Leroy Storage Facility, Uinta County, Wyoming: A Case History of Attempted Gas-Migration Control
Authors: Araktingi R.E. , Benefield M.E. , Bessenyei Z. , Coats K.H. , Tek M.R. 
Published: December 2007

Summary
Leroy gas storage facility, an aquifer storage development by Mountain Fuel Supply Co. in Uinta County (WY), is presented as a case history. This field represents a complex problem in aquifer storage because of the uncontrolled migration of gas to the surface. Considerable effort in reservoir engineering and planning has resulted in apparent and probable arrest of uncontrolled gas migration, although more time will be needed to evaluate and monitor the results of recent efforts. Incorporated in the evaluation of the leakage problem has been updated geological information, location and correction of possible well problems, computer simulation, extensive logging, tracer surveys, surface monitoring, and related engineering evaluation. A computer program developed to simulate a unique history match, including the effect of a time- and pressure-dependent leak, and a comprehensive analysis of data leading to the possible control of the leak by proper operating storage pressures are described.

Introduction
Through continuous attention from the Reservoir Engineering Dept. and support by management, the operations have continued with reduced scope and it now appears that the quantity migrating away from the storage horizon is definitely decreasing This paper briefly documents the history and presents the status of the storage field, relating efforts to isolate and control the leakage. The geography, geology, historical background, reservoir description, reservoir performance, and related engineering studies relevant to the case history are included. The computer simulation of history match including the leak, analysis of the results, subsequent reservoir engineering work, discussion of results, and conclusions are presented.

Background
The Leroy gas storage field is located in Uinta County (WY) in Township 16 North, Range 117 West. The field lies 80 miles [129 km] west of Rock Springs, WY, and approximately 100 miles [161 km] northeast of Salt Lake City. UT. This particular location was of interest to Mountain Fuel Supply Co. because it was favorably located relative to the pipeline supplying Salt Lake City. Interest in this area for prospective storage facilities dates back to 1969. Evaluation of data obtained from exploratory Leroy Well 3, originally drilled by the Shell Oil Co. in Summer 1951, suggested two potential storage formations-the Nugget and the Thaynes. The Nugget sandstone was rejected because of the questionable integrity of the caprock, the faulted structure, and abnormally high-pressure gradient. Testing of the Thaynes formation in Leroy Well 3 began in Oct. 1970 following re-entry and deepening to 3,135 ft [956 m]. An extended flow test was run, producing 4,000 to 8,700 B/D [636 to 1383 m /d] water with an initial gradient of 0.508 psi/ft [11.4 kPa/m]. Initial test evaluation indicated excellent transmissibility and an expanded drilling/reentry program followed. Leroy Wells 4, 5, and 6 were drilled and completed by Mountain Fuel Supply Co. in 1971. Leroy Wells 1 and 2 were also re-entered and completed as pressure observation wells. Interference and caprock integrity tests were conducted concurrently with the re-entry and dulling programs. During initial development, lost-circulation problems occurred while the Nugget sandstone was drilled across with water-base mud. This problem was alleviated by allowing the Nugget water to flow to the surface during drilling and an intermediate string of casing was run when the underlying Ankareh formation was encountered. Two potential horizons were initially considered for storage in the Thaynes formation, designated as the T-10 (upper horizon) and the T-20 (lower horizon). Preliminary testing with gas injection into the T-20 zone showed direct communication with the T-10 zone. Subsequent injection into the T-10 zone suggested favorable conditions for development. Wells 7, 8, and 9 were completed by Aug. 1971, and 2.0 Bcf [0.06 × 10 m ] of gas had been injected. Upon completion of the surface facilities, application was made to the Federal Power Commission (FPC) to begin storage operations. The application was approved Nov. 17, 1972. Following FPC approval, storage operations proceeded for the 1972–73 heating season. The following year, inventory had been increased to 3.5 Bcf [9 × 10 m ] and Wells 9 and 10 were completed as injection/withdrawal wells.
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