A New Gas-Content-Evaluation Method for Organic-Rich Shale Using the Fractionation of Carbon Isotopes of Methane

Abstract

Summary Gas content is a key parameter for the determination of the gas resources in unconventional reservoirs. In this study, we propose a novel method to evaluate the gas content of shale through a new perspective: fractionation of carbon isotopes of methane. At first, a bicomponent gas–convection/diffusion/adsorption model (BG–CDAM) is successfully built with consideration of the flow and adsorption difference between 13CH4 and 12CH4 in the nanoporous shale. A detailed understanding of the mechanism of fractionation of the isotopes is obtained for the first time and the Knudsen–diffusivity difference is identified as the dominant factor for fractionation according to the different molecular weights of the isotopes. Then, the simulated results of BG–CDAM and the measured data from an isotope–logging test are combined to determine the unknown parameters for gas–content calculation. The proposed method for organic shale is valid and useful to obtain the quantitative component proportion in gas content, such as lost gas, degassing gas and residual gas, or free gas and adsorption gas. Thus, this method could provide a promising means for the identification of sweet spots in shale–gas reservoirs. Moreover, the method might have the potential to economically and rapidly evaluate the remaining resources in producing wells in future applications.