Study On The Effect Of Carbon Dioxide And Nitrogen Flooding In Low Permeability Reservoir

The reserve and production of high quality oil-gas field are decreasing with the improvement of exploration and development.The low permeability oil-gas field accounts for larger and larger proportion of oil production.Due to the poor physical property,small size of pore and throat,high capillary force,the water absorption capacity is poor and the speed of water injection response is slow.With breakthrough of water in oil wells,the oil and liquid productivity index decrease rapidly,which cause great difficulty to the stabilization and enhancement of oil production.How to develop effectively the ultra low permeability reservoir is the major problem confronted in the world at present.In this paper,the basic parameters of bubble point pressure,gas-oil ratio and surface degassing oil density in Gaotaizi reservoir of Longhupao oilfield are fitted by PVTi module in Eclipse software.The physical properties of formation oil after injecting different molar amounts of CO₂ and N₂ are analyzed.Using the component model of Eclipse software and the related parameters of tubule experiment method,a one-dimensional numerical model of tubule was established.The minimum miscibility pressure?MMP?of CO₂ and crude oil was calculated.The effects of CO₂ injection with different PV numbers on MMP were analyzed.42 CO₂ and N2 slug flooding schemes were designed,and the injection sequence and size of slug were optimized.A three-dimensional conceptual model for reservoir physical properties of Gaotaizi reservoir in Longhupao Oilfield was established,and the effects of reservoir permeability,porosity,heterogeneity?permeability gradient,coefficient of variation?,injection pressure and gas injection speed on oil displacement were studied.Eight CO₂ and N₂slug displacement schemes were designed,and the effects of the type and size of the pre-slug on oil recovery were analyzed by using grey correlation theory.The factors affecting the effect of gas injection flooding are discussed and analyzed.The results show that:With the increase of molar amount of CO₂ and N₂ injection,the bubble point pressure and gas-oil ratio of formation oil increase,and the viscosity decreases;with the increase of molar amount of N₂ injection,the formation oil density decreases first and then increases;the MMP calculated by numerical simulation method is 20.5 MPa,while the MMP calculated by CO₂ injection is 0.4 PV,0.6 PV,0.8 PV and 1 PV,and the MMP calculated by numerical simulation method is the same as that calculated by 1.2 PV.During slug flooding,the effect of CO₂ flooding is better than that of N₂.The optimal size of CO₂slug in this design is 0.6PV.The highest recovery of the two slug schemes is 0.6PVCO₂ slug+0.6PVN₂ slug,which is better than pure CO₂ flooding.The smaller the reservoir permeability,the bigger the porosity,the more serious the heterogeneity,the smaller the injection pressure and the slower the gas injection speed are.The lower the recovery of CO₂and N₂ flooding is.The recovery of 0.6PVCO₂ slug+0.6PVN₂ slug in three-dimensional conceptual model is higher than that of pure CO₂ flooding.The greatest correlation between CO₂ flooding and N₂ flooding is injection pressure.