EXPERIMENTAL STUDY ON THE FLOW LAW OF SLICKWATER IN THE NEAR WELL AREA OF HYDRAULIC FRACTURE BASED ON PIV/POD

In unconventional reservoir fracturing construction, the flow pattern of slickwater fracturing fluid in the near-well region is complex, and its flow pattern affects the proppant delivery and fracture extension process. Based on the similarity theory, the study builds a flat plate fracture experimental setup, and designs the experimental program in terms of the discharge volume, the viscosity of the slickwater and the injection position. Particle image velocimetry (PIV) is used to carry out the flow field test experiments in the near-well region to obtain the velocity and vorticity fields in the near-well region, and the transient velocity vector field data of the flow field obtained by PIV measurements are subjected to dominant mode decomposition by using the proper orthogonal decomposition (POD) method, which is a linear combination of finite-order intrinsic functions to study the coherent structure of the complex flow of the slick water in the near-well zone. The experimental results show that, comparing with clear water, lowering the 20° jet deflection angle of the slickwater can promote the development of jet strips and vortex structures in the flow direction, inhibit the pulsation of turbulence in the fracture, and act on the low-order, large-scale energy-containing vortex structures, which make the flow field relatively ordered and the coherent structures larger in scale. With three-hole injection, the larger the displacement and viscosity, the smaller the percentage of fluid velocity in the reflux zone, and the fluid is more inclined to flow forward quickly, which is easy for the proppant to be transported to the further end, where the viscosity has the largest effect, changing from 1 mPa·s to 6.0 mPa·s, and the percentage of fluid velocity in the reflux zone is reduced by 8.7%. Under single-hole injection, the vortex structure scale is large, and the coherent structure configuration in the flow field is relatively single and ordered; with the increase of the number of modes, the vortex structure scale of the large-scale coherent structure in the flow field gradually decreases, and the number of small-scale vortex structures increases, and the coherent structure in the flow field gradually becomes disordered from ordered.