Study On Influencing Parameters And Effects Of CO₂ Huff And Puff In G5 Ultra-low Permeability Reservoir

Ultra-low permeability tight reservoirs have huge reserves and are widely distributed in China.Due to the characteristics of ultra-low permeability pore throat structure,conventional water injection development is greatly limited,and CO₂huff and puff is an effective method to improve the recovery of ultra-low permeability reservoirs.Based on the analysis of the geological characteristics of G5 ultra-low permeability reservoir in Jidong,this paper analyzes the phase state change of crude oil after CO₂injection and determines the minimum miscible pressure,and evaluates the mechanism of CO₂ huff and puff by establishing a core-scale matrix-fracture unit model.The sensitivity analysis and optimal parameter optimization of the influencing factors of horizontal well huff and puff are carried out.Finally,the huff and puff effects of different well groups are analyzed and compared.The research shows that the phase diagram of the crude oil system shifts to the upper left as the proportion of injecting CO₂ increases,and the system becomes lighter,the minimum miscible pressure in this block is of 28.9MPa by one-dimensional thin tube simulation.The action mechanism of CO₂ is qualitatively characterized by simulation,and compared with N₂ and H₂O,it has better oil-increasing effect.Secondly,the coupling effect of start-up pressure gradient and stress sensitivity effect is studied and analyzed.It is found that both of them will reduce oil recovery,but the effect of start-up pressure gradient is greater than that of stress sensitivity,the influence of fracture properties on huff and puff is further studied and the fracture parameters are optimized.Finally,the single factor sensitivity analysis of huff and puff is carried out to study the influence law of huff and puff timing,bottom hole pressure,injection volume,injection speed,soaking time and huff and puff rounds on huff and puff effect.The huff and puff parameters are optimized by using the orthogonal design test method.Taking the cumulative oil production and oil change rate as the evaluation index,the gas injection has the greatest impact on the cumulative oil production;For the oil change rate,the biggest impact is the huff and puff rounds.The optimal parameters for this block are huff and puff after 2 years of elastic development,and the gas injection volume of 150×10⁴m³,injection rate of 6×10⁴ m³/d,soaking time of 60d,5 cycles of huff and puff.For the gas injection huff and puff of well group,the best simulation effect is asynchronous huff and puff,followed by interval huff and puff,and the worst is synchronous huff and puff.