Effect of Reinjected Flowback Water into Deep Coal Seams on Coalbed Methane Production: Low-Field Nuclear Magnetic Resonance and Molecular Dynamics Studies on Methane Desorption and Diffusion

The application of large-scale horizontal well fracturing technology has enabled the efficient exploitation of coalbed methane (CBM) in deep coal seams; however, the increased water consumption and large volumes of flowback water involved in these activities have induced new challenges. Recycling and reinjecting flowback water for hydraulic fracturing constitute a potential solution. However, the effect of reinjecting flowback water with different salinities on CBM production remains unclear. In this study, experimental low-field nuclear magnetic resonance (NMR) measurements and molecular dynamics (MD) simulations were integrated to compare (1) variations in spontaneous imbibition (SI) and forced imbibition (FI) capacities with different fluids, (2) changes in the amount of methane (CH₄) in fracturing fluids with different salinities in the stages from injection to depressurized flowback, and (3) shifts in the adsorption capacity of CH₄ and water (H₂O) at different salinities. The results show that fluids are primarily confined in micropores, the SI saturations for fluids with salinities of distilled water (DW), medium-salinity brine (MSB), and high-salinity brine (HSB) are 90.49%, 44.72%, and 13.73%, respectively, while their corresponding CH₄ displacement efficiencies are 23.13%, 11.05%, and 2.46%. As the imbibition capacity and the competitive adsorption capacity of H₂O gradually decrease with increasing salinity, the CH₄ displacement efficiency also decreases steadily. During the depressurized flowback process, the diffusion coefficient of CH₄ in dry coal samples and those containing DW, MSB, and HSB are 9.29×10⁻⁵ s⁻¹, 0.44×10⁻⁵ s⁻¹, 1.88×10⁻⁵ s⁻¹, and 8.59×10⁻⁵ s⁻¹, respectively. As salinity increases and fluid volume decreases, the water-blocking capacity gradually decreases, and the diffusion ability of CH₄ gradually increases. Given that low-salinity fracturing fluids exhibit stronger displacement capacity, while high-salinity fracturing fluids enhance CH₄ diffusion, we propose a novel fracturing fluid injection strategy. This strategy involves initially injecting high-salinity fracturing fluid to induce fractures, followed by low-salinity fracturing fluid to enhance CH₄ displacement. This approach aims to optimize CBM production while simultaneously addressing the challenges related to the management of high-salinity flowback water.