A Practical Approach for Determining Postfracturing Permeability in Unsaturated Coal Reservoirs

Permeability, a fundamental parameter for evaluating fluid flow characteristics in porous media, has been extensively researched for several decades. Nevertheless, methods to determine the permeability of coal reservoirs impacted by hydraulic fracturing remain limited. This study aims to quantify the posthydraulic fracturing permeability in unsaturated coal seams and investigate its evolution. A procedure involving an analytical well testing technique and a nonlinear regression approach was proposed, principally employing a hyperbolic decline curve identified through the production time for the coal seam. Dynamic permeability scatter points were derived principally using the water rate and bottom-hole pressure (BHP) from the initial dewatering stage. Validation against a constant-rate drawdown case from Spivey confirms the model’s reliability. When applied to the Junlian coalbed methane (CBM) field case in the southern Sichuan Basin of China, the results suggest that (1) compared with support vector regression (SVR) and Duong’s decline model, the hyperbolic regression was found to be the most suitable method to determine the postfracture permeability (PFP), considering statistical error parameters. (2) The PFP derived using dewatering data and hyperbolic fitting, also as the initial permeability during production, correlated with results from after-closure analysis. (3) A significant enhancement in PFP for the coal reservoir was observed, increasing from 0.02 mD prefracturing to 122.3195 mD postfracturing. (4) The PFP linearly increased with the cumulative gas produced over a decade, while the instantaneous permeability decline rate (IPDR) showed a linear negative correlation with the cumulative gas. (5) A hydro-mechanical numerical model exhibited an “opened book” shape in the cross-sectional displacement of the coal seam over time, in response to the hyperbolic trend in permeability. This displacement pattern contributes to casing deformation over time. These findings aim to enhance our understanding of coal reservoirs and promote more effective exploitation of CBM resources.