A Comparison of Minimum Miscibility Pressure Determinations for Weyburn CO2 Solvent Design

Abstract

Abstract In a miscible displacement process, me condition of miscibility between oil and solvent can be achieved at pressures greater than a certain minimum. This minimum miscibility pressure (MMP) is one of the essential factors affecting oil displacement efficiency. The MMP of an oil-solvent system is often used as a key criterion for screening and selecting suitable solvents for enhanced oil recovery (EOR) projects. This paper compares the suitability of various solvents on the basis of their MMP determined for three oil samples collected from Weyburn pool (a reservoir located in southeast Saskatchewan). Three different methods were employed for determining MMP, namely, slim tube experiments, rising bubble apparatus (RBA) tests, and correlations. The solvents investigated included pure CO2, several blended or impure CO2 streams, and a wellhead gas from the Steelman gas plant near the pool The contaminants in the impure CO2 streams considered were nitrogen (from flue gas) and methane (from recycled CO2). Tests were carried out to determine the various amounts of impurities in. CO2 that can be tolerated for a miscible flood in the Weyburn field. Results of the study indicated that the MMP values measured by the RBA technique agreed well with those measured using the slim tube tests and those predicted using a published correlation. However, MMP values measured by REA were generally lower than those determined by slim tube. The difference in the values may be attributed to the method of interpretation. RBA tests are fast, precise, and less expensive for determining MMP but may require a knowledge of the mechanism of miscibility development for efficient operation. This study demonstrated that for the Weyburn reservoir, promising EOR agents (having an MMP below 80% of the reservoir fracture pressure) are pure CO2 and blended CO2 containing up to about 12 mol% CH4 or 5 mol% N2. The wellhead gas from Steelman gas plant requires further enrichment (with CO2 or hydrocarbon gases) to become a suitable EOR agent for southeast Saskatchewan reservoirs. Introduction The majority of light and medium oil (LMO) reservoirs in Saskatchewan have reached their economic limit of production under primary and secondary melhods.1 There is limited opportunity to discover additional reservoirs in askatchewan Therefore, it is essential to develop proper tertiary enhanced oil recovery (EOR) techniques if light and medium oil production in Saskatchewan is to be maintained. Miscible flooding with carbon dioxide or hydrocarbon solvents is considered to be one of the most effective EOR processes applicable to LMO reservoirs. It is estimated that an additional 15 to 25 percent of initial-oil-in-place (IOIP) will be recovered if EOR is successfully applied to Saskatchewan LMO reservoirs.2 Such a gain will triple the existing LMO reserves and extend the producing life of these reservoirs by two decades. With this in mind, the Saskatchewan Research Council (SRC) started a five-year multiclient research project in September 1988 to assess the suitability of CO2 and/or a hydrocarbon gas as a miscible flooding agent for Saskatchewan light and medium oil reservoirs. A reservoir located in southeastern Saskatchewan. Weyburn, was selected for the study.