Research Of N₂ Combination Huff And Puff To Improve Oil Recovery In Low-permeability Trapped Reservoirs

Low permeability reservoirs have become an important position in China's proved reserves and production capacity construction.There are so many problems in developing lowpermeability trapped reservoirs,like poor property of formation,low degree of well network,fast decline of natural energy,et al,which will be solved well by nitrogen gas huff and puff.But the oil recovery becomes lower and result becomes not satisfactory as cycles increase.It is necessary to start further study about the mechanism of nitrogen gas huff and puff,and put forward the improvement measures of nitrogen gas compination huff and puff.The research can provide theoretical guidance for improving oil recovery of low-permeability trapped reservoirs.This research studies several experiments of nitrogen gas huff and puff,including physical simulation and numerical simulation,based on the investigation of national and international development of huff and puff in low-permeability trapped reservoirs.By studying the regulation of oil,gas and pressure in different production stages,the seepage characteristics of oil-gas twophase flow at puff stage was obtained,and the influence factors of exploitation at different cycles were analyzed.Based on phase behavior matching,the classical well model has been built to simulate and optimize nitrogen gas huff and puff in different reservoir conditions.According to the limitation of nitrogen gas huff and puff,this research studied nitrogen gas compination huff and puff.The physical and numerical simulation of nitrogen gas plus water,nitrogen gas plus surfactant,nitrogen gas plus carbon dioxide have been carried out,and the mechanism of which were obtained.The results show that the seepage states at puff stage presents four different conditions,which are gas channeling state,oil-gas phase flow,gas-sparged oil state.By comparing the oil production rate and gas production rate at puff stage,three seepage states were verified and the transport tendency chart of oil and gas phase in core was built.As cycles increase,the pressure gradient duration maintained by the injected gas decreases,the rate of gas production increase,the oil recovery of each cycle decreases.Improving injection pressure and soak time can expand the effective oil region,and then enhance oil recovery of nitrogen gas huff and puff.Nitrogen injection volume has a great effect on nitrogen huff and puff,and with the increase of nitrogen injection,the increase of oil production in the cycle is obvious.Well-head pressure would decrease along with soak time,and there is an optimal soak time when the well-head pressure is equal to the average pressure of the formation.Intermittent injection can decrease head pressure,increase nitrogen injection volume,improve the cumulation of oil production,and there is an optimal injection time.Nitrogen gas plus water huff and puff has the mechanism of promoting gas diffusion and slowing down gas channeling;Nitrogen gas plus surfactant huff and puff has the mechanism of pluging top gas channeling;Nitrogen gas plus carbon dioxide huff and puff has the mechanism of reducing oil viscosity and enhancing oil swelling capacity.