Prediction model for gas adsorption capacity of the Lower Ganchaigou Formation in the Qaidam Basin, China

Adsorbed gas is prevalent in shale, especially in plays in China. Quantitatively predicting the adsorbed gas content is significant for resource assessment and selection of favorable areas for shale-gas production. This paper focuses on methods to optimize dominant factors and establish a prediction model for shale gas adsorption capacity, considering the Palaeogene Lower Ganchaigou lacustrine shale in the western Qaidam Basin in China as an example. Using total organic carbon (TOC), Rock-Eval, X-ray diffraction, N2 adsorption measurement, and methane isothermal adsorption assessments, the basic characteristics of a shale reservoir were analyzed. Through the obtained data, the relations among shale gas adsorption capacity and TOC, maturity, mineral composition, temperature, and pressure were determined. Gas adsorption capacity has an evident relationship with TOC and quartz content. However, no apparent correlation was observed in the presence of clay minerals, indicating that clay minerals do not contribute significantly to gas adsorption capacity in the shale samples studied. Gas adsorption capacity also varied with temperature and pressure. Based on these variations, several principles were established for the selection of dominant factors that are easy to obtain, have evident relationships with adsorption capacity, are supported by sufficient data, and accurately reflect regularity. TOC, quartz content, temperature, and pressure were eventually optimized through multiple regression analysis to establish a prediction model. The predicted results were compared with actual measured results, yielding a correlation coefficient as high as 0.9192. Although the geological conditions of the Lower Ganchaigou shale do not completely meet the standards expected from a typical case, the obtained function and research process can be applied to any play to illustrate the impacts of pressure and temperature regimes on the gas adsorption capacity of shales with varying abundances of organic matter and matrix mineral compositions.