Numerical modeling and history matching of deep coalbed methane considering multiple effects

Abstract

Deep coal seams are characterized by geological conditions of high in-situ stress, high reservoir temperature, low permeability, and complex tectonic features, resulting in significant differences in gas occurrence states and migration mechanisms compared with shallow coal seams. Conventional commercial numerical simulators are generally based on simplified dual-porosity models, making it difficult to accurately describe the complex stress sensitivity and multiphase flow dynamics of deep coal seams during production and drainage. This leads to considerable discrepancies between simulation results and actual production data. To address this issue, this study established an efficient numerical simulation method suitable for deep coalbed methane (CBM) characteristics, aiming to accurately characterize single-well gas production, water production, bottom-hole flowing pressure, and produced gas composition. Based on the geological and engineering characteristics of deep coal seams, a fully coupled numerical model was developed considering a quasi-triple-porosity medium (matrix pores, microfractures, and large fractures), competitive adsorption/desorption of multicomponent gases (CH4, CO2, etc.), multiscale diffusion, gas-water two-phase seepage, and coal matrix shrinkage/deformation effects. A typical deep CBM Well JS6-7P01 in the Daning-Jixian block of the Ordos Basin was selected as a case study for history matching and analysis. In terms of computational performance, comparisons under different computational conditions showed that, compared to conventional decoupled or simplified simulation methods, the proposed method increased the computation time by only 0.1-0.2 hours, and the number of Newton iterations and linear iterations by only 10.5% under 4-core and 8-core CPU parallel computing conditions. Regarding matching accuracy and geological understanding, the model performed history matching of gas production, water production, bottom-hole flowing pressure, and produced CO2 molar fraction variations for Well JS6-7P01, achieving a comprehensive history fitting rate of 92%. Based on the numerical simulation results, the contribution proportions of free gas and adsorbed gas during production were further quantitatively calibrated, clarifying the sources of productivity contribution in deep CBM reservoirs. Different stages of fluid flow were finely divided. Finally, the seepage characteristics of deep CBM under multi-field coupling effects were characterized based on the evolution patterns of six fields, including pressure field, saturation field, desorption degree field, permeability field, CO2 adsorption mass density field, and CH4 adsorption mass density field. This method overcomes the limitations of conventional numerical simulation methods and provides a preliminary exploration and an effective tool for the prediction and decision-making of deep CBM development.