Mechanisms of Proppant Pack Instability and Flowback During the Entire Production Process of Deep Coalbed Methane

Deep coalbed methane (DCBM) reservoirs often experience severe proppant flowback during large-scale hydraulic fracturing, which undermines fracture conductivity and limits long-term recovery. The critical flowback velocity (CFVP) is the key parameter controlling proppant pack instability and flowback. In this study, the instability and flowback behavior of proppant packs throughout the entire production process, from early water flowback to late gas-dominated stages, were systematically investigated. Proppant flowback under closure stress was simulated using a CFD–DEM approach to clarify the flowback process and mechanical mechanisms. Laboratory experiments on coal fracture surfaces under gas-liquid two-phase and gas-liquid-solid three-phase conditions were then conducted to quantify CFVP and its variation across different production stages. Finally, a semi-empirical CFVP predictive model was developed through dimensional analysis. Results show that proppant flowback proceeds through three distinct stages—no flowback, gradual flowback, and rapid flowback. Increasing fracture width reduces proppant pack stability and lowers CFVP but allows higher flow capacity, and within the typical gas and water production ranges of deep coalbed methane reservoirs, flowback is significantly reduced when the width exceeds about 8 mm. Closure stress enhances CFVP below 15 MPa but has little effect above this threshold, while higher stresses progressively stabilize the proppant pack and minimize flowback. Larger average proppant size raises CFVP and preserves conductivity, whereas higher gas–liquid ratios elevate CFVP and reduce flowback, with ratios above 40 sustaining consistently low flowback levels. These findings clarify the mechanisms and threshold conditions of proppant flowback, establish quantitative CFVP benchmarks, and deliver theoretical as well as experimental guidance for optimizing DCBM production.