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Abstract The Banzala Field, located offshore Cabinda, Angola, contains over one billion barrels of oil in place, in shallow water, at shallow depth and close to an existing infrastructure. The field was discovered in 1981, but the well was lost due to a blowout after reaching total depth. The blowout resulted in the charging of a very shallow interval with over-pressure gas that now completely overlies the deeper oil reservoirs. The gas pressure was unknown, but believed to be very close to fracture pressure. Exact areal extent of the gas was unknown and it was not known if the system was static or continuing to charge from a deeper gas sand. The field has not been developed because of the extent and uncertainty of shallow overpressured gas that overlies the deeper oil reservoirs and because of limited individual well productivity. It was surmised that the outer limit of the shallow gas or gas/water contact (GWC) should correspond to a structural contour elevation such that the corresponding hydrostatic head of sea water exactly balanced the pressure of the gas bubble. Hydrostatic pressure data dating from 1983 was used to boundgas pressure between a maximum and minimum value. Consequently, it was believed, the vertical subsea depth of the GWC was bounded. A dynamically positioned geotechnical vessel was chartered to drill along the calculated gas boundary. The effort was successful and appraisal wells have been drilled and evaluated. Low well productivity will be solved by closely spaced wells on electric submersible pump (ESP). Well downtime and pump pulling costs are being minimized by deploying theESPs on coil tubing. A "hard landing" area is being designed into the platforms to support coil tubing equipment spreads. Introduction The Banzala Field is located in Blocks 34, 35, 45, and 46 in Area A of the Cabinda Concession, offshore Angola, West Africa (Figure 1) in approximately 125 to 175 ft of water. The hydrocarbon accumulation at Banzala occurs in three primaryreservoirs (Mesa, Lago, and Azul) between -1600 ft and -2000 ft tvdss. It is estimated that these sands contain over one billion barrels of oil in place. The productive oil bearing reservoirs are overlain by the Frio and Landana gas sands. The Frio is a naturally occurring overpressured gas sand at -1000 ft tvdss. The Landana was originally normally pressured, but has been supercharged with gas as a result of the discovery well blowout in 1981. The Landana is between -425 and -600 ft tvdss. The Banzala Field is an anticline with four-way dip closure. The anticlinal structure (Figure 2) is well defined by six wells and 2seismic data. The crest of the anticlinal structure is difficult to resolve seismically because seismic data quality is locally degraded by shallow gas. Although amaximum of three faults have been mapped, additional faults that are not imaged with the current seismic data may be present.

F. Friedmann, Chevron Petroleum Technology Company, T.L. Hughes, Schlumberger Cambridge Research, M.E. Smith, Chevron Petroleum Technology Company, G.P. Hild, Chevron USA Production Company, A. Wilson and S.N. Davies, Schlumberger Cambridge Research Abstract Thirty six thousand four hundred barrels of CO2 gel led foam were successfully placed via an injector well into the Rangely Weber Sand Unit in Colorado. The treatment objectives were 1) to improve volumetric conformance in this CO2 flood by reducing excessive CO2 breakthrough through fractures and 2) to increase oil recovery from the associated producers. Local reservoir characteristics indicate the need for a large-volume treatment to achieve these goals. The required treatment volume is beyond the economic limits of standard gel systems. Foam-gel technology is one way to economically achieve in- depth conformance improvement at Rangely by replacing 60% to 80% of the liquid phase by the less expensive CO2 phase. A new gelled foam system was specifically designed for application to Rangely conformance problems. The field-tested surfactant/gel system was designed according to the following criteria:to produce strong and robust gel led foams under the harsh pH conditions of a CO2 flood,to provide enough gelation delay to achieve the injection of a large foam volume with manageable injectivity reduction andto considerably reduce the unit cost of the treatment fluid relative to standard non-foamed gel systems. This paper describes the methodology used to design and test the optimum gel led foam system for Rangely. Laboratory results are presented to support the chemicals system selection, including gelation kinetics experiments, surfactant selection and core floods with supercritical CO2 at field conditions. The candidate well selection process is described, including injection profile surveys, offset well response and bypassed reserves calculation. Data taken during the injection phase of the program, including injectivity history and on-site quality control monitoring of the chemical system behavior are given. Finally, a preliminary assessment of the impact of the treatment on CO2 cycling rates and incremental oil production is presented. Introduction Oil recovery efficiency in CO2 floods can be substantially reduced if premature CO2 breakthrough occurs at offset producers through fractures. Injector polymer gel treatments have successfully improved volumetric sweep in some CO2 field projects. CO2 diversion from the fracture network to the adjacent matrix rock resulted in the recovery of additional oil reserves. However, the volume of injector polymer gel treatments can be limited by cost which in turn limits the potential impact on reservoir sweep. If there is significant crossflow between the thief zone and the remainder of the reservoir, larger treatment volumes impact sweep over a large volume of the reservoir thereby, improving incremental oil recovery. Gelled foam technology is one way to economically increase the treatment volume when reservoir characteristics dictate the need for in-depth conformance improvement. Fluid costs can be substantially reduced if 60% to 80% of the expensive aqueous phase can be replaced by a cheap readily available phase such as CO2. P. 883^

The Rangely Weber Sand Unit is giant oil field located in northwestern Colorado currently under miscible CO2 and water flooding operations. Field development has evolved through many stages, including infill drilling, waterflood expansions, and CO2 injection. Conformance improvement has become a major strategy as the CO2 project matures and production and oil cuts decline. The success of a conformance improvement effort requires development of links to the business objectives of the field's operating teams. The effort to improve conformance, although not new, has taken on significant strategic importance to insure continued strong economic performance and maximum oil reserves. A multi-disciplinary team was developed to focus on this issue. As with any effort attempting to change the focus of an operation, conformance improvement required a detailed study to scope the potential benefits, and to communicate those benefits to decision-makers. This paper summarizes the Rangely Conformance Improvement Team's efforts, including: Organizational approach to the effortStrategic alignment of conformance improvement with operating team prioritiesAssessment of the state of conformance and the tools developed for problem identification and opportunity rankingConformance improvement method selectionResults to dateFuture conformance improvement efforts and challenges.