Methodology for Miscible Gas Injection EOR Screening

Abstract

Abstract Miscible gas injection is the most widely applied light oil enhanced oil recovery (EOR) process.A methodology, based on well-established physical principles, has been developed for estimating the conditions under which gas will be miscible with oil.The methodology enables rapid screening of a range of potential gas injectants (such as CO2, enriched hydrocarbon gas, N2, or H2S) through the use of readily available gas and oil properties.The methodology has been applied to numerous reservoirs worldwide.An overview of the methodology is provided along with an illustration of its application to screening of gas injection EOR for the Malay Basin. The methodology makes use of a correlation1 that can be used to rapidly estimate miscible or near-miscible residual oil saturation, Sorm, minimum miscibility pressure (MMP), or minimum miscibility enrichment (MME) for a wide range of injected gases, crude oils, temperature and pressure conditions.The correlation is based on representation of the physical and chemical properties of the crude oil and injected gas through Hildebrand solubility parameters. Thirty-four offshore reservoirs operated by either ExxonMobil Exploration and Production Malaysia Inc. (EMEPMI) or production sharing contract (PSC) partners were screened for gas injection EOR.Potential injection gases included pure CO2, CO2 diluted with methane, field separator gas, CO2 enriched with LPG, and separator gas enriched with LPG.A spreadsheet was created to facilitate calculation of MMP or MME using the solubility parameter correlation for a large number of reservoirs and potential injection gases.The correlation was useful for ranking reservoirs and potential injectants with respect to gas injection EOR potential and identifying which reservoirs to carry forward for in-depth laboratory and simulation evaluation. In general, the predictions of the correlation compare reasonably with the more costly experimental data obtained in subsequent detailed evaluation. Introduction Gas injection is the most widely applied EOR process for light oils.Oil recoveries for gas injection processes are usually greatest when the process is operated under conditions where the gas can become miscible with the reservoir oil.The primary objective of miscible gas injection is to improve local displacement efficiency and reduce residual oil saturation below the levels typically obtained by waterflooding.Examples of miscible gas injectants are CO2 or N2 at sufficiently high pressure, dry gas enriched with sufficient quantities of LPG components, and sour or acid gases containing H2S. The conditions under which gas becomes miscible with oil (MMP or MME) are most commonly determined in the laboratory using slim-tube experiments.Phase behavior measurements, in combination with compositional simulation, can also be used to determine miscibility conditions.However, slim-tube and phase behavior measurements can be time consuming and costly, especially in situations where many potential gas injectants and reservoirs need to be evaluated. A number of correlations have been proposed for estimating MMP or MME when experimental data are not available.The correlations can also be used to define the conditions to be used in PVT and slim-tube tests, thereby minimizing the time and cost of the laboratory program. Lange1 proposed a correlation that can be used to predict MMP or MME for a wide range of injected gases, crude oils, temperatures, and pressures.The correlation is based on representation of the physical and chemical properties of the crude oil and injected gas through Hildebrand solubility parameters.It uses readily available gas and oil properties and appears to be applicable to any injected gas composition, which are distinct advantages over many other predictive schemes. This paper describes the application of the solubility parameter correlation to screening of 34 offshore reservoirs in the Malay Basin.Potential injection gases included pure CO2, CO2 diluted with methane, field separator gas, CO2 enriched with LPG, and separator gas enriched with LPG.The predictions of the correlation are compared with the predictions of other correlations and with available experimental measurements of MMP and MME for the reservoirs of interest.