The Lost Soldier Tensleep field tertiary performance has a noteworthy case history that demonstrates how an incremental 13% of original oil in place will be recovered from this sandstone reservoir using carbon dioxide. Located in south central Wyoming, the Lost Soldier Tensleep has been under carbon dioxide (CO2) injection since 1989. From 1989 through 1995, a 61% hydrocarbon pore volume (HCPV) slug of CO2 was injected into the reservoir to recover an incremental 13.6 million barrels of tertiary oil. Prior to CO2 injection, the reservoir was producing 2,500 bopd at a 97% watercut. Within one year, oil production exceeded 10,000 bopd and is currently 6,000 bopd.
Since its start-up in 1989 with 47 producers and 40 injectors the subject flood has used a water-alternating-gas (WAG) method and line drive pattern to process approximately 840 acres of reservoir. Illustrations of actual WAG injection cycles and resultant offset production response are included in this case history. The Lost Soldier flood has been so successful that one producing well had an incremental rate of 1700 bopd, carrying the unique distinction of having the highest peak incremental CO2 response that the authors could find documented in U.S. literature.
Information contained in this paper provides observations and conclusions about field performance optimization as well as reservoir management philosophy. Several actions were completed during the early to mid-1990's to maximize recovery and profitability. These included maintaining miscible reservoir pressure, increasing natural gas liquid recovery, converting to sour reinjection, and exploiting downdip oil potential.
Introduction
The Lost Soldier Tensleep field is located in the Great Divide Basin of Wyoming approximately 40 miles northwest of the city of Rawlins (Figure 1). Lost Soldier is the larger of two fields located near the town of Bairoil, Wyoming, the other being Wertz field. Combined Lost Soldier and Wertz field production is 10,000 bopd and 1,500 bbl/day of natural gas liquids. The majority of current production comes from the Pennsylvanian age Tensleep and the Mississippian age Madison carbonate. The original oil in place (OOIP) in all horizons in both fields approaches 1 billion barrels.
Currently, besides the Lost Soldier Tensleep, CO2 is being injected into the Wertz Tensleep1 and the Lost Soldier Darwin/Madison reservoirs.
CO2 for the Bairoil fields is supplied from Exxon's LaBarge project2. The CO2 is transported via pipeline 120 miles from its Shute Creek plant in southwest Wyoming to a point 19 miles northwest of Bairoil, where it is transferred to an operator-owned spur line for final delivery.
Field History and Development
The Lost Soldier Field was discovered in 1916 when Bair Oil Company drilled a well 265 feet into the first Frontier formation. The well produced 200 bopd. The Dakota, Lakota, Morrison, and the Sundance sands were discovered prior to 19263. In 1930, the Tensleep was discovered, and the initial well flowed 2,435 bopd. In 1947, the Darwin sandstone and Madison carbonate were discovered and produced at a combined rate of 1,045 bopd. In 1948, the Flathead sandstone was discovered immediately above the granitic basement. Amoco purchased the properties in 1975.
Development was slow for the Lost Soldier Tensleep until 1942. During the early 1940's, 16 wells were completed, producing between 2,500 and 8,000 bopd. Since its discovery, the Tensleep has been the most prolific of the nine productive horizons in the Lost Soldier Field. Primary production is attributed to a combination of fluid expansion, water influx, and gravity drainage. Peripheral water injection began in 1962, and pattern waterflood was initiated in 1976. The pattern development in the late 1970's resulted in a 16 acre north to south line drive pattern still used today.
Cumulative production from the Lost Soldier Tensleep through 1995 is 120 million barrels of oil (50% of OOIP).
