Coalbed methane desorption characteristics controlled by coalification and its implication on gas co-production from multiple coal seams

In this work, CH4 isothermal adsorption measurements were carried out on 64 coal samples collected from western Guizhou Province of China, and the coalbed methane (CBM) desorption processes were quantitatively analyzed. The results show that the Langmuir volume and the Langmuir pressure are controlled by coalification, and tend to increase as the vitrinite reflectance changes from 0.98% to 4.3%. Based on a division method of CBM desorption stages, the CBM desorption process were divided into four stages (inefficient, slow, fast and sensitive desorption stages) by three key pressure nodes (the initial, turning and sensitive pressures). The fast and sensitive desorption stages with high desorption efficiency are the key for achieving high gas production. A theoretical chart of the critical desorption pressure (<inline-formula><math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="mathml_Z-20220418124620" overflow="scroll"><mml:msub> <mml:mi>P</mml:mi><mml:mrow class="MJX-TeXAtom-ORD"><mml:mrow class="MJX-TeXAtom-ORD"><mml:mi mathvariant="normal">c</mml:mi><mml:mi mathvariant="normal">d</mml:mi></mml:mrow></mml:mrow></mml:msub></math></inline-formula>) and its relationship with different pressure nodes was established. The higher-rank coals have the higher initial, turning and sensitive pressures, with larger difference between pressure nodes. Most CBM wells only undergo partial desorption stages due to the differences in <inline-formula><math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="mathml_Z-20220418124646" overflow="scroll"><mml:msub><mml:mi>P</mml:mi> <mml:mrow class="MJX-TeXAtom-ORD"><mml:mrow class="MJX-TeXAtom-ORD"><mml:mi mathvariant="normal">c</mml:mi><mml:mi mathvariant="normal">d</mml:mi></mml:mrow></mml:mrow></mml:msub></math></inline-formula> caused by the present-gas content. Under the same gas content conditions, the higher the coal rank, the less desorption stages that CBM needs to go through. During coalbed methane co-production from multiple coal seams within vertically superposed pressure systems, the reservoir pressure, the <inline-formula><math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="mathml_Z-20220418124640" overflow="scroll"><mml:msub><mml:mi>P</mml:mi> <mml:mrow class="MJX-TeXAtom-ORD"><mml:mrow class="MJX-TeXAtom-ORD"><mml:mi mathvariant="normal">c</mml:mi><mml:mi mathvariant="normal">d</mml:mi></mml:mrow></mml:mrow></mml:msub></math></inline-formula>, the initial working liquid level (WLL) height, and coal depth are key factors for evaluating whether coal seams can produce CBM simultaneously. It must be ensured that each production layer enters at least the fast desorption stage prior to that the WLL was lower than the depth of each layer. Only on this basis can all layers achieve the maximum gas production.