Matthews, T.A., Phillips Petroleum Co., Bartlesville, Okla.
The South Burbank Unit of Osage County, Okla., produces from in an offshore-bar-type reservoir which has been subjected to both intensive gas injection and to water flooding. The Unit was formed early in the primary life of the reservoir, and development involved only about one-half the well densities of nearby pools in the same-type reservoirs. Gas injection was started immediately after unitization, and was continued to near its economic limit. Ultimate recovery by gas injection would have been about 34 per cent of the initial oil in place, compared to about 22 per cent recovery of the initial oil in place by primary depletion in nearby areas. Initial water flood development of the South Burbank Unit was made in 1951 on an irregular five-spot pattern with about 40 acres per producing well. Recoveries will be as high as nearby regular five-spot pattern on 20 acre spacing. The water flood will ultimately recover an additional 12.8 per cent of the initial oil in place over that which could have been recovered by gas injection. The combination of gas injection and water flooding will recover 46.8 per cent of the original oil in place. However, early application of water injection, without the intervening period of gas injection, would have recovered as much total oil in a much shorter total operating life.
The South Burbank Unit, in Osage County, Okla., affords an opportunity to compare the recovery of oil by each of the three most common energy mechanisms: solution gas, gas injection and water flooding. Since the Unit was formed and gas injection started some 17 months after discovery of the field, performance of the field under primary solution-gas drive can only be determined on the basis of the performance in nearby, similar reservoirs. Early unitization permitted development with about one-half the well density of nearby areas.
The South Burbank pool was discovered on Jan. 5, 1934, with completion of the Mead Oil Co., et al., No. 1 De Noya in the NE-NE-SW part of Section 10–25N-6E, at a total dept of 2,857 ft. in the Burbank sand. Other publications have discussed the geology of the pool, and the information presented here is but a brief summary. The South Burbank field is located off the southeast flank of the North Burbank field and, like the latter field, is a sand lens within the Cherokee shale formation which immediately overlies the Mississippi lime. Following post-Mississippian erosion, the general area of southern Kansas and eastern Oklahoma was covered by the Cherokee Sea in which shales were deposited upon the eroded surface of the Mississippi lime. As the Cherokee Sea advanced and receded upon land, offshore sand bars were formed. Major sand bodies such as North and South Burbank were laid down over an extended period of time. As conditions were altered by shifting currents, severe wind and wave action, and changes in the shore line, there were periods of deposition of silt and mud, resulting in inhomogeneities in the sand bars. The general attitude of the base of the sand in the South Burbank pool is that of an irregular plane dipping at the rate of about 40 ft/mile toward the west. The productive sand thins to the south and west as it is pinched-out by sandy shales. Permeability of the Burbank sand ranges from less than 1 md to about 2 darcies, and porosity ranges from 11 to about 29.5 per cent. Estimated average reservoir physical properties are tabulated in Table 1.
Fig. 1 presents the gas solubility and shrinkage curves for a sample of the reservoir oil. The gas in solution at the estimated original reservoir pressure of 1,200 psi was 380 cu ft/bbl, and the original formation volume factor was 1. 2. Gravity of the produced oil is about 39 to 40 API.