Abstract
The effective proppant transport and setting in hydraulic fractures will largely determine the well productivity. During hydraulic fracturing, proppants can be transported into formation by slickwater with a high concentration of 420 kg/m3 at low viscosity of 2-5 mPa·s. Fracturing fluid viscosity, which was always highly valued before, seems not that important in these cases. Moreover, the proppant transport and setting law in complex fractures during slickwater fracturing need in-depth investigation.
In this paper, proppant transport experiments and simulation were conducted on a large-scale complex slot system. The slot system consists of one primary slot and several secondary slots. This novel apparatus has features such as tortuosity, rough fracture wall surfaces, fluid leak-off and adjustable perforation density.
The results demonstrated a smaller inclination, longer dune length and smaller dune angle in front of the dune peak during the injection of larger mesh proppants with a higher pump rate. The transport of large mesh proppants was less affected by the flow rate bypass and secondary fracture angles, and most of the settled proppants in both primary and secondary fractures were small mesh proppants. Proppant transport occurs not only by the suspension mechanism in frac-fluids but also by the rolling-migration mechanism on already formed proppant dunes or slopes. Under the drag and lift forces, proppant particles that have been descended can still roll upward along the proppant dune slope and roll downward to form a new proppant pack after the dune peak. The results and analysis showed that viscosity does impact the dune shape, dune equilibrium height and dune equilibrium time, while when considering a high pump rate and the rolling-migration effect, viscosity is no longer the primary factor influencing proppant transport and setting. A joint consideration of the pump rate and fluid viscosity should be the parametric and integrated indicators. Our results also highlight that proppant particle collision, agglomeration and overlapping largely reduce proppant transport and setting efficiency when using large mesh size distribution range proppants.
The results of our study can provide ideas and methods for hydraulic fracturing design optimization through the selection of proppants, fluid properties and pump rate. This study reveals the proppant transport and setting behaviors in complex fractures for slickwater hydraulic fracturing.
