Study On Shut-in Mechanism And Shut-in Time Optimization For High-Clay Condensate Shale Gas Reservoirs

The Jurassic shale in the Sichuan Basin is regarded as an important formation that is expected to start China’s continental shale oil and gas revolution.However,the Jurassic shale has high clay mineral content,and the overall thermal evolution of the formation is in the light oil-condensate window,which causes the complex fluid properties and seepage characteristics.Regarding the shut-in mechanism of fracturing fluid in shale reservoirs with high clay content,the current researches are still in the early exploration stage,and the research on the related mechanisms of imbibition displacement,water locking,and hydration damage needs to be further studied.In addition,the research on the oil-gas-water three phase seepage characteristics and oil and gas production characteristics under the influence of fracturing fluid of fractured condensate shale gas reservoirs is still blank.Further,for Jurassic condensate shale gas reservoirs with high clay content,the process of shut-in time optimization relies on field experience and lacks theoretical guidance,and systematic,modeling and quantitative research work is urgently needed.Based on laboratory experiments and numerical simulations,this paper conducts the researches on shut-in mechanism of high clay condensate shale gas reservoirs.The cores of the Jurassic shale were selected and gas-water imbibition and drainage experiments were carried out.The results of experiments were used to quantify the water locking damage and hydration damage characteristics of fracturing fluid during imbibition and drainage process.Reverse condensate experiments and oil-gas-water three phase seepage experiments in cores were carried out,which were used to quantify the occurrence and seepage characteristics of the three phases.For different fluid types in the Fuxing area shale condensate gas/volatile oil reservoirs,a series of component models were established describe phase change characteristics,and a clay hydration expansion mechanism model was established to characterize the clay hydration characteristics.Based on the multi-phase multi-component seepage equations,the numerical model of shut-in and flowback after fracturing for shale condensate gas/volatile oil reservoirs was constructed to simulate the distribution characteristics of fracturing fluid during well shut-in process and oil and gas production characteristics during the well flowback process.Finally,this paper put forward the optimization concept and design ideas of well shut-in time,and forms a set of well shut-in time optimization technology for multi-stage fractured horizontal wells in unconventional reservoirs,based on the numerical simulation method and conclusion,and taking into account the reservoir heterogeneity,the difference in fracturing effect and real shut-in time of each stage,and apply this technology to horizontal wells on site in Fuxing area.Research shows that the shut-in mechanism in high clay content shale reservoirs is mainly fluid energization,imbibition displacement and clay hydration.Fluid energization is the fastest.The effect of imbibition displacement can be divided into an early rapid imbibition stage and a late slow imbibition stage.Clay hydration damage can be divided into a fracturing fluid injection stage,an early rapid hydration stage and a late slow hydration stage.The influence of shut-in time on production is jointly restricted by the negative effects of clay hydration damage and the positive effects of fracturing fluid imbibition and displacement.For Jurassic condensate shale gas reservoirs,the optimal well shut-in time is concentrated in 10 to 20 days.The specific optimal time is affected by differences in fluid and reservoir characteristics.A shut-in time optimization method and a set of plates available for inquiry for multi-stage fractured horizontal wells in Fuxing area were developed that meet the requirements of mine scale and calculation speed,which were successfully applied to the Jurassic shale reservoir fractured horizontal well group.The research results are helpful to further clarify the shut-in mechanism in shale reservoirs and the seepage characteristics of condensate and gas in shale.And this research has direct engineering significance for guiding the flowback operation and optimization of well shut-in time in Jurassic condensate shale gas reservoirs in the Fuxing area.