Research On Improving Efficiency Of CO₂ Huff-Puff In Tight Reservoir Based On Nano-pore Confinement Effect

With the growth of global energy demand and the dependence on foreign crude oil of China exceeding 70%,the efficient development of tight oil reservoirs is of great significance to alleviating the energy crisis and ensuring national energy security.Due to the enrichment of nano-scale pores,tight reservoirs have low porosity,low permeability,rapid the productivity decline,and low oil recovery.Existing studies have shown that CO₂ injection after conventional hydraulic fracturing can effectively improve the recovery of tight oil reservoirs,which has great application potential and dual values of economy and environmental protection.However,the theoretical foundation of tight oil reservoir development is still weak,that is,CO₂/oil interaction mechanisms and oil flow characteristic in nano-pore are lacking,which restricts the productivity of tight oil reservoir.Accordingly,the equation of state of fluid in nano-pore is modified according to the phenomenon of critical physical property migration.The confinement effect of nano-pores is characterized by physical property migration,capillary pressure,adsorption behavior.And then,the calculation method of vapor-liquid equilibrium is proposed for nano-porous media,which is solved by iterative algorithm based on updating of the compositions of components in the bulk phases.This developed model of vapor-liquid equilibrium calculation has been validated by experimental results.Besides,a novel model for describing viscosity distribution of confined fluid in nano-pores was proposed on a basis of molecular kinetic mechanism of liquid/solid interaction.Then,the developed models of velocity profile and flowrate on the cross-section of liquid hydrocarbon in nanopores with diameters greater than 2 nm are derived coupled with proposed viscosity distribution function,slip velocity model,and the law of Newton’s internal friction,and have validated with experimental results and molecular dynamic simulation results from the literature.According to this model,the apparent permeability is derived for tight oil reservoir simulation.And then,combined with the research results of phase equilibrium model and fluid flow model of confined fluid in nano-pores,a typical tight reservoir geological model with a single-fractureing section was established,to simulation CO₂ huff-puff development process of tight oil reservoir.Based on the CO₂ huff-puff process,CO₂ huff-puff to flooding development model for tight oil reservoirs is proposed combing with the same well injection and production method,and CO₂-alternating-N₂ injection scheme is evaluated.For those development modes,the mechanisms and corresponding key parameters are studied.The results show that the confinement effect of nano-pore promotes the dissolution of CO₂in crude oil,and greatly affect the interaction mechanisms of CO₂ and tigh oil in nano-porous meida:decadence of fluid saturation pressure,weakness of light component extraction,decadence of minimum miscible pressure,ascendence of diffusion coefficient increases,increase of CO₂ solubility in crude oil,weakness of viscosity-reducing effect.From the proposed flow model of oil in nano-pore,the confined flow characteristics is revealed:the viscous flow away from wall is approximately parabolic similar to Hagen-Poiseuille flow,and the maximum velocity in center is related to pore size and Hamaker constant which characterizes liquid/solid interaction strength.The thickness of low-velocity region could be quantitatively described,which could be considered as the action scope of solid surface.The critical radius of nano-pores to distinguish the confined flow and bulk flow is obtained from the analysis of flowrate enhancement factor and ascends with the descendant of liquid/solid interaction strength.The flow characteristics of confined flow curve from this proposed flow model with certain external pressure driving conditions are consistent with the non-Darcy flow curve of tight reservoir,which means this proposed flow model can be used to explain the nonlinear seepage low of tight reservoir.Besides,reducing wettability of pore wall is beneficial to improving oil fluidity in tight reservoir,and increasing temperature can reduce the action range of nano-pore wall on the confined fluid flow.The simulation results of CO₂ huff-puff process show that the effective throughput is 3 turns,and the problems such as as the small range of matrix utilization and the deficit of replenishing reservoir pressure are prominent.According to this,the development mechanism of“injection-production in same well and inter-fracture displacement”is estabilished,and the effect production model of CO₂ huff-puff to flooding is proposed,which could greatly improve the production rate of tight oil in inter-fracture matrix.Beside,the development model of“nitrogen injection to increase reservoir energy and production efficiency”is optimized,and the CO₂-N₂ slug huff-puff process is selected,which could effectively improve the scope of CO₂ diffusion and reservoir pressure,and could increase the CO₂ retention rate in tight reservoir.