Research On Anti-Channeling Effect Of Foam In Huff-n-Puff Gas Injection

China’s shale oil resources have huge potential and broad development prospects.Especially with the rapid progress of volume fracturing technology in horizontal wells,the production of shale oil has increased significantly,and it has become an important position for increasing oil and gas reserves and production in China.A large number of practices and studies have shown that the huff and puff gas injection process can greatly improve the recovery rate of shale oil reservoirs,and is an important technology for shale oil production.However,due to the existence of artificial fractures and natural fractures generated by fracturing,gas channeling often occurs during the huff-n-puff gas injection process,which affects the production and leads to poor development effects.How to reduce or prevent gas channeling during huff-n-puff gas injection is an important challenge to effectively improve shale oil recovery.In this paper,foam is used as an anti-channeling agent in the process of huff-n-puff gas injection.Taking the anti-channeling effect of foam in huff-n-puff gas injection as the research problem.The related research is carried out using a combination of experiments and numerical simulations.The foaming agent was screened out with foam stability as the index,and the concentration of the foaming agent was optimized.By means of micro-flow experiments,the migration law and the oil recovery mechanism of foam-assisted gas in fracture-matrix porous media were studied,and the anti-channeling mechanism of foam during gas injection was revealed.We mastered the gas channeling law in the process of huff-n-puff gas injection in fractured shale cores.On the one hand,in view of the difficulty in measuring the recovery factor of shale cores,we use NMR equipment to monitor the oil content in microscopic pores,and calculate the recovery factor of the core,to evaluate the anti-channeling effect of foam in huff-n-puff gas injection in fractured shale cores.On the other hand,the reservoir pressure,gas saturation,oil saturation and recovery factor were analyzed by numerical simulation method.The anti-channeling effect of foam in the process of huff and puff gas injection in shale reservoirs is evaluated,and a specific foam injection scheme is given.The research results provide an effective method for efficient gas injection development of shale oil.The research results show that:in the process of gas injection in porous media,the foam can form a plug in the high permeability area,effectively control the flow of gas,expand the sweeping range of gas in heterogeneous porous media,and improve the oil recovery efficiency of gas.So,the foam has good anti-channeling ability.Foam has good anti-channeling effect in the process of huff-n-puff gas injection in fractured shale cores,which can prolong the gas channeling time and significantly improve the oil recovery of the core.The anti-channeling effect of the foam is reflected in the use of oil in macropores,mesopores and micropores.The numerical simulation results further verify the anti-channeling effect of foam and the potential of enhancing oil recovery in huff-n-puff gas injection.Taking an actual shale reservoir as an example,combined with the research results of laboratory core experiments,the foam injection process and parameters were optimized,and the optimal foam-assisted N₂ huff-n-puff process scheme was proposed,providing guidance and basis for efficient development of shale oil.The results show that in the experiment of injecting foam into fractured shale core,when the volume fraction of foam is 50%and the volume flow rate is 0.05 ml/min,the resistance coefficient is large and the plugging ability is strong.Foam can significantly increase the gas channeling time of huff-n-puff gas injection,and prolong the gas channeling time from 3 min to 386 min.Compared with N₂ huff-n-puff,the oil recovery of one cycle of foam assisted N₂ huff-n-puff is increased by 4.50%.It is proved that foam has good channeling prevention effect in huff-n-puff gas injection from the aspects of gas channeling time and oil recovery.After a cycle of N₂ huff-n-puff test,the oil saturation in the macropores decreased by 0.07%,and the oil saturation in the mesopores and micropores did not decrease;After a cycle of foam assisted N₂ huff-n-puff experiment,the oil saturation in the macropores decreased by 2.33%,and the oil saturation in the micropores and mesopores decreased by 0.18%and 1.99%respectively.From the perspective of oil content in micro pore,it shows that foam has a significant anti channeling effect in huff-n-puff gas injection.The core experiment of foam assisted N₂ huff-n-puff shows that when the volume fraction of foam is 50%,the volume number is 0.4V_frac,the gas injection rate is 2 ml/min and the production time is 5 h,the core oil recovery after one cycle of huff-n-puff is 4.57%.The simulation results of foam assisted N₂ huff-n-puff show that when the volume fraction of foam is 50%,the volume number of foam is 0.04 PV,the surface activator concentration is 0.7%,and under the mode of one foam and one gas injection per cycle,when the time ratio of foam to gas injection is 1:5,the time of foam injection is 30 days,the time of gas injection is 150 days,and the production time is 180 days.The channeling prevention effect of foam in the huff-n-puff gas injection of shale reservoir is the best,and the maximum oil recovery is30.2226%.