Direct method to estimate the gas loss characteristics and in-situ gas contents of shale

Abstract

The commonly used direct methods for the lost gas and in-situ gas content estimation of shale usually follow the assumptions and mathematical models that were originally established for coalbed. The essential differences in formation fluid compositions, coring and on-site desorption processes, governing equations on gas release, and model parameters between shale and coal are not comprehensively analyzed for these available direct methods, therefore, they cannot fully reflect the gas loss characteristics of shale and are also inaccurate for geological reserves calculation. In this paper, the modified lost gas content calculation method is constructed by analyzing the general coring and on-site desorption processes of shale. This method simultaneously depicts the loss characteristics of free and adsorbed gas. It can be summarized that the initial adsorbed gas concentration, time-dependent gas diffusion coefficient, time-dependent desorbed gas concentration at the core boundary, core size, porosity, methane saturation, formation temperature, and formation pressure are the influencing parameters for accurate in-situ shale gas content calculation. Herein, the estimation or optimization methods for some of these parameters are presented. The fitting results manifest that the newly presented method shows satisfactory applicability in gas loss curve prediction and in-situ gas content estimation based on single and twice heating on-site desorption data. It is found that some existing direct methods may exhibit anomalies or stability problems in shale gas loss curve characterization, therefore, the assumption reasonability and mathematical-physical law consistency of these methods should be further confirmed. By contrast, the assumptions of the new method are closer to the practical shale coring, especially the simultaneous loss of free and adsorbed gas as well as the introduction of time-dependent parameters, laying the foundation for a more accurate gas loss curve prediction and geological reserves calculation for shale.