Phase behavior characteristics and water-flooding development technical policy of weakly volatile oil in carbonate reservoirs

A fracture-pore carbonate reservoir in eastern Pre-Caspian basin was taken as an example to analyze the oil phase behavior change and seepage physical characteristics change of weakly volatile oil reservoirs with the decrease of formation pressure. Based on the analysis, the water-flooding development technique policy of the reservoir under different formation pressure was presented. Experiments show that the weakly volatile crude oil degasifies as the formation pressure decreases, with methane and intermediate hydrocarbons separated out successively, and the crude oil gradually transforms into ordinary black oil. With the separation of light hydrocarbons, the saturation of in-place oil drops rapidly, the viscosity increases, and the oil permeability reduces, leading to lower well productivity. Retention of formation pressure is a vital factor controlling the reservoir development effect. The lower the formation pressure, the lower the ultimate recovery. Given low formation pressure, water-flooding is required to recover the formation pressure. In water-flooding, the lower level the formation pressure is retained at, the lower the reasonable pressure to be recovered is, and the lower the ultimate recovery is. Compared with zones without fractures, the zone with fractures provides lower injection-production ratio when water-flooding is conducted under the same formation pressure, and its water-flooding time has more impact on oilfield recovery. Therefore, it is recommended to develop weakly volatile oil reservoir by early water-flooding in a moderate way. Key words: carbonate reservoir, weakly volatile oil, phase behavior change, seepage characteristics, formation pressure, water-flooding time