Study On Adaptability Evaluation Method And Optimization Of CO₂ Huff-and-puff Process In Tight Oil Reservoirs

In recent years,tight oil has become a hot spot of unconventional oil and gas development around the world.Due to ultra low porosity and permeability,the primary oil recovery in these reservoirs is very low.Therefore,it is more than imperative to develop an enhanced oil recovery method for tight oil reservoirs.Given this,CO₂huff-and-puff technology has great potential.In this paper,the production pattern of fractured horizontal wells in tight oil reservoirs is analyzed,and the impacts of reservoir properties on the the performance of CO₂ huff-and-puff are studied.Based on this,a method to evaluate the adaptability of CO₂ huff-and-puff in tight oil reservoirs is established.Using this method,the adaptability of CO₂ huff-and-puff in Lucaogou tight oil reservoir is evaluated.In addition,the CO₂ huff-and-puff process is optimized for tight oil reservoirs and a optimal scheme is brought up for Lucaogou tight oil reservoir to improve oil recovery.The results show that the first four reservoir properties affecting the CO₂huff-and-puff in tight reservoirs are permeability,porosity,oil viscosity and reservoir temperature,followed by reservoir thickness,oil saturation,reservoir heterogeneity and reservoir pressure.Permeability and reservoir temperature are positively correlated with the performance of CO₂ huff-and-puff,while reservoir pressure,thickness,porosity,and heterogeneity are negatively correlated with the performance of CO₂ huff-and-puff.The oil viscosity and saturation have optimal values for CO₂ huff-and-puff.Calculated by the established adaptability evaluation method,the adaptability of CO₂ huff-and-puff in Lucaogou tight oil reservoir is ranked as“About good”.For tight oil reservoirs,it is recommended to conduct CO₂ huff-and-puff after reservoir pressure drops by 30%.For CO₂ huff-and-puff process,gas injection volume has the optimal value from the perspective of CO₂ utilization.In additon,less gas injection time will seriously diminish the performance of CO₂ huff-and-puff.The impact of soaking time on CO₂huff-and-puff is insignificant.Longer production time is favorable to CO₂ huff-and-puff process.For Lucaogou tight oil reservoir,the optimal operational parameters are gas injection volume of 5.6 million cubic meters,gas injection time of 20 days,soaking time of 5 days,production time of 500 days in single cycle and bottom hole pressure of10 MPa.The incremental oil recovery of six-cycle CO₂ huff-and-puff is about 2.01%.