New Coupled Apparent Permeability Models for Gas Transport in Inorganic Nanopores of Shale Reservoirs Considering Multiple Effects

The study of seepage mechanism in shale gas reservoir has been paid more and more attention. The shale gas reservoirs are rich in nanosized pores. The pores in shale matrix can be divided into organic nanopores and inorganic nanopores. At present, there are many literature studies focusing on establishing a model to analyze the gas transport mechanism in shale organic pores. Some researchers also considered the difference of gas transport in organic and inorganic matrix pores, and a mathematical model of inorganic nanopores has been established. However, for inorganic nanopores, most of the models ignore the effect of irreducible water distribution on gas transport, which leads to overestimating of gas transport capability. In this paper, first, based on the weighting coefficient proposed by Wu, the apparent permeability models are established for inorganic nanopores with two different cross-sectional shapes, which are known as cylindrical capillary and slit nanopores. The influence of irreducible water distribution, real gas effect, and stress dependence is also taken into account in the models. Then, the proposed model is verified, and the results show that the model and the experimental data can be well fitted. Finally, the effect of each factor on apparent gas permeability is analyzed and discussed. The results indicate that the apparent permeability of nanopores with different cross-sectional shapes decreases with the increase of relative humidity. When the relative humidity increases to a critical value, the apparent permeability decreases sharply, and the pores will be blocked with capillary water. The gas transport capability in cylindrical capillaries and slit nanopores at the same cross-sectional area is different, and the pore pressure, pore size, effective stress, and aspect ratio of the slit nanopores are important factors affecting the transport process. Under high-temperature and low-pressure conditions, methane transport capacity is significantly higher than those of ethane and carbon dioxide. The results of this paper can provide a reference for researchers in the study of the gas seepage mechanism in shale gas reservoirs.