Study of factors affecting shale gas adsorption by simplified local density-Peng–Robinson method

Abstract

Shale gas has become an important source of natural gas, and a significant portion of gas in the reservoir exists in an adsorbed state. Thus, knowledge of gas adsorption behavior is important to estimate original gas in place and shale gas production. In this paper, shale gas adsorption affecting factors which include temperature, pressure, pore type, and diameters were studied quantitatively and systematically using simplified local density model. It was found that the main affecting factors were temperature, pore type, diameter, and pressure; type I kerogen has the lowest and type III kerogen has the largest excess adsorption gas capacity; due to the interaction energy between gas molecules and pore wall molecules, gas has the highest local density near the pore wall and the lowest density in the middle of the pore under a certain pressure and temperature; the increment of gas density as temperature decreases near the pore wall is significant, but the bulk methane gas density shows a very small change due to temperature change; the ratio of excess adsorption gas to free gas declines and its rate slows down as pressure increases; when the pore diameter increased from 1.5 to 5 nm, the ratio of excess adsorption gas to the total gas amount decreases rapidly; the excess adsorption gas of type III kerogen takes up 89% of the total gas amount with diameter of 1.5 nm and takes 70% of the total gas amount with diameter increases to 5 nm; the excess adsorption gas of type I kerogen takes 71% of the total gas amount with diameter of 1.5 nm and takes up 43% of the total gas amount with diameter increases to 5 nm. Local gas density in different kinds of pores under different conditions also provided a way to calculate gas transport in pores that take adsorption gas into consideration.