Coalbed methane extraction in multibranch horizontal wells: evolutions of reservoir characteristics with different geometric parameters

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ABSTRACT Despite effective application of multibranch horizontal wells (MBHW) in the development of coalbed methane (CBM) fields, fewer studies have explored the variation of coal reservoir parameters under MBHW CBM extraction with different geometric parameters, which markedly restricts the maximum MBHW productivity. This study aims to develop a thermal–hydraulic–mechanical (THM)-coupled model, including coal deformation, gas migration, and thermal transport. Using the finite element simulation software, COMSOL Multiphysics, we analyzed the main geometric parameters of MBHW, including branch structure, branch angle, branch length, and branch quantity on the reservoir pressure, gas production, and reservoir temperature. In the entire CBM extraction process, the reservoir pressure and temperature at various points in the coal seam declined gradually; the closer to the wellbore, the faster the decline of the gas pressure and temperature in the coal seam. Typically, the cumulative gas production proportionally increases with an increase of branch length. The impact of the branch quantity on the productivity of unit length wellbore correlated with the extraction time. When the extraction time was relatively short (within 500 days), the gas production rate unit length of the wellbore increased gradually with an increase of the branch quantity.