Assessment of Oil Reservoirs Suitable for CO2 Flooding in Mature Oil Reservoirs, Changqing Oilfield, China

Abstract Most oil reservoirs in Changqing Oilfield area are low permeability. After several ten years of production, the effect of water flooding becomes worse and worse as the reservoir enters middle development stages. CO2 flooding is an important method for oil fields in the future to enhance recovery factor. In this paper, based on the data of 253 mature oil reservoirs, a set of potential evaluation method for CO2 flooding were builded to assess the potential of CO2 EOR in Changqing Oilfield. A numerical model was established to study the effect of the related factors to CO2 miscible flooding and immiscible flooding. These factors are reservoir properties (thickness, Angle, temperature, pressure, depth), rock physical property (porosity, permeability, saturation, permeability differential, fracture development degree and permeability) and fluid properties (viscosity, density). Technical and economic benefits is assessed, to determine screening criteria and index for CO2 miscible flooding, their relative importance, and the index weights. Using comprehensive fuzzy evaluation method, this paper sets up a comprehensive evaluation method of CO2 miscible flooding and immiscible flooding, Which suit for the different blocks of Changqing Oilfield. Comprehensive evaluation value of reservoir was calculated, according to the evaluation method. With evaluation value of 0.53 as a boundary, CO2 flooding potential assessment of Changqing Oilfield has been done. Among 253 reserviors, 213 reservoirs are suitable for CO2 flooding EOR, which accouts for 84% of the total number of reservoirs within the block. The geological reserves which is suitable for CO2 flooding could be 25.05×108 t, equaling to 80% of the total reserves of Changqing Oilfield. Therefore, CO2 flooding has a good application prospect in Changqing Oilfield, and it would bring huge economic benefits to Changqing Oilfield.