Optimization Of Fracturing Fluid Flowback Rules For Liquid CO₂ Pre-Pad Fracturing In Shale Reservoir

The complex pore-roar structure and strong water sensitivity of this type of reservoir could cause problems such as reservoir damage and low flowback rate during the hydraulic fracturing of horizontal well.Liquid CO₂ pre-fracturing is a stimulation technology that liquid CO₂ as pre-pad fluid is firstly injected and then water-based fracturing fluids such as slickwater and gels are injected for reservoir stimulation.Compared with conventional hydraulically fracturing,The technic has the advantages of increasing formation energy,rapid flowback,less formation damage and high productivity of single well,and plays an important role in the efficient development of unconventional oil and gas reservoirs.However,the flowback of liquid CO₂pre-fracturing mostly depends on field experience,and there is really little scientific basis.Therefore,it is necessary to research the flowback system after liquid CO₂ pre-fracturing.In this paper,based on the investigation of domestic and foreign literature on liquidfied CO₂ fracturing technology and fracturing fluid flowback in shale reservoirs,the fracture propagation during the initial fracturing of shale reservois with liquid CO₂ was simulated,and the fracture parameters were obtained.Then the effects of Young’s modulus and Poisson’s ratio on fracture propagation were analyzed by single factor analysis.The liquid CO₂ pre-fracturing energy enhancing flowback laboratory experiment was carried out.Through the displancement and pressure drop flowback experiment,the energy increase and flowback conditions of the reservoir under different injection modes and different CO₂ injection volumes were studied.The soaking time and flowback pressure were optimized with the goal of optimal production capacity and flowback rate.Combined with the experiment and fracturing simulation,the relationship between the flowback rate of shale reservoir and capillary pressure,matrix permeability and injection pressure was investigated,On this basis,the flowback parameters and the flowback system were discussed and analyzed.According to the results,the fracture half-length,the average fracture width and the dimensionless conductivity increase with the increase of Young’s modulus and decrease with the increase of Poisson’s ratio,and there is no linear relationship the increase/decrease at different stages.CO₂ prepad can obviously increase the formation pressure.The higher the matrix permeability and injection pressure are,the higher the flowback rate of fracturing fluid is.And the capillary pressure is inversely correlated with the flowback rate,and when the capillary force of the reservoir reaches a certain value,the flowback rate of fracturing fluid will significantly reduce with the further increase of the capillary force.The flowback rate and cumulative production of prepad CO₂ and low-concentration linear rubber fracturing both increase first and then decrease with the soaking time.There is an optimal soaking time under the condition of L zone,30 days for the prepad CO₂ fracturing,and 15 days for the large-scale low-concentration linear rubber fracturing fluid.The cumulative oil production does not change much with the flowback rate,but the flowback rate increases with the increase of the flowback rate.The optimal flowback rate from both the productivity and the flowback rate is 40 m³·d^-1.The cumulative production and the flowback rate of fracturing fluid increase with the decrease of the flowback pressure.In order to achieve the best development results the flowback pressure should be as low as possible within a controllable range.