Evaluation of Coalbed Methane Well Productivity Variation Based on Production Indicator Curves: A Case Study for the Northern Part of K Block, Southern Qinshui Basin

At the same structural position within the northern part of Block K, the coalbed methane (CBM) wells in the 3# and 15# coal seams exhibit marked productivity disparities. This study integrates a correlation analysis of engineering parameters with dynamic diagnosis of production indicator curves to examine two dimensions: static engineering compatibility and dynamic seepage interference. The results show that the degree of matching between engineering parameters and geological conditions is a key factor controlling the productivity variation. The 3# coal seam has favorable engineering performance, and boosting fracturing scale and drainage rate both help promote gas output. Compared with the 3# coal seam, the 15# coal seam has a notable deficiency in engineering compatibility: its effective horizontal section length and coal encounter rate decrease by 8.4% and 5%, respectively, while sidetracking frequency increases approximately fivefold; moreover, enlarging fracturing scale does not bring better gas results. Production indicator curves were established based on the relationship between cumulative water production per unit effective horizontal length and production pressure difference and were classified into upward-convex and downward-concave types. The resulting chart for exogenous water interference enables quantitative characterization of key metrics, including theoretical ultimate gas production, interference level, and gas production efficiency. The theoretical ultimate gas production of the 3# coal seam reaches 69.454 m3/(d·m), over ten times that of the 15# coal seam, but the critical interference threshold of the 3# seam is lower than that of the 15# seam. Most 3# wells are stably located in the non-interference zone, indicating high production capacity and strong sensitivity to exogenous water interference. For the 15# seam, the production indicator curves of most wells deviate toward the interference zone at an early production stage, reflecting strong exogenous water leakage recharge. Therefore, for this block, CBM development must shift from the traditional approach of blindly enlarging stimulation scale to geological compatibility and recognition of exogenous water interference.