Pressure Transient Analysis Of CO₂ Flooding In Low Permeability Reservoirs

Low-permeability reservoirs have low permeability,and conventional water flooding requires large-scale fracturing operations to achieve production,but the cost is high and the effect is slow.Carbon dioxide has the characteristics of reducing the viscosity of crude oil and forming miscibility with crude oil.The injection of carbon dioxide can greatly improve the degree of crude oil recovery in low-permeability reservoirs,making it an important development method for low-permeability reservoirs.At present,the research on carbon dioxide flooding mostly adopts the experimental and numerical software simulation methods,and there are few studies on the dynamic analysis of carbon dioxide flooding pressure,and the research on the mathematical model is mostly concentrated on the traditional two-zone composite model.Therefore,it is necessary to establish a three-zone composite pressure dynamic mathematical model of carbon dioxide flooding in low-permeability reservoirs according to the characteristics of low-permeability reservoirs,carbon dioxide physical properties,and flow mechanism,and form a pressure dynamic analysis method.In this paper,the basic physical properties of carbon dioxide,oil displacement mechanism and flow characteristics under miscible and immiscible conditions with crude oil are given first.A mathematical model of composite flow in low permeability reservoirs with carbon dioxide flooding in three zones is established without considering the starting pressure gradient,and the bottom-hole pseudo-pressure solution is obtained by using Laplace transform and Stehfest numerical inversion.A mathematical model of composite flow in low permeability reservoirs with carbon dioxide flooding in three zones considering the starting pressure gradient is established,and the bottom-hole pseudo-pressure solution is obtained by using Laplace transform and Stehfest numerical inversion and introducing Green's function.The typical curve of CO₂ flooding pressure dynamic in low permeability reservoir is drawn,the stages of the typical curve are divided,and the sensitivity analysis of the main influencing parameters is carried out.The research results show that the carbon dioxide flooding flow in low permeability oil reservoirs is divided into 7 stages:pure wellbore storage,transition,radial flow in CO₂ area,transition from CO₂ area to two-phase oil and gas area,radial flow in two-phase oil and gas area,transition from two-phase oil and gas area to pure oil area and radial flow in pure oil zone.Compared with the pressure dynamic curve without considering the pressure gradient,the pressure gradient will cause the upturn of the radial flow phase in each region.The larger the starting pressure gradient is,the larger the pressure is consumed,and the more obvious the upturn of the pressure derivative curve is in the radial flow stage.The greater the mobility ratio of the oil and gas two-phase region,the closer the mobility of the oil and gas two-phase region is to the pure oil region,the more difficult the fluid flow,and the greater the pressure consumed when the CO₂ region transitions to the oil and gas two-phase region.The more obvious the transition between the oil and gas two-phase area,the smoother the transition from the oil and gas two-phase area to the pure oil area.The higher the mobility ratio of the pure oil area,the more obvious the transition from the oil and gas two-phase area to the pure oil area.The smaller the radius of the CO₂ area,the shorter the duration of radial flow in the CO₂ area,and the earlier the transition to the oil and gas two-phase area;the smaller the radius of the oil and gas two-phase area,the shorter the duration of radial flow in the oil and gas two-phase area,and the pure oil area The sooner it appears.The dynamic interpretation method of measured pressure is given.The measured data of A and B injection wells were interpreted and analyzed,and parameters such as reservoir permeability,regional radius,well-reservoir skin,and pressure gradient were obtained.It provides an effective method for the dynamic analysis of carbon dioxide flooding pressure in low permeability reservoirs.