Supercritical methane adsorption on shale gas: Mechanism and model

Abstract

Shale gas is different from conventional natural gas stored in sandstone and carbonate formations because the shale formation is often both the source and the reservoir of the natural gas itself. The adsorbed gas accounts for 20%–85% of the total amount based on current studies in five major shale formations in the United States. Studying on the mechanism and model of shale gas adsorption is of great importance for reserve evaluation and making development plan. The micropore structure and isothermal adsorption curves of Longmaxi shales were analyzed by low-temperature N2 adsorption and high-pressure CH4 adsorption experiments. The results show that its specific surface area and pore volume are mainly controlled by mesopores (2–200 nm). The excess adsorption capacity ( n ex) increases to a maximum value at the pressure of approximate 10 MPa, and then begins to decline with pressure. This abnormal phenomenon has also been observed in some previous studies, but most researchers stated that the adsorption isotherm of methane in shale monotonically increased with pressure and reached a constant value at a high pressure (i.e., type I isotherm). The main reason for these two different types of adsorption isotherms is whether the volume of the adsorbed phase is assumed to be negligible. To simulate the enrichment and production of methane in shale gas reservoirs, an accurate model of gas adsorption is needed. The most commonly used adsorption model for shale gas reservoirs is the classic Langmuir equation and its extended Langmuir-Freundlich (L-F) equation, which assumes that methane is a monolayer adsorbate and the surface of the solid adsorbent is homogeneous with constant adsorption heat. Many physical adsorption phenomena can be described by the Langmuir model, including coalbed methane adsorption, but the assumptions are too ideal to match the complicated situation of shale gas reservoir. Several multilayer adsorption theories also have been proposed and applied, such as the Dubinin-Radushkevich (D-R) and the Dubinin-Astakhov (D-A) equations based on micropore filling mechanism. The D-R and D-A adsorption model have been widely used in methane adsorption on shale because of its quite good fitting quality. The comparative analysis of the four commonly used adsorption models shows that the fitting effect of D-A model is better than D-R model for micropore filling mode, and the fitting effect of L-F model is better than Langmuir model for monolayer adsorption mode. Further through the two hypotheses, it is proved that the adsorption mode of methane is neither single micropore filling nor single monolayer adsorption, and the adsorption mechanism is the coexistence of micropore filling and monolayer adsorption. Based on the adsorption mechanism, a new model of supercritical adsorption of shale gas—DA-LF model was proposed. The new model has better fitting effect than the commonly used models, and can calculate the adsorption amount of micropores and mesopores, respectively. The results show that the adsorption capacity of micropore filling is larger than that of monolayer adsorption, which accounts for about 76% of the total adsorption capacity, indicating that the supercritical adsorption mechanism is mainly micropore filling and monolayer adsorption coexists.