The Cooper Reservoir is located in the north-eastern end of the Wind River Basin. Wells are completed in the lower Fort Union and Lance formations at true vertical depths from 3300 to 8200 ft. Fracture treatments are complicated by water producing sands interspersed among up to a dozen or more gas producing sands as well as mobile water within the gas producing zones. An extensive data set of core analysis, rock mechanics testing, pre- and post-fracture well logs, pressure build up analysis, detailed fracture modeling, and detailed production analysis was compiled on 56 completions in order to better understand and evaluate well performance and stimulation effectiveness in this field. The evaluation is based primarily on the use of the reciprocal productivity index (RPI) graphical production analysis method. RPI uses daily production and wellhead pressure data to determine the effectiveness of the treatments by several parameters including flow capacity (kh), reservoir flow geometry, drained volume (area), and effectiveness of produced liquids management. Significant variations in reservoir quality and completion effectiveness were observed.