Study On The Flowback Behavior Of Clean Fracturing Fluid Under Different Stresses In Tight Reservoir

Tight reservoirs have developed nano-micron pore throats,and fracturing is necessary to increase productivity.Flowback is an important part of fracturing construction.For the clean fracturing fluid,a reasonable choice of the flowback timing and control of the flowback volume are essential to maximize its displacement efficiency and increase the productivity after fracturing.From the closed well to the liquid flowback process,the fluid pressure is in a dynamic change,resulting in the continuous change of the net stress on the reservoir rock.Meanwhile,the liquid seepage capacity will also change accordingly,which affects the flowback effect.This paper studies the influence of flow pressure changes on the flowback behavior of fracturing fluids,and provides a theoretical basis for the optimization of the flowback system of the pressure drive system.The paper taked the Chang 6 tight reservoir as the research object,firstly measured its basic physical parameters and evaluated the performance of the clean fracturing fluid and the influence of the gel breaking fluid on the properties of the interface between crude oil and rock.Then the characteristics of formation fluid pressure change during actual production were simulated to investigate the influence of different factors on rock permeability,including oil nozzle replacement level difference and circulation switch well on.Finally,using low-field nuclear magnetic resonance equipment,the flowback behavoir of fracturing fluid and the fluids migration characteristics in different sizes pores during the flowback process were analyzed.By reasonably changing the flow pressure to control flowback rate,a better drainage effect can be achieved.The results show that the clean fracturing fluid has good oil-breaking performance,the viscosity of the gel breaking fluid is less than 5 m Pa·s,and the minimum IFT value is 0.21m N·m^-1 at 70℃.When the concentration exceeds 0.3%,the core wettability changes from wet to weak wet-neutral.During the flowback process of the initial well opening,the flow pressure decreases and the net stress on the core increases.The permeability first decreases rapidly and then gradually stabilizes.The net stress at the turning point is 10 MPa.When the reservoir energy is insufficient,the well is closed to restored pressure and the well is re-opened for production.The permeability change pattern is the same as the first time,but the damage rate is reduced（46.3%-38.6%）.The core damage mainly occurs in the initial production,and the pressure change needs to be controlled during this process to reduce the damage of the reservoir.During the closed well time before flowback process,the gel breaking fluid replaces the small-hole crude oil.As the increase of closed well time,the change of oil saturation in the mesopores rises to 15.4%.Before the flowback,the closed well time should be enough so that the surfactant in the gel breaking fluid can play a role and increase the production of the oil phase.During the flowback process,as the net stress increases,the flowback rate of fracturing fluid decreases（14.1%-6.3%）.At this time,affected by the compression and deformation characteristics of pores of different sizes,the small pores show a gradient decline,while the flowback volume of mesopores decreases greatly before the net stress turning point of 10 MPa,and then the amplitude decreases.Therefore,during the production stage,the nozzles are replaced from small to large.As the pressure difference increases,the matrix permeability damage rate decreases.The replacement level can be appropriately increased to increase productivity;the flow pressure is restored during the closed well time,and the permeability recovery rate decreases with the increase of pressure difference.The appropriate closed well time should be selected to maintain a reasonable pressure difference range,and then give full play to the system’s drainage performance.