Controlling Effect of Shale Pore Connectivity on the Adsorption State of Shale Gas: A Case Study of Continental Shale of Jurassic Ziliujing Formation, Sichuan Basin

The pore connectivity of shale directly affects the migration and distribution of oil and gas within shale reservoirs and controls the adsorption state of shale gas by determining the accessible pore spaces for methane molecules. However, previous studies mostly focused on the surficial properties, pore size, and specific surface area, ignoring the significant impact of pore connectivity on the adsorption state of shale gas. Accordingly, taking the continental shale of the Jurassic Ziliujing Formation from the northeastern Sichuan Basin as the research object, this study mainly investigates the effects of shale pore connectivity on the adsorption state of shale gas by conducting methane adsorption and field emission scanning electron microscopy experiments together with image processing software. The results show that the methane adsorption capacity per unit specific surface area of type III kerogen is higher than that of type II2 kerogen, indicating type III kerogen has a stronger affinity to methane. However, the total methane adsorption capacity of type II2 kerogen in Ziliujing Formation shale samples is about four times that of type III kerogen, which is due to the much higher specific surface area, providing more methane adsorption sites. The development of OM and clay minerals in the shale of the Ziliujing Formation has a certain orientation and promotes the pore connectivity of the shale. Furthermore, a better pore connectivity of shale samples, which is usually accompanied by more developed microfractures of OM and clay minerals, higher roughness of pore surface, and stronger randomness of pore arrangement, results in a stronger methane adsorption capacity. Based on the close relationship between pore connectivity and methane adsorption capacity, a new pore connectivity index is proposed, considering the difference between the actual and the theoretical methane adsorption capacity of shale samples. Finally, a shale methane adsorption model is established based on pore connectivity.