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This paper summarizes the comprehensive reservoir characterization effort for the foam pilot area and discusses the response to foam injection in the CO2 Foam Field Verification Pilot Test conducted in the East Vacuum Grayburg San Andre S Unit (EVGSAU) in New Mexico. A detailed study of the pilot pattern geology provided an understanding of the major controls on fluid flow in the foam pattern. Pattern performance data, falloff testing, profile surveys, and interwell tracer results were integrated into the geologic model to guide project design work and provide a framework for interpretation of foam performance. Localized regions of high permeability resulting from solution enhancement of the matrix pore system appear to be the primary cause of the early CO2 breakthrough and channeling of injected CO2 toward the problem production well in the foam pattern. Positive response to foam injection is indicated by reduced injectivity and injection profile data in the foam injection well; by results from time sequence monitor logging in the observation well; and by changes in production performance in the high GOR, "offending" production well in the foam pattern. Hall plots and pressure falloff testing were used to measure in situ changes in fluid mobility near the foam injection well. Time sequence logging responses at an observation well located 150 feet from the foam injector provided evidence of changes in fluid flow patterns in response to foam injection. Positive response to foam injection is further evidenced by changes in the CO2 production and oil rate performance at the "offending" production well in the foam pilot pattern.
EVGSAU GEOLOGIC SETTING
The Vacuum Field, located about 15 miles northwest of Hobbs in Lea County, New Mexico, is comprised of several large Units and leases. The East Vacuum Grayburg-San Andres Unit (EVGSAU) covers more than 7000 acres on the eastern side of the Vacuum Field. The primary productive interval at EVGSAU is comprised of the dolomitized carbonate sequences in the upper few hundred feet of the San Andres Formation, at a depth of approximately 4500 feet. The San Andres structure is an east-west trending anticline with more than 400 feet of closure above the original oil/water contact. The reservoir section is informally subdivided into a "lower" San Andres section and an "upper" San Andres section, separated by the more siliciclastic Lovington Sandstone Member.
Stratigraphy and Lithofacics
The San Andres reservoir section is comprised of a series of repeated, anhydritic, dolomitized, fining-upward, carbonate sequences composed of grain-rich dolostones which grade upward into dolomudstones. The subtidal, grain-rich carbonate facies form the primary reservoir units; the dolomudstones contain little effective porosity. Repetition of these depositional packages upwards through the formation results in a San Andres section composed of cyclical, shallowing/shoalingupward parasequences. Commonly occurring reservoir pore types include primary intergranular porosity, intercrystalline porosity (related to dolomitization), grain-moldic porosity, and vugular porosity. All of these pore types show varying degrees of solution enhancement.