Study On The Mechanism Of Proppant Backflow After Fracturing And Its Influence

Hydraulic fracturing technology is an effective method for low permeability reservoir development,which can improve oil well production and oil recovery.Flowback after fracturing is one of the important links affecting fracturing effect.The flowback system has great influence on later production and development.In the process of flowback,if the working system is unreasonable,the proppant will flow back to the bottom of the well,leading to the reduction of fracture conductivity and productivity after fracturing.In order to study the critical velocity of proppant backflow and the influence of backflow on fracture conductivity,this paper takes fracturing wells in Shenfu block of Ordos Basin as the research object.This paper establishes the mathematical model of critical velocity of proppant backflow,the simulation model of proppant distribution during backflow and the calculation model of fracture conductivity,studying the solution method of the model,developing relevant calculation programs,and analyzing the factors which can affect proppant critical velocity and backflow on fracture conductivity.(1)This paper establishes the critical velocity calculation model of proppant backflow during the flowback of fracturing fluids according to the analyzation about the stress of proppant.This model is established based on the principle of stress balance,including two conditions before and after fracture closure.Using C# software to program the calculation model and taking the fractured horizontal well in Shenfu block as an example,the critical flowback velocity of proppant initiating backflow is calculated.(2)The simulation calculation model of proppant backflow is established according to the studies of proppant backflow.Based on the calculation of fracture geometric parameters,the model simulates the distribution of proppant particles along with the fracturing fluid migration,and calculates the number of proppant particles when the flowback velocity is higher than the critical velocity by the finite element method.The simulation results indicate that the proppant particles migrate towards the bottom of the fracture,gather at the wellhead,and enter the wellbore through the perforation.(3)The calculation model of fracture conductivity during proppant backflow is studied.The calculation formulas for the tortuosity and pore throat radius of propped fracture after fracture closure are developed.The proppant retention in the fracture is substituted into the fracture conductivity calculation model,and the corresponding calculation program is developed to calculate the change of fracture conductivity after proppant backflow.The calculation results reveal that the flowback velocity higher than the critical velocity will lead to proppant particles backflow,which will reduce the fracture conductivity,and even lead to fracture closure and fracturing failure in severe cases.(4)The theoretical research results and the developed calculation program in this paper are applied to the calculation of fractured horizontal wells in the Shenfu block,in which the factors affecting the critical flowback velocity and the fracture conductivity are analyzed.The analysis results demonstrate that the radius and density of proppant and the pressure difference between the inside and outside of the fracture are positively correlated with the critical flowback velocity;the density and viscosity of the fracturing fluid and the saturation of the wetting phase are negatively correlated with the critical flowback velocity;the main factors affect fracture conductivity are the proppant quantity,proppant radius,pressure difference between inside and outside the fracture and flowback velocity.