Influences of Primary Moisture on Methane Adsorption within Lower Silurian Longmaxi Shales in the Sichuan Basin, China

Abstract

Moisture significantly affects the adsorption capability of gas shales for methane (CH4), the main component of shale gas. Primary moisture, i.e., the moisture that exists in in situ shale reservoirs, is therefore crucial to estimate and produce shale gas resource. Aiming to understand the influences of primary moisture on CH4 adsorption on shale samples, the occurrence of primary moisture in shales gathered from the Lower Silurian Longmaxi Formation located in southern Sichuan Basin of China was experimentally investigated. Additionally, the primary moisture dependence of CH4 adsorption equilibrium and thermodynamics of shales was investigated. Results indicate that the primary moisture contents of the four shale samples vary between 0.64 and 0.82% (mass percentage), positively correlated with the clay mineral content of shale samples. The adsorption equilibrium behavior regarding water vapor on shales well follows the modified Brunauer–Emmett–Teller (BET) equation. The water vapor is typically adsorbed onto the primary adsorption sites of shale samples, i.e., the oxygenic functional groups consisting of COOH, conjugated CO, and highly conjugated CO, and the secondary adsorption sites, i.e., the previously adsorbed water molecules and clay minerals. The pores with pore diameter less than 4 nm of shales are the main accommodation space for primary moisture. The adsorption equilibrium of CH4 on primary moisture-containing shales well obeys the Ono–Kondo lattice equation. On the basis of the modeling results, the primary moisture causes a remarkable reduction in maximum CH4 adsorption capacity of shale samples by 12.86–45.45%. Moreover, the primary moisture reduces the isosteric heat of CH4 adsorption on shale samples. In summary, the primary moisture in gas shales decreases the adsorption affinity between CH4 and shale samples. Therefore, focusing on the effects of primary moisture on CH4 adsorption on shales is vital to better estimate and produce shale gas resource.

Impacts of Long-Term Exposure to Supercritical Carbon Dioxide on Physicochemical Properties and Adsorption and Desorption Capabilities of Moisture-Equilibrated Coals

“…In general, the scCO 2 –H 2 O exposure alters the CO 2 adsorption capability of all of the coals. To further investigate the effect of scCO 2 –H 2 O on the CO 2 adsorption capability of coals and to better explore the equilibrium adsorption behavior, in this study, we applied the Ono–Kondo lattice model provided with high predictivity for reservoir fluid adsorption equilibrium. , …”

“…To further investigate the effect of scCO 2 −H 2 O on the CO 2 adsorption capability of coals and to better explore the equilibrium adsorption behavior, in this study, we applied the Ono− Kondo lattice model provided with high predictivity for reservoir fluid adsorption equilibrium. 19,77 The simplified Ono−Kondo equation is 78 Energy & Fuels 12 and Table 6, respectively. The modeling curves agree well with the experimental data.…”

Role of H2O of Gas-Bearing Shale in Its Physicochemical Properties and CH4 Adsorption Performance Alteration Due to Microwave Irradiation

“…As illustrated in Figure a, microwave irradiation promotes rapid increase in temperature of the shale matrix, therefore significantly promoting physically adsorbed CH 4 desorption from shale. , Furthermore, microwave irradiation decreases adsorption pores for accommodating CH 4 of the dry shale matrix with a diameter below 10 nm, while it increases meso- and macropores with a diameter greater than 10 nm (Figure b) . Such transformation in full-scale pores of a dry shale matrix can not only promote CH 4 desorption but also strengthen diffusion capability of previously desorbed CH 4 within gas-bearing shale reservoirs. , In addition, the shale matrix contains a wide variety of inorganic minerals mainly including nonclay minerals, such as quartz, pyrite, calcite, feldspar, dolomite, and plagioclase, and clay minerals, such as illite and smectite. , The dielectric properties of typical inorganic materials of shale are listed in Table . , As shown in eq , the dielectric constant is measured by the real part of relative permittivity, indicating how much energy can be stored in the material, and by the loss factor, that is, the imaginary part of the relative permittivity, indicating the ability of converting the stored energy into heat. Among these minerals, pyrite, illite, and smectite with a greater dielectric constant and loss factor always show better microwave heating performance .…”

“…It is worth noting that the actual gas-bearing shale reservoir contains primary moisture. , Numerous studies have confirmed that H 2 O shows remarkable impact on CH 4 adsorption, desorption, and migration within the reservoir. Given the strong electric dipole and high polarity of the H 2 O molecule, the adsorption strength of H 2 O on shale is higher than that of CH 4 .…”

“…Given the strong electric dipole and high polarity of the H 2 O molecule, the adsorption strength of H 2 O on shale is higher than that of CH 4 . In addition, formation of H 2 O clusters and capillary condensation of H 2 O often occur in the mesoporous space of the shale matrix . The aforementioned two aspects weaken CH 4 adsorption performance of the shale matrix.…”