The Buffalo field air-injection units, located in northwest South Dakota, are the oldest high-pressure-air-injection (HPAI) projects currently in operation. Air injection began in January 1979, and as of December 2007, approximately 240 Bscf of air has been injected into the field. A total of 17.2 million bbl of incremental oil has been produced by the HPAI process, which is equivalent to 9.4% of the original oil in place (OOIP). The cumulative air/oil ratio (AOR) after 29 years of air injection is approximately 14 Mscf of air/bbl of incremental oil.

This paper summarizes the performance of the 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 caused by air injection and discusses how the air use has changed over time

To date, the three HPAI projects in the Buffalo field continue to be a commercial success. In the last 3 years, horizontal laterals have been drilled out of more than 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 openhole laterals cross a porosity zone that would have taken away some of the injection into nonproductive reservoir.

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