Study Of CO₂-Viscoelastic Fluid Synergistic Oil Drive Mechanism In Tight Sandstone Reservoirs

Due to insufficient energy in tight oil reservoirs,small reservoir porosity,extremely low permeability（Kg<1m D）,and low natural depletion production production,it is necessary to de-densify and increase energy in the reservoir to increase the mobility of crude oil and improve its performance.crude oil recovery.Conventional water injection is difficult to replenish energy,water injection pressure is too high,water flooding development production decline fast,low recovery factor,need to inject medium to replenish formation energy for a long time;CO₂ flooding is an important technical means to enhance oil recovery and achieve effective CO₂ storage in tight reservoirs,but the reservoir heterogeneity is strong,resulting in CO₂ flooding is prone to gas channeling,and the effect of enhancing oil recovery is not good.Therefore,it is urgent to study a new oil displacement method or technology suitable for CO₂ flooding to effectively enhance oil recovery in tight sandstone reservoirs.The viscoelastic fluid flooding constructed by viscoelastic surfactant with low interfacial tension has the similar function of surfactant/polymer binary composite flooding.The viscoelastic surfactants can meet the needs of low/ultra-low permeability reservoirs and improve water/gas injection sweep volume and oil displacement efficiency due to its small molecular weight.In order to further improve the effect of CO₂ flooding,a low interfacial tension viscoelastic fluid flooding system was introduced,and the mechanism of CO₂-low interfacial tension viscoelastic fluid synergistic flooding was studied in tight oil reservoirs.In view of the tight sandstone reservoir environment and the performance requirements of the control and flooding system,the molecular structure of anionic gemini surfactants（hydrophilic head group type,hydrophobic chain carbon number,linking group carbon number）on the solution micelle size,interfacial activity was systematically explored,microstructure and rheology,the optimal molecular structure of viscoelastic surfactant with low interfacial tension was determined.On this basis,a GOBT/inorganic potassium salt low interfacial tension viscoelastic fluid（GOBTK for short）was constructed and evaluated.The adaptability of GOBTK system in complex reservoir environment was discussed,and the relationship between CO₂ and low interface in tight sandstone reservoirs was discussed through thin tube experiments,dynamic core flooding,microscopic visualization,emulsification washing effect and wettability testing.Synergistic oil displacement effect and mechanism of tension viscoelastic fluid.The main research results obtained are summarized as follows:（1）The viscosity,viscoelasticity and oil-water interface activity of carboxylate Gemini surfactants were the best when the number of hydrophobic carbon chain and the number of connecting group were the same.The viscosity,viscoelasticity and oil-water interface activity of Gemini surfactant solution were improved by increasing the number of hydrophobic carbon chain with the same number of bonding group carbon.When the number of hydrophobic carbon chain is unchanged and the number of bonding group is increased,the solution viscosity and viscoelasticity show a tendency of strengthening first and then weakening.The evaluation of oil-water interfacial tension and rheological properties shows that GC18-2-18（carboxylate Gemini surfactant）is more advantageous.（2）GOBT is an asymmetric salt-resistant anionic Gemini surfactant developed on the basis of GC18-2-18.When the concentration is higher than 0.3%,its solution has good viscoelasticity,interfacial activity and viscosity enhancement,especially GOBT16-4-16.Adding 0.01%～1%inorganic potassium salt can effectively increase viscosity of 0.5%GOBT solution,and it has good temperature resistance and viscoelasticity.GOBT inorganic potassium salt can be used to construct low interfacial tension viscoelastic fluid,called GOBTK,which has strong adaptability to mineralized water（6788mg/L～8000mg/L）in tight reservoirs.（3）The mechanism of CO₂-GOBTK cooperative flooding to improve oil recovery in tight reservoirs can be summarized as GOBTK and CO₂ foam cooperate to block high-permeability channels or fractures and expand the sweep volume of CO₂ flooding.The CO₂-GOBTK flooding system improves the water wettability of porous media and is beneficial to GOBTK injection and crude oil stripping start-up.CO₂ dissolution and extraction of low carbon components of crude oil,strong hydrophilic effect of CO₂/GOBTK cooperative oil flooding system and GOBTK deformation displacement capacity of pore residual oil lay a foundation for emulsifying and dispersive recovery of full residual oil components to overcome the deposition risk of high carbon components.（4）CO₂/GOBTK in tight reservoirs（<1m D）,and the smaller the size of the slug,the better the injectivity.When the water cut reaches 90%in homogeneous and heterogeneous models,continuous CO₂ flooding has the worst EOR effect,0.3PV GOBTK slug add subsequent water flooding has the best effect,and CO₂/GOBTK alternate flooding has the best effect,which has obvious synergistic oil displacement effect.CO₂-GOBTK alternately flooding is better than GOBTK-water alternately flooding at the same slugging ratio,and CO₂-GOBTK alternately flooding has more effective turns in enhancing oil recovery,which is a new technology more suitable for enhancing oil recovery in tight reservoirs after water flooding.This paper reveals the CO₂-viscoelastic fluid co-flooding mechanism in tight sandstone reservoir,and the CO₂/GOBTK co-flooding technology proposed is a new efficient EOR technology that can not only solve the problem of CO₂ flooding gas migration in heterogeneous tight reservoir,but also decrease the risk of asphaltene deposition,which has excellent potential application prospect.