Numerical Modeling of Multi-Stranded Hydraulic Fracture Propagation: Accounting for the Interaction Between Induced and Natural Fractures

Abstract

Recent examples of hydraulic fracture diagnostic data suggest complex, multi-stranded hydraulic fractures geometry is a common occurrence. This reality is in stark contrast to the industry-standard design models based on the assumption of symmetric, planar, bi-wing geometry. The interaction between pre-existing natural fractures and the advancing hydraulic fracture is a key condition leading to complex fracture patterns. Performing hydraulic fracture design calculations under these less than ideal conditions requires modeling fracture intersections and tracking fluid fronts in the network of reactivated fissures. Whether a hydraulic fracture crosses or is arrested by a pre-existing natural fracture is controlled by shear strength and potential slippage at the fracture intersections, as well as potential debonding of sealed cracks in the near-tip region of a propagating hydraulic fracture. We present a complex hydraulic fracture pattern propagation model based on the Extended Finite Element Method (XFEM) as a design tool that can be used to optimize treatment parameters under complex propagation conditions. Results demonstrate that fracture pattern complexity is strongly controlled by the magnitude of anisotropy of in situ stresses, rock toughness, and natural fracture cement strength as well as the orientation of the natural fractures relative to the hydraulic fracture. Analysis shows that the growing hydraulic fracture may exert enough tensile and shear stresses on cemented natural fractures that they may be debonded, opened and/or sheared in advance of hydraulic fracture tip arrival, while under other conditions, natural fractures will be unaffected by the hydraulic fracture. Detailed aperture distributions at the intersection between fracture segments shows the potential for difficulty in proppant transport under complex fracture propagation conditions. Introduction Large volumes of natural gas are stored in low-permeability fractured reservoirs around the world. Because of the low permeability of these formations and the low conductivity of the natural fracture networks, stimulation techniques such as hydraulic fracturing are necessary to make economic production possible. The low conductivity of the natural fracture system could be caused by occluding cements that precipitated during the diagenesis process (Laubach 2003, Gale et al. 2007). The fact that natural fractures might be sealed by cements does not mean that they can be ignored while designing well completion processes, however. Cemented natural fractures can still act as weak paths for fracture growth. New diagnostic tools developed during the last decade strongly suggest multiple fracture propagation or multi-stranded hydraulic fractures in naturally fractured reservoirs (Fisher et al. 2005). Dynamic fracture mechanics theories (Freund 1990) indicate that crack tip branching will occur only in cases where fracture propagation speed is comparable to the seismic velocity of the material (more precisely, the Rayleigh wave speed). However, field data demonstrate that hydraulic fractures propagate much more slowly than seismic wave speeds (Valkó and Economides, 1995), so multi-branched fracturing should not occur in a homogeneous, isotropic, intact rock mass. On the other hand, the present day in-situ tectonic stress direction can be rotated from the time of the formation of natural fractures (Laubach et al. 2004). So, natural fractures are not necessarily aligned with the present day direction of maximum compressive stress. Thus, natural fractures may not be parallel with the hydraulic fracture and might be intersected by the hydraulic fracture. Intersection with geological discontinuities such as joints, bedding planes, faults and flaws in reservoirs might render fractures non-planar and multi-stranded.

A three-phase XFEM model for hydraulic fracturing with cohesive crack propagation

“…The lubrication equation is a nonlinear partial differential equation that relates the fracture width and the fracture pressure gradient. The fracturing flow model can also be extended to include power-law fluids [1,12] and slightly compressible fluids. However, in these models the fracture is assumed to be fully saturated, involving a single-phase fluid flow, which is not consistent with the actual field conditions where the fracturing fluid can have properties substantially different than those of the host fluid.…”

“…In reality, fracture pressures are felt along the fracture faces in their entirety, without reduction from Biot's coefficient. Dahi Taleghani [12] used XFEM to model the fracture discontinuity, but simulated fracture flow through a one-dimensional laminar flow. The coupling between fracture flow and mechanical model was achieved weakly through a successive procedure where the results of fracture flow were used to update the solution from the mechanical model and vice versa.…”

Coupled Finite Volume Methods and Extended Finite Element Methods for the Dynamic Crack Propagation Modelling with the Pressurized Crack Surfaces

“…Khoei et al [25] simulated the crack growth in saturated porous media using XFEM. Taleghani [26] also developed an XFEM code to simulate fracture propagation, initiation, and intersection, and the presented coupled fluid flow-fracture mechanics simulations extended available modeling efforts and provided a unified framework for evaluating fracture design parameters and their consequences. Salimzadeh and Khalili (2015) [27] proposed a three-phase hydromechanical model for hydraulic fracturing and they handled discontinuity by using XFEM while cohesive crack model was used as fracturing criterion.…”

Coupled flow-stress-damage simulation of deviated-wellbore fracturing in hard-rock

“…Such as the finite element method (FEM) (Guo et al, 2015;Zhang et al, 2010;Chen et al, 2009;Tang et al, 2009aTang et al, , 2009b, finite difference method (FDM) (Nagel and Sanchez-Nagel, 2011), boundary element method (BEM) (Hossain and Rahman, 2008;Rahman et al, 2002), displacement discontinuity method (DDM) (Zhang et al, 2007, discrete element method (DEM) (Nagel et al, 2012;Tang et al, 2014Tang et al, , 2013 and the extended finite element method (XFEM) (Taleghani, 2009). Table 1 summarizes some numerical methods and software products for hydraulic fracturing simulation.…”

“…Guo et al (Guo et al, 2015) gave a new perforating cluster space optimization method based on PPCE to simulate horizontal well multi-stage fracturing in extremely thick unconventional gas reservoir. Taleghani (Taleghani, 2009) developed an XFEM-based code to simulate the propagation of hydraulic fractures in natural fractured reservoirs. Gordeliy and Peirce (Gordeliy and Peirce, 2013) used the XFEM to solve the propagation problem of a planar hydraulic fracture in an elastic medium-hard reservoir.…”