The effect of oil and gas molecular diffusion in production of fractured reservoir during gravity drainage mechansim by CO₂ injection

During the primary production of fractured reservoir most of oil is produced from fractures and a lot of oil remains in matrix. Trapped oil in the matrix can be recovered by gas injection by activating gravity drainage mechanism. In addition there is a big impact of molecular diffusion of oil and gas in total oil recovery from fractured reservoir. The experimental work can by used to model this mechanism in combination of numerical simulator to investigate this phenomenon more accurately. A fully compositional model has been applied to a numerical experiment in literature to investigate the drainage of CO2 from a core with artificial fractures and the effect of molecular diffusion included. The same study has been applied for a synthetic fractured reservoir model to investigate the effect of CO2 injection in oil recovery mechanism in the field scale. In this work we found that at early stage we have oil swelling and gravity drainage followed by a slow extraction mechanism which recovers the intermediate and heavy components from the residual oil. The combined effect of diffusion and gravity suggests that application of the oil and gas diffusion coefficients is critical in any field scale simulation of a fractured reservoir and the correct diffusion coefficients should be applied. A lot of oil reservoirs in the world are fractured and with primary recovery we can produce maximum 30 % of original oil in place therefore we need to have an accurate observation of secondary and tertiary oil recovery in these reservoirs. The understanding of oil recovery mechanism also is crucial for reservoir management especially when developing field management plans. A low miscibility-pressure requirement often is a significant advantage of CO2-miscible flooding. This process could have significant future application in areas with economical CO2 supplies from natural deposits or surface sources.