Coalbed methane production of a heterogeneous reservoir in the Ordos Basin, China

Abstract

Macrolithotypes control the pore-fracture distribution heterogeneity in coal impacting gas adsorption, diffusion coefficient, and the gas flow. However, the behaviors that are impacted by coal macrolithotype are traditionally ignored and the drainage radius that linked to the macrolithotype contributions are lack of systematic research all the way. Thus, to evaluate the gas production relationship for macrolithotype, four blocks (bright, semi-bright, semi-dull, and dull) were obtained from the same seam. The methane absorption, diffusion, and permeability data were obtained and used in mathematical modeling to identify well production, drainage radius, and well-interference locations. Production data for 104 wells and 11 test wells were also obtained and with COMET3 simulation software to validate the mathematical model. From the bright to dull coal: there was a lower micropore contribution, a lower gas capacity, and lower gas diffusion coefficient. The cleat frequency and aperture differences impacted the reservoir permeability with bright coal having 195% higher than the dull coal. As these macrolithotype differences impact the coalbed methane (CBM) development and cause heterogeneous gas flow systems within the resource, a coupled mathematical model for comprehensive consideration of the adsorbent-diffusion-seepage was established capturing the macrolithotype contribution to the pressure propagation, gas flow, and gas drainage radius estimation. The bright coal had the higher Langmuir volume, CH4 diffusion coefficient, and permeability thus, producing a higher gas production volume, and a larger gas drainage area than the dull macrolithotype. Thus, the macrolithotype diversity and its stratification cause partitioning of the gas flow with multi-stage pressure drop effecting exploration and development. In addition, the well interference was also observed and show that the wells of bright and semi-bright coal have a continuous pressure drop, which achieving the purpose of overall pressure reduction.