Research On Reaction Mechanism Of Enhanced Extra-Super Heavy Oil Recovery By Carbon Dioxide

As a kind of oil displacement agent CO₂ is used widely in the petroleum industry. It is proved in both laboratory and field tests that using CO₂ to displace oil is an effective method of greatly increasing well productivity. With the discovery of CO₂ source and the development of match technology in our country,enhanced oil recovery by CO₂ injection is widely used as a new improvement technique applicable to low permeability and heavy oil reservoir and is good prospect in field application.In this paper we mainly carry on research to the problems induced production of extra-super heavy oil more difficult and recovery ratio lower in Shengli Oil Field. Based on information investigation and field data analysis we carried on the relevant experiments of extra-super heavy oil, such as viscosity-temperature and rheological characteristics, reservoir fluid-CO₂ expansion and viscosity experiment, interfacial tension experiment and oil displacement in sand pack experiment. The preliminary conclusions are as the following:(1) It is very sensitive to temperature for extra-super heavy oil: the viscosity of extra-super heavy oil reduces sharply with temperature rising, and changes into the Newton fluid from non-Newtonian fluid gradually, exists a critical temperature;(2) CO₂ is very easy to dissolve in the extra-super heavy oil: the solution of CO₂ gas oil ratio is up to 59.04 m3/m3(s);(3) With more CO₂ dissolving in crude oil, the volume factor of the crude oil and shrinkage ratio obviously increase, can help the remaining oil to break away from formation water and constraint in the surface of rock, thus increase the well yield;(4) The viscosity of formation oil reduce by a large margin with more CO₂ dissolving in crude oil: under the condition of reservoir temperature and pressure, viscosity is reduced to 6581.93mPa.s, viscosity reducing rate up to more than 90%, when CO₂ gas oil ratio reaches 23.9m3/m3(s);(5) The viscosity of reservoir oil increases along with increase of the pressure, while with quantity of CO₂ solution increase, the sensitiveness of the viscosity to pressure is obviously low; (6) CO₂ can obviously reduce the interfacial tension of oil/water, there is no obvious influence to the interfacial tension of oil (saturated CO₂)/water system when the degree of mineralization increases;(7) Based steam flooding experiment, we arrive at the mechanism of EOR by CO₂, such as: CO₂ dissolving in crude oil decrease viscosity and interface tension, CO₂ can reduce not only interface tension , but also form weak foam system to improve sweep efficiency after CO₂ combines with surfactant.