Fully Coupled Hydromechanical Simulation of Hydraulic Fracturing in 3D Discrete-Fracture Networks

Abstract

We developed a hydraulic-fracturing simulator that implicitly couples fluid flow with the stresses induced by fracture deformation in large, complex, 3D discrete-fracture networks (DFNs). The code is efficient enough to perform field-scale simulations of hydraulic fracturing in DFNs containing thousands of fractures, without relying on distributed-memory parallelization. The simulator can describe propagation of hydraulic fractures and opening and shear stimulation of natural fractures. Fracture elements can open or slide, depending on their stress state, fluid pressure, and mechanical properties. Fracture sliding occurs in the direction of maximum resolved shear stress. Nonlinear empirical equations are used to relate normal stress, fracture opening, and fracture sliding to fracture aperture and transmissivity. Fluid leakoff is treated with a semianalytical 1D leakoff model that accounts for changing pressure in the fracture over time. Fracture propagation is modeled with linear-elastic fracture mechanics. The Forchheimer equation (Forchheimer 1901) is used to simulate non-Darcy pressure drop in the fractures because of high flow rate. A crossing criterion is implemented that predicts whether propagating hydraulic fractures will cross natural fractures or terminate against them, depending on orientation and stress anisotropy. Height containment of propagating hydraulic fractures between bedding layers can be modeled with a vertically heterogeneous stress field or by explicitly imposing hydraulic-fracture-height containment as a model assumption. Limitations of the model are that all fractures must be vertical; the mechanical calculations assume a linearly elastic and homogeneous medium; proppant transport is not included; and the locations of potentially forming hydraulic fractures must be specified in advance. Simulations were performed of a single propagating hydraulic fracture with and without leakoff to validate the code against classical analytical solutions. Field-scale simulations were performed of hydraulic fracturing in a densely naturally fractured formation. The simulations demonstrate how interaction with natural fractures in the formation can help explain the high net pressures, relatively short fracture lengths, and broad regions of microseismicity that are often observed in the field during stimulation in low-permeability formations, and that are not predicted by classical hydraulic-fracturing models. Depending on input parameters, our simulations predicted a variety of stimulation behaviors, from long hydraulic fractures with minimal leakoff into surrounding fractures to broad regions of dense fracturing with a branching network of many natural and newly formed fractures.

Stress‐Induced Anomalous Transport in Natural Fracture Networks

“…In addition, this work studied 2‐D fracture system, and there could be important 3‐D effects. While several studies have investigated fluid flow in stressed 3‐D fracture networks (Garipov et al, ; Lei et al, ; McClure et al, ), the effects of geological stress on tracer transport through 3‐D fracture networks remain to be investigated. A recent study has shown that the Bernoulli CTRW model can capture particle transport through 3‐D fracture networks (Hyman et al, ), and the natural extension of this study will be the effects of geological stress on particle transport through stressed 3‐D fracture networks.…”

The effect of stress boundary conditions on fluid‐driven fracture propagation in porous media using a phase‐field modeling approach

“…In all of our computational results, the length of the phase-field diffusive zone is set to be = 2h min and so the initial thickness of phase-field variable is 2h min , where h min is the minimum mesh size. In addition, we set the regularization parameter as = 10 −10 × h min , and the parameter defined in (16) as D = 0.5. Except the last example using a field data in Section 4.6, we assume homogeneous and isotropic reservoir domains, and the following parameters are consistently used: K R = 1.0 × 10 −12 m 2 , R = F = 1 cp, and c F = 4.0 × 10 −10 Pa −1 .…”

“…5 However, modeling fluid-driven fracturing processes in porous media is challenging because of its complexity involving solid-fluid interaction and various fracture geometry scenarios. There have been a lot of studies to model the fracturing process by using cohesive (interface) elements, [6][7][8][9] extended finite element method (XFEM)/generalized finite element method (GFEM)/partition of unity method (PUM), 8,[10][11][12] displacement discontinuity methods, [13][14][15][16] peridynamics, 17 and others.…”

Statistical Method for Tracing Hydraulic Fracture Front Without Evaluation of the Normal

“…The next sections present the numerical studies performed by using three benchmark solutions. These are the solutions to (i) the Khristianovic-Geertsma-de Klerk problem (KGD problem) [5,6,17], given, e.g., in the papers [7,8]); (ii) the axisymmetric problem [9,10]; and (iii) the truly 3D problem for a pay-layer between half spaces with symmetric stress contrast [11,18]. The numerical results show that the method avoiding explicit evaluation of the normal may be used in practical calculations.…”