Study On The Enhanced Oil Recovery Via CO₂/H₂O Emulsion Displacement

CO₂-EOR technology has attracted an increasing attention resulting from its unique characteristics.CO₂ has a good solubility in oil,and CO₂ is used for oil displacement can signifiactantly improve the oil recovery.Meanwhile,as the main greenhouse gas,CO₂ can be sequestrated in the oil formation by CO₂-EOR technology,which has environmental benefit.As the world’s largest emitter of CO₂,China’s use of CO₂-EOR technology is an important tool to achieve the strategy of“Carbon Dioxide Emissions Peak and Carbon Neutrality”.However,there are some challenges in CO₂ flooding such as premature breakthrough and low sweep efficiency aroused by the low density and high mobility of CO₂.Furthermore,the cost of CO₂ remains high because of the lack of natural CO₂ source and convenient transportation pipeline of CO₂ in China.In this work,a new method of CO₂/H₂O emulsion for enhaced oil recovery was proposed,and the relevant basic research was carried out.The main research contents are as follows:（1）The interfacial tension of CO₂-H₂O systems with various surfactants were studied by the high pressure interfacial tensiometer.The effects of surfactant concentration,temperature,pressure and the formula of surfactants on CO₂-H₂O interfacial tension were investigated.The mechanism and emulsifying potential of different surfactants at CO₂-H₂O interface were studied,and APG and AEO were selected as emulsifier for the preparation of CO₂/H₂O emulsion.Moreover,the interfacial tension of CO₂/H₂O emulsion-diesel was measured,and the factors affecting the interfacial tension of CO₂/H₂O emulsion diesel oil were investigated.A preliminary understanding of the interfacial interaction of CO₂/H₂O emulsion and diesel oil was obtained.（2）The PVT cell was used to study the morphology and stability of CO₂emulsion.The inversion point of CO₂ emulsions morphology stabilized with APG or AEO changed with water cut was determined.The macro and micro stability of CO₂/H₂O emulsion was evaluated by the glass visual cell designed by ourselves.The stability process and destabilization mechanism of CO₂/H₂O emulsion are clearly understood.The effects of surfactants,temperature,pressure,water cut and salt content on the stability of CO₂/H₂O emulsion were investigated,and the dosage and ratio of APG and AEO were determined.At the same time,the properties of CO₂/H₂O emulsion in contact with crude oil were preliminarily studied,which provides a theoretical basis for the application of CO₂/H₂O emulsion in oil displacement.（3）The oil displacement characteristics and mechanism of CO₂/H₂O emulsion were comprehensively studied by the slim tube displacement experiment and oil-water emulsification experiment.It is found that the CO₂/H₂O emulsion flooding has the similar displacement characteristic with that of CO₂ flooding,and there are also two characteristics of immiscible and miscible flooding in CO₂/H₂O emulsion.The CO₂/H₂O emulsion flooding shows more advantages under the condition of relatively low pressure,and it can reduce the minimum miscible pressure（MMP）during the displacement and the decrease was 12.8 to 16.2 percentage points.In addition,the displacement characteristics of the two crude oils were compared.The results show that CO₂/H₂O emulsion displacement performs better for displacing heavier crude oils.In addition,it is determined that oil-water emulsification is an important mechanism of CO₂/H₂O emulsion displacement,and it has the best emulsification efficiency for crude oil with moderate density.（4）The stability and seepage characteristics of CO₂/H₂O emulsion in porous media were studied,and the apparent viscosity of CO₂/H₂O emulsion was measured.The stability of CO₂/H₂O emulsion in two sizes of porous media was evaluated by visual cell.The results show that CO₂/H₂O emulsion in porous media obviously expresses better stability compared with the stability of CO₂/H₂O emulsion in visual cell.The smaller the pore size,the better the stability of CO₂/H₂O emulsion.In addition,the study on the seepage characteristics of CO₂/H₂O emulsion shows that CO₂/H₂O emulsion manifests the best plugging performance with the water cut of 50v%,but the apparent viscosity of CO₂/H₂O emulsion decreases with the increase of water cut.It is preferred to prepare CO₂/H₂O emulsion for oil displacement experiment with 1.0 wt%APG+0.5 wt%AEO and 40 v%water cut.（5）The CO₂/H₂O emulsion displacement experiments were carried out by simulating high permeability reservoir with long sand pack and low permeability reservoir with long core.Under the high permeability miscible displacement in long sand pack,the displacement characteristics of CO₂ displacement and CO₂/H₂O emulsion displacement in the system without initial saturated water and the system with initial saturated water were compared.It shows that CO₂/H₂O emulsion displacement can significantly improve crude oil recovery.Through the component analysis of the effluent oil at different displacement stages,it is found that CO₂/H₂O emulsion can extract the components of crude oil just like CO₂ displacement.Under the long core low permeability immiscible displacement,comparing the displacement effects of CO₂ displacement and CO₂/H₂O emulsion displacement under various displacement modes,it is concluded that CO₂/H₂O emulsion displacement can improve crude oil recovery approximately 12%.The NPV method was used to evaluate the economical efficiency of CO₂ displacement and CO₂/H₂O emulsion displacement.The CO₂/H₂O emulsion displacement shows the better economical efficiency than CO₂ displacement,and the economical efficiency of CO₂/H₂O emulsion displacement increases with the increase of project period and displacement scale,which provides further theoretical support for the application of CO₂/H₂O emulsion displacement.