The injection of water or immiscible gas into an oil deposit only leads to average recovery rates. It was, thus, natural to try to improve these recovery rates, and research has increased in this field over the last twenty years.
Recovery with conventional injections is imperfect for two technical reasons. First, incomplete sweeping of the reservoir space (macroscopic trapping). Second, trapping of residual oil by capillary action in the swept areas (microscopic trapping).
To improve the recovery it is necessary to improve the spatial sweep by reducing the mobility ratio between the two fluids and to reduce or better drop the capillary forces by obtaining the miscibility of the two fluids.
The miscible method is the most efficient when two fluids are miscible -when they can mix to form a single fluid-. it is interesting to use an injected fluid miscible with the residing oil. The interfacial forces being suppressed, there is no more saturation in residual oil and the efficiency of displacement is theoretically 100% from a considerable improvement of the recovery, which can reach 30 to 40% for injected gas miscible with the oil in place.
The gas injection can be miscible or immiscible depending on the injection pressure. Miscibility can be achieved when the pressure exceeds the minimum miscibility pressure (MMP). Temperature and pressure are important factors that usually affect MMP. The properties of the oil play an important role in the success of miscible injection, with miscible gas injection working best when the petroleum is light.
Miscible gas injection is the most widely applied enhanced oil recovery (EOR) process. Based on well-established physical principles, has been developed to estimate the conditions under which gas will be miscible with oil. Of a range of potential gas injection (such as CO2, enriched hydrocarbon gas, N2, or H2S) through the use of readily available gas and oil properties. It has been applied to many reservoirs around the world. With an illustration of its application to the screening of EOR gas injection for the Malaysian basin.
The method uses a correlation that can be used to quickly estimate miscible or near-miscible residual oil saturation, Sor, 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 the representation of the physical and chemical properties of crude oil and injected gas through Hildebrand solubility parameters.
Thirty-four offshore reservoirs operated by ExxonMobil Exploration and Production Malaysia Inc. (EMEPMI) or Production Sharing Contract (PSC) partners were selected for EOR gas injection. Potential injection gases included pure CO2, CO2 diluted with methane, field separator gas, LPG-enriched CO2, and LPG-enriched separator gas. A spreadsheet was created to facilitate the calculation of MMP or MME using the correlation of solubility parameters for a large number of potential reservoirs and injection gases. The correlation was useful in classifying potential reservoirs and injectors respecting the potential EOR of gas injection and identifying reservoirs to carry forward for further evaluation in the laboratory and by simulation.
In general, the predictions from the correlation compare reasonably with the more expensive experimental data obtained in a subsequent detailed evaluation.