| Low permeability reservoirs,generally featured with low porosity,low permeability and low productivity,are widely distributed in China.The conventional methods of water injection and gas injection will improve the development effect.However,those may also generate gas channeling in the middle and late development periods and therefore leads to low recovery efficiency.Combing the advantages of foam flooding and that of CO2 flooding,CO2 foam flooding not only enhances sweeping area,but also improves the efficiency of washing oil,oil expansion as well as oil extraction.The paper is based on the research of ultra-micro CO2 foam system,which can resist high salinity(11.8 × 104mg/L)and high calcium and magnesium ions(2.9 x 104 mg/L).The optimization of Micro CO2 foam preparation methods was carried out while the quantitative characterization was carried out on the bubble size to analyze the influence factors of bubble diameter.The performance of CO2 foam while injecting into core and displacement effect evaluation of Micro CO2 foam were conducted.Besides,the microscopic etching model was built to study the microcosmic oil displacement mechanism of CO2 foam.The distribution of CO2 foam bubble was simulated by Eclipse software and the comparison of flooding effect of water flooding and CO2 foam flooding was completed.The major conclusions of this dissertation can be summarized as follows:(1)The ultra-micro CO2 foam system has been successfully developed for high salinity,high calcium and magnesium ions in ultra-low permeability reservoirs:0.25wt%HTM+0.75%wt%LSB+0.54wt%ST3.While being stirred at the speed of 6000r/min,the foam system will generate foaming volume of 980 ml with half-life of 22 min and foam composite index of 16170 mLˇ min.(2)Comparison of five kinds of CO2 foam preparation methods show that the constant speed mixing foam model at the speed of 6000 r/min and quartz sand filling tube model(K=5.56 mD)generates the largest number and the most uniform micro bubbles that are generally within the size of 20?100um while ranging from 6.25 to 31.26 um under the condition of formation temperature and pressure.Micro CO2bubbles which are limited in the confined space are pretty stable.(3)Through the research of influence factors of micro bubble diameter,we find that the constant speed mixing foam model at the speed of6000 r/min produces the most exquisite micro bubbles.Quartz sand filling tube foam model shows that the optimal gas liquid ratio of CO2 micro foam injection is 3:1,injected slug is 0.5 PV,gas flow rate is 3 ml/min,liquid injection rate is 1 ml/min.When the temperature increases,the foam diffusion improves while foam inflation accelerates,which leads to bigger bubble diameter.As the concentration of foamer increases,the bubble diameter increases,and the original stable structure of protective shell will be damaged.When the back pressure increases within a certain range,micro bubble diameter displays a decreasing trend becoming more stable.When the content of crude oil exceeds 5%,the bubble diameter will be affected and presents certain oil resistance.(4)The experiments on micro CO2 bubbles shows that the foam system can be injected well into the extra low permeability cores where the drag coefficient is about 8.32?15.33 and the residual resistance coefficient is 1.46?2.71.The performance of CO2 foam flooding demonstrates that gas-liquid injection delivers best result,the best ratio of gas liquid is 3:1,and the injection rate of liquid is 1.6 mL/min for oil displacement,which can improve the oil recovery more than 25%.(5)The micro etching model shows that the mechanisms of CO2 foam flooding mainly includes viscoelastic displacement function,expanding crude oil and reducing oil viscosity,inhibition of viscous fingering,gas resistance effect,the function of the emulsion,carrying and peel oil film.(6)From the black oil model simulation of Eclipse,we find that CO2 foam flooding can enhance oil recovery 29.97%for extra low permeability reservoir,which is 14.79%higher compared with the water drive.CO2 foam flooding can improve the injection performance and swept area of low permeability layer,and replenish the formation energy,keeping a good and stable production,controlling the water production well. |
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