| Popis: |
Abstract A CO2 foam field trial was conducted in the North Ward-Estes field in Texas to evaluate the effectiveness of foam in improving CO2 sweep efficiency. This paper describes the design, results, and analysis of foam treatment with emphasis on the design methodology of applying foam in grossly heterogeneous reservoirs, characterized by uneven areal and vertical sweep in most patterns. Over a period of nearly two years, foam was emplaced into an injector four times by alternately injecting CO2 and surfactant solution, and followed by continuous CO2 injection after each emplacement. Foam was generated in situ, which reduced CO2 injectivity by 40 to 85%. Gas production in the problem producer decreased dramatically, while gas and oil production in other offset producers increased, indicating areal diversion. Vertical diversion also occurred, as evidenced by significant increase in oil production in the problem producer, where tertiary oil response had peaked prior to the foam treatment. The field test shows that a property designed foam treatment can significantly improve the CO2 sweep efficiency and be economically successful. The methodology developed in the design and analysis of the foam test may be applied to other conformance improvement processes. Introduction The North Ward-Estes (NWE) field is located in Ward and Winkler counties, Texas. Primary production began in 1929, waterflood began in 1955, and CO2 flood started in 1989. The oil-in-place for the CO2 project area at the beginning of CO2 flood was estimated at 77 million barrels, or 54% of original oil-in-place. The CO2 project area, covering 3,840 acres, was developed in 20-acre patterns within six sections. The producing formation in this area is Yates, which consists of A, B, C, D, E, F, Stray, J1, and J2 sands, separated by dense dolomite. The sands vary from fine-grained sandstone to siltstone, containing various amounts of clays. Fig. 1 is a type log of this formation. Rock permeability varies greatly between and within sands, with a Dykstra-Parsons coefficient of about 0.85. Table 1 summarizes NWE reservoir properties. Additional information about the NWE CO2 flood has been documented previously. Poor sweep efficiency has been a concern since the start of the CO2 flood. Poor sweep was manifested by fast CO2 breakthrough and low tertiary oil production. In some areas, CO2 breakthrough occurred in less than a month, with tertiary oil response peaking in less than three months. Several methods were considered to improve the CO2 conformance. Mechanical treatments were impractical in NWE because most wells were completed as open holes, "shot" with nitroglycerine, and hydraulically fractured. Drilling new wells would be risky and expensive. Foam treatment was the most promising alternative because foam can selectively retard gas flow. A field trial was conducted to evaluate its potential in this reservoir. This paper describes the design methodology, lab study, results, and analysis of the foam field trial. PROJECT DESIGN Objective of Foam Treatments In past studies, foam has been considered mainly for in-depth mobility control purposes. This was not a viable objective in NWE because high CO2 mobility was not the culprit for poor sweep. Lab and field injectivity studies showed that the CO2 mobility was only marginally higher than the water mobility and that the mobility of a 1:1 WAG was about half of the water mobility. P. 129^ |
