Research On Percolation Mechanism In Super-deep Heavy Oil Reservoir With Fractured Well By CO₂ Huff And Puff

The total reeserve of heavy oil amounts to 6 trillion barrels in the world.In the process of thermal recovery for heavy oil reservoir,it was widely used with steam huff 'n' puff and steam flooding technology.However,for the limitation of reservoir pressure and heat loss,the above technologies were not suitable for the recovery of super-deep heavy oil reservoir with buried depth over 2000 meters.It was known that the buried depth has little effect on the development effectiveness of gas injection recovery.In addition,for the higher solubility of carbon dioxide in heavy oil and the greater effect of viscosity-reducing and volume expansion than other gases,the L heavy oil reservoir will be developed with carbon dioxide huff 'n' puff in early stage and thermal recovery later when the reservoir pressure decreases to appropriate value.The phase behavior of formation oil from L reservoir with and without carbon dioxide injection had been studied in the paper.In addition,three-dimensional large-size artificial core was made with physical property test.At last,the carbon dioxide huff and puff laboratory experiment had been carried out with the above artificial core.The study results and conclusion of the paper are as follows:?1?With the increasing of carbon dioxide injection into the formation oil,the formation volume factor,bubble point pressure and solution gas oil ratio of formation oil increased while the viscosity and density decreased.?2?The viscosity of heavy oil will reduce sharply after carbon dioxide injection.A new model for calculating the viscosity of heavy oil saturated with carbon dioxide has been established,which could represent the experimental data with good accuracy.?3?It was found that the water relative permeability was very low and the residual oil saturation was high while the water relative permeability at residual saturation was at lower level in the oil-water relative permeability curves for heavy oil.Similarly,the gas relative permeability was very low and the residual oil saturation was high while the gas relative permeability at residual saturation was at lower level in the oil-gas relative permeability curves for heavy oil.For the effect of gas channeling and slippage,the displacement efficiency of gas was lower than that of water,which resulted the higher residual oil saturation for gas drive.?4?From the relationship between cumulative oil and average formation pressure,it was found that there were different slopes in every cycle,which indicated that there existed different percolation process in the formation.After multi-cycle carbon dioxide huff and puff,the impact region of carbon dioxide gradually increased.Meanwhile,the radius of super-saturation and pseudo-saturation region increased while the under-saturation region was compressed continually and even disappeared.?5?For the existence of fracture near wellbore,the pressure-measuring point near the fracture would respond earlier to the injection well with larger amplitude in the huff and puff process.In the process of soak,the pressure of pressure-measuring point near the fracture was higher than that of pressure-measuring point away the fracture.With the prolongation of soak time,the pressure of every pressure-measuring points tend to be uniform.However,in the early stage of production,the pressure of pressure-measuring point near the fracture was lower than that of pressure-measuring point away the fracture.The fracture near wellbore could improve the oil and gas percolation condition and increase the drainage area near wellbore area,which would enhance the drainage efficiency and sweep efficiency.?6?It was found that foamy oil was produced in the production process of CO₂ puff 'n'huff,which proved the foamy oil solution gas drive occurring in formation seepage process.The formation and EOR mechanism of foamy oil solution gas drive were also studied in the paper.At last,the affecting factors of foamy oil solution gas drive had been analyzed.