Optimization Of CO₂ Huff-n-puff Technology In Tight Reservoirs

The conditions of tight reservoirs in China are poor.At present,the development of volume-fractured horizontal wells is generally used,but the production declines rapidly and the primary recovery rate is low.After the volumetric fracturing,the crack system and the matrix system have a larger gap in the reservoir and seepage capacity.The conventional water injection development method makes it difficult to use the crude oil in the matrix,and the waterlogging phenomenon is intensified.Therefore,enhanced oil recovery in tight reservoirs is in urgent need of finding new ways.CO₂ has special physicochemical properties.When it enters the formation and mixes with crude oil,it can reduce the interfacial tension,make the volume expansion of crude oil increase the formation pressure,reduce the density and viscosity of crude oil,and extract the light components in crude oil.At the same time,greenhouse gases are injected into underground reservoirs,reducing emissions to the atmosphere and achieving the effect of reducing emissions.Tight reservoirs have great potential for enhanced oil recovery using CO₂ huff-n-puff.Based on the geological characteristics and reservoir parameters of the Yuan 284 area in Changqing Oilfield,a numerical model was established and the process of CO₂ stimulation in tight reservoirs was analyzed.The results show that CO₂ huff and puff in tight reservoirs can effectively supplement formation energy,expand the range of use,increase the sweep coefficient,and extract light components in crude oil.The technical policy of volumetric fracturing horizontal wells and vertical wells for CO₂ stimulation is optimized.The results are: horizontal well injection time 40 days,injection speed 150t/d,soak time 30 days,huff-n-puff cycle 3-4 rounds;vertical well injection time 30 days,injection speed 75t/d,and soak time 30 days.The influence degree of CO₂ huff-n-puff parameters was analyzed by CMOST.The influence degree of horizontal well production was from large to small: cycle number of huff-n-puff,injection speed,injection time and soak time;the degree of influence on vertical wells was from large to small: cycle number of huff-n-puff,soak time,injection speed,injection time.Finally,according to the preferred horizontal well CO₂ huff-n-puff parameter combination,the CO₂ huff-n-puff production forecast is carried out in the horizontal well group of Yuan-28 area.It is estimated that the cumulative oil increase in one cycle of huff-n-puff is 2351 t,and the stage recovery degree can be increased by 1.01%.