A hydraulic fracturing operation involves fluid injection into a naturally fractured rock mass. Yet the presence of these natural fractures and their influence on fracture propagation is often not accounted for in hydrofrac design calculations. Presented here are the results of a set of simulations carried out using the discontinuum-based distinct-element method. The simulations have been carried out applying a transient, coupled hydro-mechanical analysis to a naturally fractured rock mass. The inclusion of a voronoi tessellation scheme adds the necessary degrees of freedom to model the propagation path of a hydraulically driven fracture as a function of its interactions with the natural fracture network and in-situ stress state. The results show that key interactions develop with the natural fractures that influence the calculated stimulated volume through additional connected surface area and fracture dilation. These interactions also have the potential to decrease the size and effectiveness of the hydrofrac stimulation by diverting the injected fluid and proppant, and limiting the extent of the hydraulic fracture.
Hydraulic fracturing is one of the primary means for improving well productivity in the oil and gas industry, guided by over 60 years of experience in its use. Accordingly, hydraulic fracturing theory was largely developed based on linear elastic fracture mechanics, with current standard hydraulic fracturing simulations adopting assumptions such as the generation of bi-wing, perfectly planar, symmetrical fractures. In situ, the actual reservoir rock conditions are much more complex. Neglected in the standard calculations are the influences of rock mass heterogeneity and natural discontinuities on the propagation of the induced hydraulic fracture. One of the key elements of a successful hydrofrac job is the effective connectivity of the hydraulic fracture, which would interact with and incorporate the natural discontinuities in the rock mass.