations are illustrated in tables and figures. The outcomes exposed the results from the USBM method and polynomial curve-fitting method as deviating from the real gas content in the Duvernay formation without the consideration of the free gas loss. The model is reliable for performing gas content evaluations because it considers dual-pore structure, PVT behaviour and free gas escape simultaneously.