Mechanics of fluid‐driven fracture growth in naturally fractured reservoirs with simple network geometries

Abstract

[1] A numerical model has been developed for fluid-driven opening mode fracture growth in a naturally fractured formation. The rock formation contains discrete deformable fractures, which are initially closed but conductive because of their preexisting apertures. Fluid flow that develops along fractures depends on fracture geometry defined by preexisting aperture distribution, offsets along a fracture path, and intersections of two or more fractures. The model couples fluid flow, elastic deformation, and frictional sliding to obtain the solution, which depends on the competition between fractures for permeability enhancement. The fractures can be opened by fluid pressure that exceeds the normal stress acting on them and by interactions with intersecting closed fractures experiencing Coulombtype frictional slip. The Newtonian fluid is assumed to flow through the conductive fractures according to a lubrication equation that relates the cube of an equivalent hydraulic aperture to fracture conductivity. The rock material is assumed to be impermeable and elastic. This paper provides the governing equations for the multiple fracture systems and the solution methods used. Flow distribution and fracture growth in conductive fracture sets are simulated for a range of geometric arrangements and hydraulic properties. Numerical results show that elastic interaction between fracture branches plays a controlling role in fluid migration, although initial apertures can give rise to a preferential fluid flow direction during the early stage. In the presence of offsets, fracture segments subject to strong compression are difficult to open hydraulically, and their resulting smaller permeability can increase overall upstream fracture pressure and opening. The patterns of fluid flow become more complicated if fractures intersect each other. A portion of injected fluid is lost into closed empty fractures that cut across the main hydraulic fracture, and this delays the pressure increases required for fracture growth past the crosscutting fracture. The nonlinear fluid loss rate depends on the geometric complexities of the fracture sets and on the fluid viscosity. Sometimes fracture growth can be accelerated by the fast fluid transport along an intersected, relatively conductive natural fracture.Citation: Zhang, X., R. G. Jeffrey, and M. Thiercelin (2009), Mechanics of fluid-driven fracture growth in naturally fractured reservoirs with simple network geometries,

Numerical modeling of porous flow in fractured rock and its applications in geothermal energy extraction

“…The existing numerical models for extracting geothermal energy are almost all based on the continuum approach (Hunsweck et al, 2013;Secchi and Schrefler, 2012;Zhang et al, 2009). Although these models have been shown to be valuable in geothermal energy exploration, as reported in a number of case studies in the literature (Zhou and Hou, 2013;Gong et al, 2011;Blocher et al, 2010;Bataille et al, 2006), they are incapable of capturing the full physics of the modeled fractured rocks.…”

Fracture propagation in a naturally fractured formation

“…In this case, the propagation of shear slip is driven by the slip-weakened friction force, and by the fast fluid pressure diffusion in the fracture [27][28][29]. Shear-induced dilation enhances fracture conductivity and accelerates the pressure diffusion in fracture, which in turn transmits the reduced shear force in the slipped part of the fracture toward the not-yet-slipped part of the fracture ahead of the slip front, causing the slip front to propagate.…”

“…The large separation between these curves indicates that fracture crossing is very sensitive to the intersection angle. Laboratory testing [27][28][29][30], to be described in Chapter 9, has been carried out for verifying the above crossing criterion. The predictions from the above extended Renshaw and Pollard crossing criterion are in good agreement with the results from laboratory testing.…”

Mechanics of edge crack growth under transient pressure and temperature conditions

“…In particular, Khristianovich and Zheltov (1955) recognized that stable crack growth is associated with the existence of an unpressurized cavity or a lag zone arising from the coupling between factors that generate the net pressure distribution in the fracture, the compressive confining stress, and the fracture toughness of the rock. A numerical scheme with implicit iterations for tracking the fluid fronts has been described by Zhang et al (2005Zhang et al ( , 2009 and the reader is referred to those papers for more details. It was found that these moving boundaries can be approximated by using our numerical method (Zhang et al, 2005(Zhang et al, , 2009).…”

“…A numerical scheme with implicit iterations for tracking the fluid fronts has been described by Zhang et al (2005Zhang et al ( , 2009 and the reader is referred to those papers for more details. It was found that these moving boundaries can be approximated by using our numerical method (Zhang et al, 2005(Zhang et al, , 2009). …”