Particle-Transport Mechanisms and Distribution in Typical Tortuous Wedge-Shaped Interwoven Fractures of Deep Coal Seams: A CFD–DEM Study

Natural weak discontinuities, such as natural fractures, bedding planes, and coal–rock interfaces, are widely developed in deep coal reservoirs. During hydraulic fracture propagation, induced fractures readily interact with these weak planes through crossing, deflection, and combined activation, thereby forming complex fracture geometries and significantly affecting proppant transport and placement. To clarify the transport behavior of proppant under different fracture geometries, four representative tortuous wedge-shaped fractures were constructed to characterize typical fracture propagation patterns in deep coal reservoirs, namely a vertical straight fracture (“|”), a horizontal straight fracture (“—”), a T-shaped fracture, and a cross-shaped fracture (“+”). On this basis, a two-way coupled fluid–particle model was established using the CFD–DEM method to systematically investigate proppant migration, settling, and placement in different fractures, as well as the effects of injection velocity, particle size, and fluid viscosity. The results show that fracture geometry exerts a significant influence on proppant transport patterns and placement performance. Specifically, proppant transport in the “|”-shaped, T-shaped, and “+”-shaped fractures can be divided into three distinct stages: rapid start-up, stratified transport, and front advancement. In contrast, particles in the “—”-shaped fracture are only weakly affected by gravity and remain almost entirely in an orderly front-advancement regime, exhibiting the most stable and continuous placement behavior. Increasing injection velocity and fluid viscosity both improve proppant placement uniformity and markedly promote branch entry in the T-shaped fracture, whereas their improvement in the “+”-shaped fracture is relatively limited. When the fluid viscosity increases from 1 mPa·s to 5 mPa·s, the placement uniformity coefficient (PUC) of the “—”-shaped, “|”-shaped, T-shaped, and “+”-shaped fractures increases by approximately 3.2%, 5.6%, 6.3%, and 7.1%, respectively. These findings provide mechanistic insight into geometry-dependent proppant transport and placement in complex fractures of deep coal seams, and offer theoretical support for hydraulic fracturing design and parameter optimization.