Geochemical characteristics of terrestrial shale gas and its production prediction significance in the Ordos Basin, China

In order to study the variation of shale gas composition as well as the carbon isotopic fractionation characteristics of terrestrial shale in the Ordos Basin, and ultimately establish a prediction model for shale gas production in order to monitor production capacity and effectively manage the development of shale gas, selected core samples from production wells were obtained for desorption analyses of their corresponding gas composition and carbon isotope. The desorption results showed that the methane content of the desorption gas is relatively low, the drying coefficient (C1/C1-5) ranges from 0.6 to 0.8, and the carbon isotope is relatively negative compared to other places. By means of the increasing desorption and production time, the drying coefficient and carbon isotope values of such samples show obvious carbon fractionation during gas adsorption/diffusion processes. The changes are more evident with increasing heavy hydrocarbons (C2+) and heavier carbon isotope values. The drying coefficient shows good correlation (R2 > 0.8) with the content of shale gas desorption per unit mass. A gas production prediction model was proposed to predict gas production throughout the life of the production well based on the relationship between the drying coefficients and gas production volume in the Ordos Basin.