Experiment of proppant backflow effect on shale artificial fracture conductivity

Key words:  shale  artificial fractures  conductivity  proppant reflux  sand paving

Abstract:

Proppant backflow is one of the key factors causing damage to the conductivity of artificial shale fractures, and a quantitative characterization of proppant backflow under actual stress loading conditions is required. In this study, an in-house developed conductivity test system was used to simulate the proppant backflow during fracturing fluid flowing back by adding a proppant backflow chamber module, and long-term conductivity comparison tests of shale artificial fractures with different stress loading methods (variable confining pressure test and variable flow pressure test) were carried out. The results show that different stress loading methods have similar effects on the amount of proppant crushed and embedded, but the change of proppant backflow rate with confining pressure is about 4.46%, which is inconsistent with the field proppant backflow data. Under the variable flow pressure test, the proppant backflow rate is 20.53% with the flow pressure 20 MPa, and the backflow rate decreases with flow pressure decrease,which is Consistent with the field proppant backflow phenomenon. In addition, ignoring the backflow of proppant will lead to a low damage rate to the fracture conductivity, which is not conducive to the formulation of construction plans. Therefore, an accurate test of the hydraulic conductivity of artificial fractures in shale should adopt the variable pressure test method, which is more consistent with the actual stress loading, in order to better reflect the influence of proppant backflow on the hydraulic conductivity.