Investigation On Exploitation Mechanism Of Water/CO₂ Huff And Puff In Tight Oil Reservoir

Numerous tight oil reservoirs in China have problems of insufficient natural energy,resulting in low oil recovery of depleted development,which is urgent to supplement formation energy.Meanwhile,some tight reservoirs have difficulties in injection performance or fluid channeling along fractures,making it difficult for displacement development in the oilfield.Taking the Ordos Basin as the research object,this research aims to solve the problems such as unclear effective distance of injected fluid,oil-water（oil-gas）flow law and oil-increasing mechanism during the huff and puff development for tight reservoirs.This paper investigates the effective distance of water/CO₂ huff and puff,enhancing oil recovery and CO₂ storage by using the combination of physical simulation,theoretical analysis,and numerical simulation.Numerical simulation research on water huff-n-puff and CO₂ huff-n-puff are conducted,taking tight blocks in Changqing Oilfield as an example,providing theory and reference for efficient development of tight oil reservoirs.（1）Countercurrent imbibition distance and saturation profile evolution law of tight cores are quantitatively characterized,and the countercurrent imbibition range is determined.Simultaneously,the influences of injection pressure,initial water saturation,matrix permeability and core wettability are investigated,and the relationship between countercurrent imbibition distance and imbibition recovery factor is revealed.Through CT online scanning device,countercurrent imbibition distance of 0.3×10^-3μm² core is quantified about 1.25～1.625 cm at 72 hours.The influence of injection pressure and initial water saturation on countercurrent imbibition distance is little,however,the efffect of matrix permeability and core wettability is significant.（2）Nonlinear countercurrent imbibition model of tight porous media is established,the dynamic evolution of imbibition saturation profile is clarified,and the reliability of the modified model is verified by experimental data.Then,the sensitivity analysis of influence parameters is conducted,including imbibition time,core length,oil viscosity,matrix permeability,relative permeability parameters,and capillary pressure parameters.Countercurrent imbibition process has critical imbibition saturation and critical imbibition time,which can be divided into three stages:the early super-diffusion stage(faster than t^0.5),the transition stage,and the late sub-diffusion stage(slower than t^0.5).（3）CO₂ huff and puff distance and dynamic saturation distribution of tight cores are quantified,the exploitation characteristics of micro-pore oil is revealed,and the stimulation range,oil recovery,and CO₂ storage ratio are determined.Through CT online scanning,CO₂ effctive distance of 0.22×10^-3μm² core is quantified as is 3.375 cm under16 MPa.CO₂ huff and puff primarily exploits crude oil in the larger pores（micron scale）,which is about twice that of the smaller pores（sub-micron and nano scale）.EOR-storage experiment quantitatively evaluates the CO₂ storage ratio of huff-n-puff under different depressurization conditions are 70.70%,42.45%,and 9.69%respectively for 0.22×10^-3μm² core.With the decline of production pressure,the oil recovery gradually increases while CO₂ storage ratio decreases,so there is a collaborative optimization of production pressure.Meanwhile,the high-temperature and high-pressure device is employed to investigate the oil recovery of CO₂ huff and puff in different phases,and the results show that supercritical CO₂ has the highest oil recovery.（4）Based on the evolution of CO₂ saturation profile in tight core,the one-dimensional diffusion coefficient of CO₂-oil in tight porous media is determined by the variable diffusion coefficient model,revealing the variation of the diffusion coefficient with time.Compared with the constant diffusion coefficient model,the variable diffusion coefficient model has a higher fitting degree,which can provide support for numerical simulation research.Simultaneously,according to the pressure data of CO₂ diffusion in different phase states,radial diffusion coefficient of CO₂-oil under different conditions are determined by combining theoretical formulas.Pressure is an important factor affecting diffusion coefficient and CO₂ huff-n-puff performance.（5）Combined with the experimental results and theoretical analysis of water/CO₂huff and puff,a reservoir-scale numerical simulation model considering the mechanism of water imbibition and CO₂ diffusion is established based on the actual geological model.Numerical simulation research of water/CO₂ huff and puff in tight reservoirs is conducted.Compared to not considering imbibition and diffusion effect,water imbibition and CO₂ diffusion during huff-n-puff development increase the stimulation range,enhancing oil production by 12.3%and 6.8%respectively,and their mechanism cannot be ignored.Based on the oil recovery and remaining oil distribution,the oil recovery and stimulation range under different injection-production parameters and artificial fractures are determined,providing theoretical basis and reference for oilfield development.