Proppant Transport in Complex Fracture Networks

Abstract

Abstract During hydraulic fracturing, natural fractures and bedding planes can intersect with growing hydraulic fractures and form complex fracture networks. This can result in the flow of fluid and proppant in convoluted fracture pathways with highly variable fracture width and height. Existing models of hydraulic fracturing assume a planar fracture geometry and are unable to simulate proppant placement in such complex networks. In this work, we investigate proppant transport in growing fracture networks using a fully three-dimensional, geomechanical fracture flow, network model with the ability to simulate proppant transport. A three-dimensional hydraulic fracturing simulator developed using the displacement discontinuity method is coupled with a network model for proppant transport. The simulator captures the effect of proppant concentration, fracture width, and fluid rheology on proppant transport. The equations for the fracture network geomechanics, the fluid flow, and the proppant transport are solved in a coupled manner. This provides an accurate estimation of both the fluid pressure and the proppant distribution as the fracture network grows. The geometry of each fracture segment affects the flow distribution in the network. Simulations are then conducted to study the redistribution of proppant as it settles in the fracture network during shut-in to get the final proppant distribution in the network. It is observed that changes in the in-situ stress due to heterogeneity and the stress-shadow induced near the intersection of a hydraulic fracture and a natural fracture may reduce the fracture width and suppress the ability of the proppant to move into the natural fracture. In low permeability formations, due to low leak-off rates, the proppant almost always forms a proppant bank at the bottom of the fracture during shut-in. For planar fractures, proppant settling may disconnect the conductive proppant bank from the wellbore, isolating the productive propped fracture from the wellbore. This problem is exaggerated in the case of fracture networks, where every intersection point between fractures can potentially act as a bottleneck for the flow of produced hydrocarbons. The increase in the surface area due to hydraulically connected natural fractures increases fluid leak-off, reduces the average width of the fracture network, increases proppant concentration, and increases the likelihood of proppant bridging. This work allows us to improve our understanding of proppant placement in three-dimensional, mechanically interacting, complex fracture networks. By coupling geomechanics with proppant transport in fracture networks, it is now possible to study the impact of the stress shadow on proppant placement in natural fractures. The results will assist in improving hydraulic fracture design for naturally fractured reservoirs.