Unique Enhanced Oil and Gas Recovery for Very High-Pressure Wilcox Sands Uses Cryogenic Nitrogen and Methane Mixture

Abstract

Summary Primary recovery is low for the volatile oil and retrograde condensate reservoirs in the hot, high-pressured, multilayered Wilcox sands in the Fordoche field, Point Coupee Parish, LA. Recovery from oil reservoirs by depletion is predicted by numerous model studies to be 26 to 36% of the OOIP. Condensate recovery by depletion is predicted to be 19% of the original condensate in place and 25% of original separator gas in place. The injection of dry natural gas at high pressures will result in a miscible displacement of reservoir fluids. The immiscible displacement of reservoir fluids. The injection of a mixture of nitrogen and dry gas is predicted to be as effective as injecting dry natural gas. Model studies of gas injection in volatile oil reservoirs predict recoveries from 47 to 54% of the OOIP, and in the retrograde condensate reservoir recoveries are predicted to be 30 and 46% of the original condensate and gas in place. Oil recovery studies at 6,955 psig indicate that gas injection will displace 89% at breakthrough and 93% at blowdown at 100% sweep efficiency. A well stimulation program has been developed that results in a 2.5-fold increase in production rates. Introduction The Fordoche field was discovered by Sun Oil Co. in 1966. The location is shown in Fig. 1. The field was developed fully on 160-acre well spacing by 1970. A total of 36 wells was drilled during the short 4-year development period. Well depths ranged from 11,800 to 16,500 ft (See Fig. 2, a typical log for the Fordoche field.) Seven reservoirs encountered between depths of 11,300 to 13,900 ft had initial bottomhole pressures (BHP's) ranging from 8,372 to 11,018 psig and bottomhole temperatures (BHT's) ranging from 226 to 278°F. The Sparta sands are undersaturated oil reservoirs. The Wilcox 4 and 5 (W-4 and W-5) sands are gas-condensate reservoirs. The W-8, W-12, and W-15 sands are undersaturated volatile oil reservoirs. All of the Wilcox reservoirs experienced rapid rates of reservoir pressure depletion. Productivity of all wells declined severely. After evaluating water injection and gas injection programs, high-pressure gas injection was selected for the two major oil reservoirs, W-8A and W-12A, and for the largest retrograde reservoir, W-5A. (See Figs. 3, 4, and 5). A very high-pressure miscible gas injection project was instituted to maintain pressure and well productivity and to enhance ultimate recovery. Initially, extraneous gas was purchased for makeup and injected with the processed natural gas produced in the field. Because of the increasing cost of natural gas and the restricted oil price of $5.35 per barrel, economics dictated that the project be changed to a partial pressure maintenance program with a lower ultimate recovery or find a substitute for natural as used for makeup.