Role of overpressurized fluid and fluid-driven fractures in forming fracture networks

Abstract

A 2-D numerical study was carried out, using a fully coupled rock deformation and fluid flow hydraulic fracturing model on fracture network formation in a low-permeability naturally fractured rock, in which new fractures are allowed to nucleate and grow driven by overpressurized fluid. Fracture seeds either are defined as small flaws or are nucleated based on a stress and fracture energy criterion. Fracture intersection, fluid flow and frictional slip along fractures are also explicitly simulated in the model. In particular, we consider a few artificial fracture network geometries with sets of finite discrete or isolated fractures in order to study the roles of fluid viscosity, injection conditions, fracture intersections and offsetting at pre-existing or natural fractures in determining the paths which hydraulic fractures follow through the network. The newly fluid-created fracture segments, which are oriented locally normal to the least compressive stress, are the most conductive parts along the hydraulic fracture path. The resulting network will allow for long-term fluid flow to occur more easily provided that these segments retain their conductivity. However, the existence of intersections and offsets in the main fracture path act to impede fracture growth and fluid flow. A fracture nucleation event associated with higher fluid pressure may allow a fracture path to bypass these barriers, thus leading to a more planar fracture geometry. If fracture nucleation does not occur, fluid may enter other cross cutting natural fractures or enter along a barrier fracture to the tip of the barrier fracture, both processes that require a larger expenditure of energy reflected in an increasing fluid pressure. The results clearly demonstrate the importance of these stress and flow barriers in forming a preferential fracture and flow path. Fluid invasion into and fracture growth of natural fractures can still continue to occur at the high pressure region near the hydraulic fracture entry, even after a complete hydraulic fracture pathway has developed, showing strong similarity to the invasion percolation process. In contrast to fixed-aperture, connected fracture network models, the fracture growth-generated patterns also depend on the in situ stresses, which affect mechanical interaction between the main hydraulic fracture and natural fractures. The numerical results provide improved understanding of fluid-driven fracture growth into and through a network of pre-existing natural fractures.