Search

Geological CO2 sequestration (GCS) can help mitigate global warming and enhance methane recovery from coal beds. However, few studies have linked the effects of CO2 to surface chemistry changes controlling wetting behavior in deep coal beds. Contact angles (CAs) of CO2/N2-high volatile bituminous coal-water systems were measured under different temperatures and pressures. The surface chemistry and physical structure of coals were characterized to investigate changes in physicochemical properties and their relations with wettability after reactions. For N2 treatment, the time-dependence of static and dynamic CAs were insignificant, ranging within 4°. For gaseous CO2 treatment, the static CAs and the average advancing angles increased slightly. With supercritical (sc) CO2, both the static and dynamic CAs increased significantly, and θ adv changed to intermediate-wet (92°). Reactions with minerals exposed to scCO2 resulted in greater surface roughness and heterogeneity, greater contact angle hysteresis and more surface sites occupied by scCO2 rather than H2O. Increases in hydrophobic functional groups and decreases in hydrophilicity were shown by FTIR spectra, reflecting the shedding of polar oxygen-containing functional groups, reduction of hydrogen bonds, and increasing percentage of hydrocarbons. XRD patterns obtained following scCO2-treatment showed that crystallite growth and molecular polymerization were higher toward graphite-like. The calculated structural parameters of functional groups and crystallites both showed elevated coal rank. Changes in crystallite structure, notably higher carbon content and decreased negative surface charge, are unfavorable for water-wetting. This study contributes to understanding surface chemistry changes responsible for decreased wettability during CO2-enhanced coal bed methane recovery and GCS in coal reservoirs.

Injection of carbon dioxide (CO2) into coal seams has been demonstrated as an effective technology for enhanced methane recovery and CO2 storage. However, the impacts of the geochemical reactions between CO2 and coal on the wettability of coal pore surfaces, which influences immiscible multiphase displacement, are not yet well understood. We studied wettability alterations of coal surfaces resulting from reactions with gas, liquid and supercritical (sc)CO2 under varied pressure (1–141 bar) and temperatures (˜25 - 60 °C) through measuring static and dynamic contact angles with anthracite coal plates. We found that reactions with gas CO2 only slightly changed the wettability of coal surfaces from water-wet to intermediate-wet with static contact angles from ˜60˚ to 70°-90°. However, reactions with liquid and scCO2 altered the coal surfaces to strongly CO2-wet, with the contact angles up to 115-180°. We also found that both static and dynamic contact angles increase significantly with increasing pressure. Temperature affects the contact angles reversely especially under supercritical pressure conditions. These relationships of contact angles with pressure and temperature may be explained by the CO2 density dependence on pressure and temperature.

In order to investigate the influence of surrounding rock characteristics on gas drainage of through layer borehole in composite reservoir, the seepage law of gas in coal seam and rock stratum is analyzed. Based on the fluid solid coupling model of gas drainage in composite reservoir and single reservoir, the gas drainage effect of cross layer borehole with different dip angles on composite reservoir and single reservoir, and gas drainage by crossing hole coupling with multiple boreholes are simulated. The results show that the influence radius and effective radius of gas drainage from composite reservoir are larger than that of single reservoir when the same drilling hole is used to extract coal seam gas from different reservoirs; the influence range of gas drainage in composite reservoir is decreased with the increase of angle, and the influence of interlayer channeling is also weakened; under the condition of multi-borehole coupling drainage, the gas pressure of the middle and lower coal seams in the composite reservoir is obviously lower than that in the same position in the single reservoir.

During the process of coal spontaneous combustion (CSC), a plethora of combustible gases alongside inert gases, such as CO2, are copiously generated. However, prior investigations have regrettably overlooked the pivotal influence of inert gas production on the propensity for methane explosions during CSC. To investigate the impact of the flue gas environment generated by CSC, containing both combustible and inert gases, on the risk of methane explosion, a high-temperature programmed heating test system for CSC was employed to analyze the generation pattern of flue gas. It was found that CO, CO2, and CH4 were continuously generated in large quantities during the process of CSC, which are the main components of CSC flue gas. The effect of the concentration and component ratio (CCO2/CCO) of the flue gas on the methane explosion limit was tested. It was found that the CSC flue gas led to a decrease in the methane explosion limit, and that the explosion limit range was facilitated at 0 < CCO2/CCO < 0.543 and suppressed at CCO2/CCO > 0.543. As the temperature of CSC increases, the risk of methane explosion is initially suppressed. When the coal temperature exceeds 330~410 °C, the explosion risk rapidly expands. [ABSTRACT FROM AUTHOR]