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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.

To realize the precise stress relief and permeability enhancement of the coal seam with non-uniformly distributed gas, the criterion and optimization method of key construction parameters of hydraulic flushing was studied by modeling, numerical simulation and field tests. First, the multiphysics coupling model for stress-relieved coal seam was developed and index of optimal coal discharged was analyzed. Then the criterion and determination method of optimal coal discharged was put forward. Finally, the determination map of key construction parameters of hydraulic flushing was drawn, and a precise permeability enhancement technique for coal seam with non-uniformly distributed gas was proposed. The main conclusions are shown as follows: ① for a given gas extraction time, the residual gas content decreases while the residual gas pressure reduces followed by an increase with a rise of coal discharged; ② the criterion of optimal coal discharged includes the constraints of gas extraction reaching standard and minimization of roadway instability risk and construction cost. When the residual gas content and pressure are less than the critical values, it is considered that the gas extraction reaches the standard. On the premise of meeting the standard of extraction, the coal discharged should be reduced as far as possible to ensure the stability of coal roadway and reduce the project cost; ③ The optimal coal discharged decreases with the increase of ground stress, and increases with the increase of cohesion, gas pressure and borehole spacing. For a given coal seam, there is a maximum value of coal discharged and a minimum value of extraction time reaching standard; ④ comprehensively considering the correlation between coal seam gas pressure, coal yield, borehole spacing and extraction time reaching standard, the optimization map of key parameters of hydraulic flushing is drawn. Based on the map, a precise pressure relief and permeability enhancement technology for coal seams with non-uniformly distributed gas is put forward. In this technique, the construction parameters of hydraulic flushing corresponding to various gas occurrence areas can be determined, with the combination of gas occurrence and optimization map of key parameters, realizing the precise permeability enhancement of the coal seams with non-uniformly distributed gas.

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.