Design and Evaluation of a Gravity-Stable, Miscible CO2-Solvent Flood, Bay St. Elaine Field

Abstract

ABSTRACT A gravity-stable miscible CO2-solvent flood is underway in the Bay St. Elaine Field, South Louisiana. A 33% pore volume CO2-solvent slug was injected into a dipping water drive reservoir and is being pushed downdip by the injection of nitrogen gas. The CO2-solvent selected for this tertiary flood was tailored by the addition of methane and n-butane to the carbon dioxide. This CO2-solvent provides the density difference required to complete a gravity-stable flood within the desired time period and also satisfies the minimum miscibility pressure requirements at reservoir conditions. The paper presents laboratory experimental work performed and process design work required to undertake this type of enhanced recovery project. The results obtained from slim tube tests to determine the CO2-solvent composition are presented as well as results of 12-foot sand pack displacement tests to evaluate the recovery efficiency of the selected CO2-solvent. Procedures used to determine the mixing zone lengths needed for CO2-solvent slug design are discussed along with the method of calculating critical velocity. Pressure pulse tests conducted to improve reservoir definition within the project area are reviewed. In situ residual oil saturations for the unconsolidated sand determined from pressure cores, log-inject-log water flood tests, single well partitioning tracer tests, and open hole well logs are presented. Field injection and current production data are also analyzed. The methods presented are being used to design CO2-solvent floods for reservoirs previously thought to be unsuited for conventional miscible CO2 flooding. The procedures and concepts discussed can be applied to flood design for numerous secondary and tertiary miscible CO2 projects.