Study On The Numerical Simulation Of Increasing Oil Recovery By CO <1>

More than half of the oil and gas reserve now is in carbonate rock reservoirs. As we all know, most of carbonate rock reservoirs are multi-fracture formations, namely fracture-matrix dual media, in which matrix is the main accumulation space of oil and gas, and fracture is the main flow path of fluid. As rock surfaces in carbonate rock oil reservoirs is more likely mix-wet or oil-wet, it is hard for the injected water to enter the smaller pores of the matrix to displace the oil in it. Recently, a large number of studies and field examples have shown that CO2injection can achieve high oil displacement efficiency and sweep efficiency and greatly improve the producing degree of crude oil in the matrix of fracture-matrix dual media.R11oil reservoir is the shallowest hilltop of Wumishan Group in Renqiu, the North China Oilfield. It is a fractured carbonate rock block oil reservoir with bottom water, strong heterogeneity and low water-flooding producing degree. Based on the geologic and development characteristics of R11reservoir and the mechanism of EOR by CO2injection, this paper performes numerical simulation research on CO2flooding in R11reservoir. The main conclusions of this research are as follows:1、Through comprehensive literature survey and study on characteristics of carbonate reservoir and its water flooding, it is shown that gas injection is more appropriate for the improvement of the carbonate reservoir’s recovery.2、Gas flooding includes miscible displacement and immiscible displacement, and the oil displacement efficiency of the former is larger than that of the latter. However, it is proved by empirical equations and slime-tube tests that the current pressure of R11reservoir does not meet the requirement of miscible displacement. So only the immiscible one can be used.3、CO2can dissolve easily in crude oil and has a significant effect on viscosity reduction, which is indicated by the phenomenon that the crude oil’s saturation pressure, volume factor and expansion factor increase and its viscosity reduce with the increase of injection volume.4、The result of long-core experiments shows that the displacement efficiency of direct injection of CO2is better than that of CO2-water alternative injection and pulse CO2injection, and the displacement efficiency of CO2gravity drive is better than that of horizontal CO2flooding.5、Through the numerical simulation of R11reservoir, we find that plenty of remaining oil now distributes in the crust of structure’s top surface and mainly exists in the porous medium. So injecting CO2directly form the top, can increase the producing degree of oil in the porous medium and significantly enhance the oil recovery.