Study On Optimal Design Of Fracturing Fluid Backflow Technology On Tight Sandstone Reservior

According to the current statistics,the tight sandstone gas resource is about9.2¹3.4×10¹²m³in China,accounting for more than 50%of the total reserves of natural gas.In recent 5 years,the reserves of tight sandstone gas has reached2³×10¹¹m³for each year.Due to the low permeability and porosity of the tight sandstone reservoirs,the combination of horizontal well and staged fracturing technology is a normal mean for increasing the production and injection.The backflow of fracturing fluid is not only an important part of hydraulic fracturing,but also the last part of fracturing operation.In the process of backflow,the formation pressure is gradually reduced,and the phenomenon of large filtration loss,propping agent backflow and slow backflow caused by experience-led backflow measures directly affects the effect of increasing production of reservoir reconstruction.Reasonable pressure backflow is able to improve the efficiency of the backflow of the fracturing fluid,to control propping agent backflow and to strengthen fracture stimulation effect.This paper views a horizontal well in the sandstone gas reservoir as the research object,considering the backflow of multiple cracks while fluid loss,volume change and the effect of pressure drop of the cracks in horizontal section.According to the theory of fluid mechanics,seepage flow mechanics theory,the volume balance principle,this paper obtained a mathematics model of fracturing fluid backflow,which demonstrated the wellhead pressure changing with the time.This mathematics model can accurately predict fracture closure time and the final balance time.To control the backflow of propping agent,this paper respectively considered the three stages of proppant particles,including fracturing construction after stopping the pump,the stress situation of proppant particles before and after fracture closure.A proppant motion model and a model of proppant backflow were established,receiving the proppant movement time after stopping the pump and displacement and critical backflow velocity before and after fracture closure.By combinating the mathematical model of fracturing fluid and the backflow model of the proppant,the nozzle size in the safe status was determined,and the real-time control and replacement of the nozzle size were feasible.Based on the previous results,this paper attained the optimization design method of fracturing fluid backflow and made the reference standards of the timing of backflow,the ways of fracture closure,and the selection of nozzle size.Meanwhile,this paper optimally designed and validated the scheme of backflow in some gas well using horizontal well staged fracturing.The application from the example shows that the optimization design method of fracturing fluid in the tight sandstone reservoir is effective and feasible.