Factors affecting coalbed methane (CBM) well productivity in the Shizhuangnan block of southern Qinshui basin, North China: Investigation by geophysical log, experiment and production data

Abstract

In order to determine the dominant factors influencing on production characteristics of coalbed methane (CBM) well, the relationships between CBM wells productivity and its various factors from the no. 3 coal seam of the Lower Permian Shanxi Formation in the Shizhuangnan block of southern Qinshui basin were studied based on geophysical log, production and experiment data. CBM wells were classified into two types according to water production curves which are related to different dewatering effects. The coal seam and its roof and floor of 75 CBM wells were identified by geophysical log data. The reservoir pressure and hydraulic-head of 55 CBM wells were estimated by working fluid level height. The calculation models of coal proximate analysis components and gas content on the study area were established using geophysical log and experiment data from 14 CBM wells. The results indicate that coal rank of no. 3 coal seam is mainly anthracite (vitrinite reflectance ranges mainly from 2.505% to 3.567%). Coal macerals is mainly vitrinite (51.0–83.1%), followed by intertinite (7.5–37.3%). The groundwater in the study area flows from east to west along the direction of the elevation decrease. The groundwater flow controls gas accumulation which resulted in high gas content in west and low gas content in east. Therefore, CBM wells with high gas production rate concentrate in west. Poor sealing ability of roof may lead to external water flowing into coal reservoir through roof. This causes high water production rate and low gas production rate in CBM wells, even if CBM wells have high gas content. The western part of study area has greater potential in CBM production than eastern part of study area. Hydrological condition and sealing ability of roof are two key factors on CBM production.