Experimental investigation of proppant settling in complex hydraulic-natural fracture system in shale reservoirs

• A complex fracture network simulation device has been developed to perform proppant settling tests.
• Proppant turn around near node, creating an abrupt change of sandbank height, which can reduce fracture conductivity.
• Fracturing fluid’s carry-over effect plays a major role during sandbank formation in secondary fractures near wellbore.
• The order of proppant migration distance in different structures is: “Line” > “T” > “H” > “TF” > “Double T” > “Crossing”.
• Using low viscosity fracturing fluid is helpful to support fractures near wellbore.

The general development mode of shale gas reservoirs, as an unconventional resource, cannot yield good results. Hydraulic fracturing is a widely used method in the development of unconventional reservoirs. This method’s nature is to create hydraulic fractures in the reservoir, providing a flow path for oil and gas. To achieve a satisfying output, fracture dimensions matter greatly, as well as the fracture effective period. Maintaining these parameters requires that proppant can be distributed in fracture systems abundantly and evenly such that reservoirs can have durable fractures with good conductivity.Shale gas reservoirs have small porosity and permeability. Gas contained in them is difficult to exploit; thus, massive stimulation methods are needed. Conventional fracturing usually aims to create a long single planar fracture in a reservoir, while in shale gas reservoirs, complicated fracture networks are needed. Thus, the fracturing method in shale gas reservoirs should take natural fractures into consideration, link original micro-fractures in the reservoir, and increase the drainage area as much as possible. This type of fracturing method is called volume fracturing. Volume fracturing can also create multiple artificial fractures perpendicular to the main fracture, improving the stimulation effect and extending the stimulation effective period.Proppant distribution is critical to maintaining fracture network conductivity and enhancing a shale gas reservoir’s output. Moreover, proppant migration and settling during fracturing can affect the activation of natural fractures and the formation of fracture networks greatly, as well as the final effective fracture geometry. The body of research concerning proppant’s distribution law in conventional single fractures is quite mature, but the distribution law in complicated fracture networks has not been thoroughly elucidated. Obviously, the current understanding cannot satisfy the demand of practical stimulation. This paper considers the difference of the proppant distribution law between complicated fracture networks and conventional fractures and uses a self-designed complex fracture network simulation device to study proppant migration and the settling law in a complicated fracture network. The research results provide theoretical support for fracturing design in shale gas reservoirs.