Mechanism Research Of CO₂ Enhanced Oil Recovery For Tight Sand Oil Reservoir

In the world（especially in North America）,the mechanism of CO₂ flooding for enhancing oil recovery（CO₂-EOR）in conventional reservoirs has been studied for decades,and a relatively mature understanding has been formed.CO₂ flooding has been effectively applied to enhance oil recovery in conventional reservoirs.But worldwide,the mechanism of CO₂ flooding for unconventional reservoirs（especially tight sandstone reservoirs）has just begun.Most of the Yanchang Formation sandstone reservoirs in northern Shaanxi belong to tight sandstone reservoirs,which are characterized by the development of fractures and micro-fractures of different scales.Therefore,the development of fractures and micro-fractures is one of the most essential geological characteristics,and is also one of the main factors controlling the displacement mechanism of CO₂-EOR,which is different from conventional reservoirs.The extensive development of fractures and micro-fractures in Yanchang Formation tight sandstone reservoirs in northern Shaanxi has resulted in the complication of the mechanism of CO₂ and crude oil and the flow mechanism of fluids in reservoirs to a certain extent.This leads to a series of questions:What is the percolation characteristics of CO₂ in tight sandstone reservoirs in northern Shaanxi?In addition to conventional mechanism,is there any other mechanism in the tight reservoir with fracture?How does the tight sandston in northern Shaanxi with fracture affect the flow of CO₂ in the reservoir?"No injection,no production"is a realistic problem in crude oil production of tight sandstone reservoirs in northern Shaanxi.Can we put forward a highly targeted injection-production plan?This paper will focus on the geological characteristics of the study area and try to solve the above problems,providing theoretical basis for efficient and low-cost CO₂ injection development of tight sandstone reservoirs in northern Shaanxi.Starting from the basic geological characteristics of tight sandstone reservoirs in northern Shaanxi,based on the reservoir characteristics,this paper grasps the basic characteristics of tight sandstone reservoirs in the study area which are different from conventional reservoirs.Taking the mechanism of formation fluid-CO₂ system and formation fluid-CO₂-reservoir system as the starting point and foothold,this paper studies fluids by means of laboratory test analysis,physical simulation and numerical simulation.Multiphase flow in dense media and their interaction,fluid flow characteristics and displacement effect in dense sandstone reservoir are studied,and CO₂ channeling rule is studied,so as to interpret the mechanism of CO₂-EOR displacement in dense sandstone reservoir.Finally,CO₂-EOR displacement technology which accords with the real reservoir characteristics of Yanchang formation in northern Shaanxi is put forward and numerical simulation of development effect is carried out to guide oilfield development.The main conclusions are as follows:（1）The study area has basic conditions for the application of CO₂ flooding technology.Because of its superior geographic location,convenient transportation and abundant gas source,and its Chang4+5 reservoir sand body is large in scale,stable in thickness,good in connectivity and wide distribution of reservoirs,Area A can be used as a model of CO₂flooding in tight sandstone reservoirs in northern Shaanxi.（2）The lithological characteristics of Chang4+5 tight sandstone reservoir（fine-grained feldspar sandstone,poor sorting,low composition maturity）and complex micro-pore structure determine its poor physical properties,and the permeability is not completely controlled by porosity.（3）Based on outcrop,core and imaging logging data,the fracture characteristics of Chang4+5 reservoir in the study area are obtained:the fracture obeys the fracture occurrence distribution characteristics of Yanchang Formation,and the fracture is mainly high-angle fracture with dip angle greater than 60 degrees.There are mainly two dominant orientations:NEE,NNE,fracture extension from 3 meters to 20 meters,height from 0.5 meters to 3 meters,and development density greater than 1/m.（4）The mechanism of CO₂ flooding in the study area is manifested in both conventional and unconventional aspects.Conventionality is mainly reflected in good expansibility（30%volume expansion under formation pressure）and good viscosity reduction effect（64.29%viscosity reduction under formation pressure）.On the one hand,unconventionality is reflected in the compactness of reservoirs,which will change the critical properties of fluids,and may cause changes in key parameters such as minimum miscibility pressure;on the other hand,molecular diffusion can not be ignored in the process of CO₂ flooding in fractured tight reservoirs,and the effective diffusion coefficients of CO₂-crude oil system in the reservoirs of the study area are obtained in the order of magnitude of 10^-6-10^-5.（5）Based on the three-parameter non-linear seepage law of tight sandstone reservoir,a calculation model of non-steady CO₂ miscible flooding is established,and the non-steady CO₂flooding experiment is carried out.The typical phase permeability curve for non-miscible CO₂flooding in tight sandstone reservoir is obtained,and the influencing factors and degree of CO₂ flooding efficiency are studied,and it is pointed out that the greatest influence is pressure on the displacement efficiency,followed by injection rate and permeability.（6）Heterogeneous linear physical model and two-dimensional mathematical model simulation results show that the factors affecting gas channeling in the study area mainly include reservoir heterogeneity and fracture.The stronger the heterogeneity is,the stronger the"predatory"of the high permeability area to CO₂ gas,the more serious the gas channeling is;the areas with better physical properties and higher permeability have wider gas coverage,but lower gas saturation in the coverage area;conversely,when the permeability is low,the longitudinal coverage is smaller,but the gas saturation in the coverage area is larger.The configuration relationship between fracture and well location,fracture length and density all have influence on gas channeling.The bigger the angle between fracture direction and injection-production direction is,the higher the cumulative gas injection volume is,the higher the utilization ratio of injected gas is,and the higher annual oil production and recovery rate are.The longer the fracture extends,the larger the gas swept area,and the less gas can be seen and broken through in production wells.The larger the fracture density is,the larger the gas sweep area is,which is conducive to slowing down gas breakthrough and channeling to production wells.Reflected in the injection-production mode,injection in the high permeability areas and production in the low permeability areas mode can first ensure the gas injection capacity,maintain a high pressure level throughout the production period,and is less likely to occur gas channeling,high oil displacement efficiency.That is to say,reasonable formulation of injection-production mode based on the geological characteristics of the study area is of great significance to the effect of gas injection development.（7）Considering the mechanism of gas flooding and the law of seepage flow,based on the high-fitting fluid model and reliable geological model,and aiming at the geological characteristics of tight sandstone reservoirs in northern Shaanxi and the practical problems faced by CO₂ flooding,the technology of CO₂ injection single sand huff and puff in the study area is put forward,and the numerical simulation is carried out.The results show that the cumulative oil increment at the end of the prediction period is 1.48 million tons,the recovery rate is increased by 25%compared with the pre-prediction period,and increased by nearly 11%compared with the water flooding rate at the end of the prediction period.The production effect is good to provide a theoretical basis for efficient gas injection production in the study area.