Stimulation of a Producing Horizontal Well Using Enzymes that Generate Acid In-Situ - Case History

Abstract

Abstract The Libwa 4 well offshore the Democratic Republic of Congo was stimulated in November 1999 using an enzyme-based process that generated acid in-situ. The results to date have been excellent. Well tests as high as 759 BOPD have been recorded after the treatment compared to less than 100 BOPD before the treatment. Introduction The Libwa field is located off the coast of the Democratic Republic of Congo (formerly Zaire) West Africa. The field was discovered in June 1981 and declared commercial in 1983. Development was initiated soon after, but was deferred due to the subsequent discovery at the Lukami field. The Libwa field was put on production in January 1990. The Libwa field was formed in response to growth-faulting. The structure is a fault block anticline plunging northwest-southeast. The faults bounding Libwa field in the Aptian to Albian age section formed in response to movement of the underlying salt. They extend upward into the lower Kinkasi, and are believed to extend downward into the underlying Loeme salt layer. Structural dip within the Libwa Upper Pinda reservoir averages about 14°. The depositional environment of the Libwa field is that of a shallow carbonate shelf, adjacent to a clastic-dominated coastline with the terrigenous material periodically supplied by the river systems of deltas, as observed in other nearby Upper Pinda fields. The Libwa field is characterized as a low permeability (2 millidarcy) limestone with about 240 feet of oil column overlain by about 310 feet of gas cap. The oil leg is laterally continuous through the field and the gas zone is very thin at the downdip portions of the field that thickens towards the crest of the structure. Core samples indicated that there is virtually no vertical permeability and minimal natural fracturing. All of the Libwa wells currently on production required acid stimulation in order to establish economic production rates. The Libwa Upper Pinda reservoir is a saturated oil system with a large gas cap. Original oil in place is estimated to be 350 MMSTB and the original gas in place in the gas cap is estimated to be 114 BCF. PVT analysis indicated a solution gas-oil ratio of approximately 470 scf/STB at a bubble point pressure of 2540 psia. Cumulative oil recovery from the field through year 2000 is 5.2 MMSTB or only 1.5% of the original oil in place. Libwa 4 is a horizontal well that was drilled in 1990 into the low permeability (2 md) limestone of the Libwa field [Figure 1].