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

Abstract

Microwave irradiation is available to produce shale gas. Practical gas-bearing shale reservoirs often contain water (H2O). Given the special physicochemical property of H2O and its occurrence rule in the gas-bearing shale matrix, the presence of H2O has potential effect on electromagnetic energy penetration and dielectric property of the shale matrix, therefore affecting application of microwave irradiation to produce the chief component of shale gas, that is, methane (CH4), from the shale reservoir. Hence, to further explore shale gas production via microwave irradiation, the impact of H2O of gas-bearing shale in its physicochemical property response to microwave irradiation was mainly investigated. The H2O dependence of CH4 adsorption capability alteration due to microwave irradiation was also addressed. The results indicate that microwave irradiation exerts minor impact on the micropore of both dry and moist shales but a noticeable impact on their mesopore. Specifically, both the mesoporous surface area and volume of dry and moist shales decrease after microwave irradiation. The presence of H2O further decreases the typical mesoporous parameters including pore surface area and pore volume of shales in general. Moreover, fractal analysis reveals that both the roughness of the pore surface and complexity of pore structure decrease for all the shales after microwave irradiation, and those of moist shale after microwave irradiation decrease even further, thus facilitating shale gas migration and diffusion within the shale matrix. As for the surface chemical property of shale, the total amount of oxygenic groups consisting of highly conjugated carbonyl, conjugated carbonyl, and carboxyl of dry shales rises after microwave irradiation, but the aromaticity drops. Also, the same changing rule in functional groups is also found for moist shale after microwave irradiation; however, the change is even more pronounced. Finally, the CH4 adsorption capacity of both dry and moist shales decreases after microwave irradiation. In comparison to dry shale, CH4 adsorption capacity of moist shale containing low total organic carbon (TOC) and H2O decreases, but that of moist shale with high TOC content and H2O content increases. These alterations in CH4 adsorption capability of various moist shales are relevant to their pore structure and functional group response to microwave irradiation. Overall, the presence of H2O is beneficial for further weakening the adsorption affinity between CH4 and the shale matrix and strengthening migration capability of CH4 under microwave irradiation, thus making microwave irradiation become a competitive technology regarding shale gas production.

Review of the Effect of Diagenetic Evolution of Shale Reservoir on the Pore Structure and Adsorption Capacity of Clay Minerals

“…Shale gas reservoirs always contain water, including bound water on the surface of matrix pores and free water in pore-fractures. 7,114,115 Water molecules fill the effective pores and separate the effective pores into multiple invalid pores (refers to the pores with diameter less than the dynamic diameter of CH 4 ), which significantly reduce the number of effective pores and increase the number of invalid pores correspondingly. 116−118 H 2 O molecules are mainly adsorbed on clay mineral particles and pore surfaces through hydrogen bonds and surface forces, form an adsorbed water film, occupy the adsorption site, and hinder the adsorption of CH 4 molecules.…”

“…Merkel et al and Li et al suggested that the adsorption capacity of H 2 O in shale is stronger than that of CH 4 and that the CH 4 adsorption amount can be reduced by 80%–90% in the presence of water. Shale gas reservoirs always contain water, including bound water on the surface of matrix pores and free water in pore-fractures. ,, Water molecules fill the effective pores and separate the effective pores into multiple invalid pores (refers to the pores with diameter less than the dynamic diameter of CH 4 ), which significantly reduce the number of effective pores and increase the number of invalid pores correspondingly. − H 2 O molecules are mainly adsorbed on clay mineral particles and pore surfaces through hydrogen bonds and surface forces, form an adsorbed water film, occupy the adsorption site, and hinder the adsorption of CH 4 molecules. When the water content is higher than 4 wt %, the CH 4 adsorption capacity in shale decreases significantly. − Water molecules are also present in the pore structure of kerogen, but it is generally suggested that the structure of organic matter is hydrophobic, so its water content is significantly lower than that in the pores of clay minerals. − …”

“…However, only a few countries such as United States, Canada, and China have successfully realized large-scale commercial exploitation. Due to the limitation of accumulation mechanism and development technology, most countries are still in the exploration process. − Shale gas occurs mainly in pores and fractures of reservoirs, in adsorbed and free states, of which adsorbed gas accounts for 20%–85%. − The efficient desorption, migration, diffusion, and production of adsorbed gas is an important aspect to enhance shale gas recovery. − Organic matter structure and pores are regarded as the primary occurrence sites of shale gas, in which total organic matter content (TOC) and thermal maturity control the CH 4 storage capacity of the reservoirs. − As another significant carrier for CH 4 adsorption, clay content also presents an apparent impact on shale gas content. − Shale is mainly composed of clay minerals. Clay content is generally higher than 20% in commercial shale gas reservoirs, and sometimes as high as 80%. − Clays are layered porous inorganic minerals with pores developed between crystal layers, inside minerals, and among particles. − The size, morphology, specific surface area, surface adsorption potential, and roughness of pores determine the adsorption capacity of clay minerals for CH 4 . − Comparatively, the contribution of brittle minerals in shale to pore structure and adsorption capacity is weak, although certain dissolution pores are found in feldspar and others. , …”

Synergistic Mechanism of Ultrasonic-Chemical Effects on the CH4 Adsorption–Desorption and Physicochemical Properties of Jincheng Anthracite

“…33 Studies have shown that the CH 4 adsorption capacity of coal is proportional to the content of aromatic groups. 34 The adsorption heat generated by the adsorption of CH 4 on coal is proportional to the content of oxygen-containing functional groups in its molecular structure. 35 The number of methyl and methylenes is proportional to the adsorption potential well of coal for CH 4 .…”

Electronic Structure Engineered Heteroatom Doped All Transition Metal Sulfide Carbon Confined Heterostructure for Extrinsic Pseudocapacitor

“…Zhang et al. [ 15 ] fabricated MOF‐derived hollow Co 3 O 4 nanocages to construct a hybrid SC device. Nevertheless, the diffusion‐controlled faradaic reactions continue to influence their reaction kinetics.…”