Factors influencing productivity and production system optimization for deep coalbed methane in the Daning-Jixian block, Ordos Basin

ObjectiveDeep coalbed methane (CBM) reservoirs are characterized by ultra-low porosity, ultra-low permeability, and super saturation, rendering it impractical to copy the productivity models of shallow coalbed methane and shale gas. Furthermore, the production performance of deep CBM varies significantly across different well sections. However, the impacts of different factors on the productivity of deep CBM remain poorly understood. This study conducted a numerical simulation analysis of the productivity of deep CBM in the Daning-Jixian block along the eastern margin of the Ordos Basin based on the geological characteristics of the block, aiming to explore the impacts of various key factors on the productivity of deep CBM. Notably, it is critical to determine the appropriate cluster spacing value and scientific drainage system. Methods Based on the geological characteristics of deep CBM reservoirs in the Daning-Jixian block, this study comprehensively considered the transformation of CBM occurrence state, multiphase fluid seepage, and the stress-strain constitutive relationships of reservoirs. To reflect the multi-physical field coupling mechanisms involved in coal reservoir deformations and fluid migration during gas drainage, this study introduced the dual-pathway gas migration pattern that combined the seepage-diffusion of free gas and the desorption-diffusion of adsorbed gas. Accordingly, a mathematical model of CBM productivity, which accounted for the coupling effects of gas-liquid two-phase flow and coal deformations, was developed. By solving the model using the COMSOL Multiphysics software and comparing the simulation results with the actual field data in the study area, this study sufficiently validated the accuracy of the model. Furthermore, major factors governing the productivity of deep CBM were analyzed using grey correlation analysis. Based on the analytical results and predicted productivity, this study revealed the influential patterns of varying well spacing values and depressurization rates on productivity. Results and Conclusions The simulation results indicate that the adsorbed gas production is directly proportional to the initial permeability ratio (k0), Langmuir volume (VL), Langmuir strain coefficient (εL), and modulus of elasticity (E) but is inversely proportional to the initial water saturation (Sw0), Langmuir pressure (pL), and Poisson’s ratio (v). In contrast, the free gas production is directly proportional to k0, pL, εL, and E but is inversely proportional to Sw0, VL, and v, with VL producing less influence on the free gas production. εL and k0 are identified as the major factors influencing the productivity of deep CBM in the study area. Analyses of the production of adsorbed gas, free gas, and water under different cluster spacing values reveal that the optimal cluster spacing and depressurization rate in the Daning-Jixian block are 20 m and 0.18 MPa/d, respectively.