Research On The Optimization Of Fracturing Flowback System Of Shale Gas Horizontal Wells

At present,To obtain the gas-production rate and total gas-production amount with economic benefits,shale gas reservoirs need to undergo large-scale hydraulic fracturing to form a large number of artificial fractures.However,During post-frac flowback,establishing a suitable flowback system can not only reduce the cost of fracturing,It also maximizes fracturing and stimulation.In this paper,a set of optimization design method of post-frac backflow system is formed through research of experiments,modeling and field application.The specific research contents are as follows:1.By means of scanning electron microscopy,Overburden porosity and permeability meter and Isothermal adsorption apparatus,experiments were designed and carried out to determine the microscopic pores,core permeability and adsorption desorption of shale with soaking time of 4-10 h and soaking pressure of 20-60 MPa respectively.Through comparison before and after the pollution and the processing of the experimental results,found under the different soaking time,with the increase of soaking pressure,The decreasing degree of shale porosity and reservoir permeability as well as the increasing degree of slippage coefficient caused by fracturing fluid pollution is increasing and the trend is obvious.The increasing range is between 1% and 20%,and the decreasing degree of gas adsorption is about 50%.Among them the slip-water fracturing fluid has more influence on shale organic matter pores,reservoir permeability and adsorption and desorption,while the linear glue fracturing fluid has more influence on shale mineral skeleton pores and slippage coefficient.2.By means of fracture conductivity testing device,self-priming and flowback devices,experiments were designed and carried out to determine fracture conductivity with grain diameter 10-70 mesh,confining pressure 30-60 MPa and determine fracturing fluid percolation and flowback with self-imbibition time 300 h,confining pressure 3MPa,displacement pressure difference 1-16 MPa.When the confining pressure is higher or the particle size is larger,the experimental results show that the fracture conductivity decreases continuously and the decreasing range is larger with the increase of measuring time.The fluid absorption amount of core in distilled water is greater than that in slippage water,and that in slippage water is greater than that in linear glue.In different liquid media,the fluid absorption amount of osmotic hydration in shale is about 0.0026g/cm2,and the pore capillary force of shale is about 13-16 MPa.3.Using the ideas of LBM and domain decomposition,and based on the experimental data,respectively established organic matter nanopores,mineral skeletons with micron holes,micro cracks and artificial fracture gas-water two-phase LBM model,and the model also considers the influence of key factors such as desorption,slipping,capillary force and osmotic pressure.Then the above gas-water two-phase LBM model is coupled to obtain the gas-water two-phase multi-scale coupled LBM seepage model of shale reservoir.Considering the change of fracture conductivity and the two-phase flow of wellbore gas-water,the coupled LBM seepage model and the shale stress sensitive model jointly constitute the mathematical model of flowback after fracturing.It is found that the calculated result of the flowback model is smaller than that of the classical analytical model,with a calculation error of < 10%.4.In the calculation of productivity,the reservoir is divided into gas-liquid two-phase flow zone,stress-sensitive zone and gas single-phase flow zone,and the corresponding seepage model is used in different areas.Using the established model,it is found that the flowback parameters such as capillary force and osmotic pressure have different effects on the daily production of shale gas wells.With the goal of maximum accumulative output,a set of optimization design method of flowback system after fracturing is developed by optimizing each flowback parameter.This method has the advantages of accurate and efficient capacity prediction.By using this method,the optimal shut-in time of a gas well in the field is given to be 1h,and the optimal nozzle combination is: 2-mm nozzle is used for 0-5 days of flowback;Use 3mm nozzle for 6-15 days for backflow;Use 3mm nozzle after 15 days of flowback.This paper not only gives us a systematic and comprehensive understanding of the fracturing fluid contaminated reservoir,but also successfully simulates the complex flow process of fluid in shale reservoir by using LBM,and its research results are a good guide for the field flowback work.