The presence of water in narrow pore spaces affects the occurrence and flow of methane, which in turn affects shale gas production. Therefore, studying the occurrence and distribution characteristics of water is of great significance to predict gas production. Based on molecular dynamics simulations, this study investigated the occurrence characteristics and influencing variables of liquid water in kaolinite nanopores in situ. Owing to its widespread distribution, kaolinite is the most prevalent clay mineral with two surfaces with different characteristics. Three systems of pure water, a CaCl 2 solution, and a H 2 O/CH 4 mixed phase were created at varied temperatures (80−120 °C) and pressures (70−120 MPa). The presence of gas and water in the nanopores was investigated thoroughly. The results showed that the adsorption of water on the Al−O octahedral surface of kaolinite was not affected by external conditions under in situ conditions, whereas the adsorption of water on the Si−O tetrahedral surface decreased with increasing temperature, but the change was small. When ions were present in the system, the water capacity decreased. Based on the aforementioned results, external conditions, such as temperature and pressure do not affect the basic state of water. However, if there are more than two fluid types in the system, the adsorption of water on the mineral surface is reduced owing to competitive adsorption. In addition, a CH 4 −H 2 O mixed system was simulated, in which methane molecules were distributed in clusters. There are two types of adsorptions in pores: gas−solid interactions and solid−liquid−gas interactions. CH 4 molecules are thought to be clustered in water molecules because of the strong hydrogen bonding interactions among the water.
Adsorbed gas ratio and gas-in-place (GIP) content are two critical parameters for determining shale gas resources and devising development plans. Existing traditional methods have their limitations in solving the above issues, while a novel approach for evaluating the aforementioned key parameters was provided by the carbon isotope fractionation (CIF) model. To investigate the stable carbon isotope fractionation and shale gas content characteristics during the degassing process, 10 shale samples from Well JY A in the Jiaoshiba area of the Sichuan Basin were chosen for on-site degassing tests. The on-site degassing patterns were recorded, and the methane, ethane, and carbon dioxide isotope values were measured. The CIF model was used to evaluate the gas degassing patterns and isotope fractionation characteristics of the complete degassing process and accurately assess the in situ gas-bearing parameters of the Well JY A shale. Research has shown that during a 20 h degassing process, the methane fractionation magnitude can reach 10‰, and the ethane fractionation magnitude is about 2‰. There are no significant fractionation characteristics for carbon dioxide. The complete cumulative degassing curve exhibits a "downward convex" shape followed by an "upward convex" shape, while the complete isotope fractionation curve shows a three-stage pattern of "initially stable, then lighter, and finally heavier". Free gas diffuses first, followed by the desorption of adsorbed gas, which lags behind significantly, and the adsorbed gas ratio controls the apparent carbon isotope fractionation. Initially, diffusion fractionation dominates, while later, adsorption−desorption fractionation gradually becomes dominant. Using the CIF model, the GIP content of shale in the Well JY A ranges 2.52−5.11 mL/g, averaging 3.56 mL/g, and the adsorbed gas ratio accounts for 4.39%−30.28%, averaging 14.64%. Using the U.S. Bureau of Mines (USBM) method, the GIP content ranges 1.49−4.14 mL/g, averaging 2.69 mL/g, which is significantly underestimated in this area.
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