Buffalo Field High-Pressure Air Injection Projects 1977 to 2007: Technical Performance and Operational Challenges

Abstract

Abstract The Buffalo Field air injection units, located in northwest South Dakota, are the oldest High-Pressure Air Injection (HPAI) projects currently in operation. In mid 1977, realizing the rapid production decline under primary recovery, the operator initiated an IOR program including laboratory tests, feasibility studies, and a pilot air injectivity test. Because the pilot results were promising, the original 3.5-section Buffalo Red River Unit (BRRU) was formed in September 1978. The results continued to be encouraging and the unit was enlarged to nine sections in September 1980 and to twelve sections in May 1981. Based upon the success of the BRRU HPAI project, the South Buffalo Red River Unit (SBRRU) of 30.5 sections was formed in June 1983 and began to respond to injection in 1985. Furthermore, given the success of the BRRU and SBRRU HPAI projects, the 7-section West Buffalo Red River Unit was formed and started air injection in November 1987. This paper summarizes the performance of the three projects and the overall experience gained by the operators after nearly 30 years of air injection. It covers almost every aspect of the entire operation since its inception; it discusses general management practices, technical and operational challenges encountered, injection and production facilities and drilling and well completion practices. It also includes estimates of incremental oil recovery due to air injection and discusses how the air utilization has changed over time To date, the three HPAI projects in the Buffalo Field continue to be a commercial success. In the last three years, horizontal laterals have been drilled out of over 40 old vertical wells to enhance production, to take advantage of accumulated reservoir energy and to improve sweep efficiency. Drilling injection wells out of old vertical wells was not possible because the open hole laterals cross a porosity zone that would have taken away some of the injection into non-productive reservoir. Introduction The Buffalo Field comprises the oldest HPAI projects currently in operation. It is located in Harding County in the northwest corner of the state of South Dakota (USA), on the southwestern flank of the Williston Basin (Fig. 1). The field produces from the Red River carbonate reservoir of Ordovician age that occurs at an average depth of 8,500 ft. The Red River is the deepest formation in which widespread commercial production has been found in the Williston Basin. The recovery mechanism under primary production was by liquid expansion and rock compaction. Ultimate primary recovery is estimated to be six percent of the original oil in place. Several different improved oil recovery techniques were considered. Waterflooding was ruled out primarily because of the anticipated limited water injectivities and also because of high water saturations. Natural gas and carbon dioxide (CO2) injection were ruled out mainly due to the excessive high cost of both gas and CO2. After conducting laboratory studies, which included miscibility studies using flue gas and a combustion tube test, the process finally selected was HPAI. Air injection in the Buffalo field began in January 1979 and, as of December 2007, approximately 240 BSCF of air have been injected into the field. A total of 17.2 million barrels of incremental oil have been produced by the HPAI process, which is equivalent to 9.4 percent of the original oil in place. The cumulative air-oil ratio after 29 years of air injection is approximately 14 MSCF of air per incremental barrel of oil. The operation of the projects has been relatively simple. The primary problems with production wells have been gas interference, carbon dioxide corrosion, emulsions, and casing collapse. The primary problems that occurred with injection wells have been near well formation plugging and annular air leaks. Production from the field has been handled with conventional surface facilities modified to treat the larger amount of fluids and emulsions.