Case History of the Tensleep Reservoir, Torchlight Field, Wyoming

Abstract

Abstract The Torchlight field is located on the east side of the Big Horn Basin in Big Horn County, Wyo. Orchard Unit Well No. 1, the Tensleep discovery well, was completed in 1947. The 33 degrees API gravity Tensleep crude was highly undersaturated, and hydraulically contained in the anticlinal reservoir by a tilted water table. After development, fluid producing rate and reservoir pressure declined rapidly. Field performance indicated the reservoir was producing by a limited water drive and would require over 25 years for depletion. Comparisons are made between field producing rates and rates predicted by water influx theory. Predicted rates are 10 to 15 per cent higher than actual rates. The difference is due to reservoir pressure being higher than that assumed in the prediction. Supplemental water injection was started in Oct., 1957, to reduce reservoir life and increase oil rates. Oil response was immediate and reached a maximum rate of 1,785 BOPD in 1959. By Jan., 1965, the reservoir had been essentially depleted after producing 2,774,000 bbl oil. Supplemental water injections have reduced reservoir life at least seven years. Oil recovery by supplemental water injections of 43 per cent of original oil in place will be comparable to recovery by natural water drive. Predicted oil recovery by the Dykstra-Parsons and Stiles methods indicate recoveries of 46 and 50 per cent original oil in place, respectively. However, these two methods assume 100 per cent areal sweep efficiency, and reduction for areal sweep would result in close agreement with actual recovery. Introduction The Tensleep reservoir, Torchlight field, Wyo., is nearing depletion by natural water drive and supplemental water injections after producing over 2.7 million bbl of oil. Collection of extensive engineering data during early development permitted thorough evaluation of reservoir properties, early recognition of the producing mechanism and initiation of a water injection program which significantly increased oil rate and reduced reservoir life. Water injections did not influence or change ultimate oil recovery but provided a decrease in operating costs as compared to depletion by natural water influx. The advanced stage of depletion, plus availability of necessary engineering information, permits comparisons between theoretical and actual reservoir performance. The fluids, rocks and producing mechanism of the Torchlight Tensleep reservoir are characteristic of many Rocky Mountain Pennsylvanian Age reservoirs. A review of the history of the Torchlight Tensleep may assist in more efficient operation of similar reservoirs. History of Development Torchlight field is located on the east side of the Big Horn Basin in Big Horn County, Wyo. (Fig. 1). Wells were drilled as early as 1904, making Torchlight one of the oldest oil fields in Wyoming. JPT P. 1159ˆ