Abstract
Deep marine shale gas reservoirs are extremely rich in the Sichuan basin in China. However, due to the in situ conditions with high temperature and high pressure (HTHP), in particular reservoir pressure being usually much higher than the test pressure, it is difficult to accurately clarify the adsorption behavior, as seepage theory plays an important role in shale gas reserves evaluation. Therefore, three kinds of sorbent, including illite, quartz and kerogen, and two simulation methods, containing the grand canonical ensemble Monte Carlo method and molecular dynamics method, are synthetically used to determine the methane adsorption behavior under HTHP. The results show that both absolute adsorption and excess adsorption decrease with the increase of temperature. When the pressure increases, the absolute adsorption increases quickly and then slowly, and the excess adsorption first increases and then decreases. The superposition of wall potential energy is strongest in a circular hole, second in a square hole, and weakest in a narrow slit. The effect of pore size increases with the decrease of the pore diameter. Under HTHP, multi-layer adsorption can occur in shale, but the timing and number of layers are related to the sorbent type.
Highlights
Shale gas is an important energy resource, and it is mainly stored with free gas and adsorbed gas in shale reservoirs, with adsorbed gas accounting for 20~85% [1,2]
Xiong et al [23] used the grand canal ensemble Monte Carlo method (GCMC) to study the adsorption characteristics of methane in quartz, and the results showed that the gas-solid interaction decreases with the increase of pressure or the decrease of pore size, and the adsorption site energy of methane is from high to low
Given the complex shale reservoirs conditions, adsorption characteristics of methane at high temperature and high pressure (HTHP) are revealed by using molecular simulation with GCMC and DM
Summary
Shale gas is an important energy resource, and it is mainly stored with free gas and adsorbed gas in shale reservoirs, with adsorbed gas accounting for 20~85% [1,2]. The results showed that the absorption capacity is superior to that of the adsorption at lower pressure, and becomes low if the pore pressure is higher than 3.5 MPa. Sun et al [29] proposed a new method to construct case-based organic nanopores, and studied the flow behavior of methane in different pore sizes and kerogen types under formation conditions by using the molecular dynamics method. The adsorption of carbon dioxide and methane on the surface of shale was studied by MD and Monte Carlo (MC) methods under drying conditions with temperature being 313 K and pressure ranging from 0.01 MPa to 17 MPa. The results showed that the adsorbed capacity of carbon dioxide is significantly higher than that of methane, and the volume of adsorbed gas is significantly more than that of free gas in the shale. This work systematically studies the adsorption mechanism of shale gas at HTHP, and makes adsorption features for deep shale gas reservoirs more definite
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