Pressure-Controlled Drainage Strategy for Deep Coalbed Methane Wells Considering Stress Sensitivity

Abstract

China’s deep coalbed methane (CBM) resources hold significant development potential, yet their commercial exploitation remains hindered by four interrelated reservoir challenges: substantial burial depth, elevated in situ stress, ultra-low matrix permeability, and pronounced stress-dependent permeability decline. These conditions collectively induce rapid reservoir energy depletion and flow-path deterioration, leading to characteristic production behavior—namely, high initial gas rates followed by steep decline and persistent instability in long-term output. Accordingly, a rigorously optimized production system is indispensable to suppress formation damage, sustain desorption-driven gas release, and maximize ultimate recovery efficiency. To address this, this study establishes a physics-based numerical model that integrates field-calibrated geological and operational parameters, gas–water two-phase flow dynamics, coupled desorption–diffusion–seepage processes, and quantitatively constrained stress–permeability relationships for coal. Implemented in the CMG-IMEX simulator, the model is validated through robust history matching against production data from ten representative wells. A comparative analysis of two drawdown management strategies—the conventional constant-decline approach and the progressively decreasing drawdown strategy—demonstrates clear performance differentiation in both cumulative production and reserve utilization. Under the conventional scheme, the average ultimate recovery factor at economic abandonment reaches only 36.42%, with adsorbed-gas recovery limited to 25.53%, underscoring substantial untapped resource potential. In contrast, the progressively decreasing drawdown strategy alleviates multiphase flow restrictions, improves pressure maintenance, and elevates the average ultimate recovery factor to 40.43%—a net gain of 4.43 percentage points in adsorbed-gas recovery. Sensitivity analysis further identifies an initial drawdown of 5 MPa as the optimal balance between early productivity and reservoir sustainability. The coupling among reservoir pressure, flowing bottomhole pressure, and casing pressure enables the bottomhole drawdown-control protocol to be converted into a surface-measurable casing-pressure decline-rate criterion. These findings improve the mechanistic understanding of gas–water co-production in deep CBM systems and provide an operational framework for production-system design.