30 Years of Successful High-Pressure Air Injection: Performance Evaluation of Buffalo Field, South Dakota
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 133494, ’30 Years of Successful High-Pressure Air Injection: Performance Evaluation of Buffalo Field, South Dakota,’ by V.K. Kumar, SPE, Hilcorp Energy Company; D. Gutierrez, SPE, Computer Modelling Group; and B.P. Thies and C. Cantrell, SPE, Continental Resources, prepared for the 2010 SPE Annual Technical Conference and Exhibition, Florence, Italy, 19-22 September. The paper has not been peer reviewed.
The Buffalo field enhanced-oil-recovery (EOR) project is the oldest still-active air-injection project in the USA. Laboratory studies (combustion-tube, miscibility-pressure, and swelling tests for various CO2/N2 mixtures) and feasibility studies including air-injectivity testing were completed in mid-1977. The result of the pilot test was promising, and the 2,240-acre Buffalo Red River Unit (BRRU) was formed in November 1978. Technical aspects of the Buffalo EOR project are presented, reviewing laboratory studies, production performance (1978–2009), and the effect of short-radius horizontal drilling.
Introduction
The Buffalo field is in Harding County, northwestern South Dakota, on the southwestern flank of the Williston basin. The field was discovered in 1954 and produces from the Ordovician Red River carbonate formation. The recovery mechanisms under primary production were liquid expansion and rock compaction. Ultimate primary recovery is estimated to be 6 to 8% of the original oil in place.
Several improved-oil-recovery techniques were considered. Waterflooding was ruled out, primarily because of an unsuccessful water-injectivity test and because of high water saturations. Natural-gas and CO2 injection were ruled out because of the excessive cost of natural gas and CO2. High-pressure air injection (HPAI) was selected after conducting laboratory studies, which included a combustion-tube test and miscibility studies. Operation of the HPAI projects has been relatively simple. Most problems have been prevented or solved with prudent engineering practices. Production from the projects has been handled with conventional surface facilities that were modified to handle the larger volume of fluids and emulsions.
Reservoir Properties
There are four porosity zones within the upper 250 ft of the Red River formation that produce oil and gas in Harding County. These porosity zones are the A, B, C, and D zones at approximately 10, 40, 100, and 170 ft from the top of the formation, respectively. Primarily, the Buffalo field is a stratigraphic trap and dips to the northeast at 100 to 150 ft/mile.
Most Buffalo field production is from the B zone, which is approximately 15 ft thick at a depth of approximately 8,500 ft. Average porosity, from core and log data, is approximately 19%. Water saturation is somewhat lower in the upper portion of the B porosity and higher in the lower portion of the B porosity, but averages approximately 50% overall. The A zone has 7 ft or less of porosity in the 6 to 8% range, and it is assumed to be a minor production contributor, mostly of water. The Red River C and D zones are, in general, avoided because of high-water-volume production.