Experimental study on alleviating water-blocking effect and promoting coal gas desorption by gas wettability alteration

Abstract

To alleviate the water-blocking effect in the hydraulic fracturing process and promote coal seam gas extraction, the influence of gas wettability alteration on the water-blocking effect and gas desorption was investigated. First, according to the contact angle, infrared spectrum, and surface energy measurement experiments, 3.5% polyacrylamide (PAM) was selected as the gas wettability alteration agent and used to treat the coal samples. After treatment with PAM, the hydroxyl and surface energy of the coal decreased, which altered the gas wettability. Second, the water injection desorption experiment results show that both 4% alkyl polyglucoside (APG) and 3.5% PAM solution can alleviate the water-blocking effect and promote coal gas desorption during water injection. The same experiment was performed by varying the gas pressure. When the gas pressure exceeds 3.25 MPa, APG inhibits gas desorption, indicating that it is unsuitable for alleviating the water-blocking effect. Furthermore, the mechanism by which gas wettability alteration relieves the water-blocking effect was analyzed from macroscopic and microscopic perspectives. Macroscopically, PAM had a high removal rate of the liquid-phase retention effect in coal and can hardly be retained in coal. Microscopically, after gas wettability alteration, the number of hydrophilic functional groups on the surface of coal decreased. The above results in the system changing from hydrophilic to hydrophobic, and correspondingly, the capillary pressure in pores or fractures of coal changes from resistance to a driving force. Therefore, compared with the original coal sample, the water in the coal is more likely to flow back, which alternates the water-blocking effect and promotes gas desorption. This study provided a laboratory prediction method for verifying the effects of gas wettability alteration agents on coal gas extraction.