Research on sequence stratigraphy, hydrogeological units and commingled drainage associated with coalbed methane production: a case study in Zhuzang syncline of Guizhou province, China | Recherche sur la stratigraphie séquentielle, les unités hydrogéologiques et le drainage combiné associés à la production de gisements houillers de méthane: une étude de cas dans le synclinal de Zhuzang, province du Guizhou, en Chine Investigación sobre estratigrafía secuencial, unidades hidrogeológicas y drenaje combinado asociado con la producción de metano de una capa de carbón: un estudio de caso en el sinclinal de Zhuzang en la provincia de Guizhou, China 与煤层气烷生产相关的层序地层学、水文地质单元及混合排水研究:中国贵州省Zhuzang向斜的研究实例 Pesquisa sobre estratigrafia de sequência, unidades hidrogeológias e drenagens misturadas associadas com produção de gás metano de carvão: um estudo de caso do sinclinal de Zhuzang na província de Guizhou, China

Exploitation of coalbed methane (CBM) involves groundwater extraction to depressurize coal reservoirs. This can involve groundwater extraction from multiple coal seams (commingled drainage). Interlayer interferences, caused by heterogeneity of hydrodynamic fields of the different coal gas reservoirs, can restrain CBM production. Understanding of the hydrogeological characteristics of each reservoir, inseparable from characteristics of the sequence stratigraphic framework, is critical for CBM exploration. Analysis of Zhuzang syncline in Guizhou province, China, found gas- and water-blocking strata near the maximum flooding surface in the upper part of each third-order stratigraphic sequence; thus, the hydrogeological units were divided vertically (SQ4, SQ3, SQ2 and SQ1) by the boundaries of the third-order sequence. The commingled-drainage CBM wells were analyzed by numerical simulation and Extenics theory, on the basis of characteristics of the hydrogeological units. Gas content, reservoir pressure and hydrodynamic parameters were found to vary between the hydrogeological units. The interlayer interference was not obvious where there was commingled drainage within single hydrogeological units with similar hydrodynamic force; this was validated by observing the consistent pressure decrease within each reservoir using historical matching. Since the source of drainage water varied from stratum SQ3 to SQ4 (containing lower hydrodynamic force compared to SQ3), it was obvious that groundwater extraction from SQ4 was restrained by SQ3, by showing obvious interlayer interference and restrained CBM production during commingled drainage across the different hydrogeological units. Reservoirs within each single hydrogeological unit tend to obtain higher CBM yield, thus take priority for commingled drainage.