Method for determining the feasibility of coalbed methane coproduction based on trace elements in produced water ——A case study of the Hancheng Block, southeastern Ordos Basin, China

Abstract

Multiseam coproduction is the primary way to improve coalbed methane (CBM) development efficiency in multiseam areas. However, the interlayer interference induced by the differences in reservoir geological conditions severely hinders the adequate pressure reduction of CBM reservoirs, resulting in poor coproduction efficiency. Deeply understanding coproduction's geological constraints and effectively determining coproduction's feasibility are critical to improving the CBM coproduction capacity. This study selected 16 CBM wells in the Hancheng Block, southeastern margin of the Ordos Basin, China, to collect water samples and production data. Geochemical tests of trace elements in water produced from CBM wells were performed to investigate their implications for CBM coproduction, and the coproduction feasibility is discussed in combination with the production data analysis. Li, Ga, Rb, Sr, and Ba were extracted as the characteristic trace elements of the produced water, whose concentrations increase with decreasing coal seam levels in a single-seam production (No. 3 < No. 5 < No. 11). The highest trace element concentration in coproduction wells with the No. 5 + 11 coal seam combination indicates their excellent coproduction compatibility. A method for determining the feasibility of coproduction based on the intersection and spider web diagrams of characteristic trace elements in produced water was constructed to distinguish the water sources and predict gas production. Additionally, a method for quantitatively identifying produced water sources between static and dynamic water was developed based on trace elements. The discrimination results were highly consistent with the gas production capacity, indicating suitability in effectively identifying the water sources and feasibility of CBM coproduction. The optimal geometric parameters of coal seam combinations for CBM coproduction were determined, and based on the superposition of the above parameters, the areas for CBM coproduction were divided into categories Ⅰ, Ⅱ, and Ⅲ, providing a theoretical basis for CBM-orderly development in the Hancheng Block. The results can provide a reference for optimizing CBM coproduction programs in relevant areas with similar geological conditions.