Experimental Study on Wettability Reversal in Coal Reservoirs Induced by the Synergy of Nitrogen and Surfactant

Reducing wettability to mitigate the water-blocking effect is a critical objective in the development of fracturing fluids for deep coalbed methane reservoirs. Herein, No. 8 gas coal from the Baode Coal Mine on the eastern margin of China’s Ordos Basin was used to investigate the influence of nitrogen-surfactant fracturing fluid systems on coal wettability. Coal samples were treated by immersing in hydrophobic (31526) and hydrophilic (OP-10) surfactants. Variations in the solid–liquid contact angles under nitrogen pressures ranging from 0.1 to 8.0 MPa and hydrophilic functional groups (oxygen-containing and hydroxyl groups) were systematically investigated. Under ambient air conditions (0.1 MPa nitrogen atmosphere), the contact angles of the original, 31526-treated, and OP-10-treated coal samples were 65.6°, 71.8°, and 41.4°, respectively. With an increasing nitrogen pressure, the contact angles of the three samples increased: a behavior independent of the hydrophilic or hydrophobic properties of the surfactants. The primary mechanism is attributed to high-pressure nitrogen occupying adsorption sites on the coal surface, forming a gas barrier that hinders the invasion of water molecules and reduces wettability. Fourier transform infrared analysis showed that treatment with 31526 decreased the contents of oxygen-containing and hydroxyl functional groups by 24.72% and 9.50%, respectively, increasing the contact angle and reducing wettability. In contrast, OP-10 treatment increased oxygen-containing and hydroxyl functional groups by 98.20% and 24.44%, respectively, decreasing the contact angle and enhancing wettability. Under the synergistic effects of 31526 and nitrogen, a wettability reversal from water-wet to gas-wet was achieved on the coal surface under a nitrogen pressure of ∼4.5 MPa. These findings provide valuable insights into the design and optimization of fracturing fluid systems for deep low-water-content coal reservoirs.