Abstract

Hydraulic fracturing in rock masses involves complex and coupled processes of fractures propagating in discontinuous media and of fluid flow in discrete channels. The Unconventional Gas Program at Lawrence Livermore National Laboratory is actively developing numerical models to gain insight in the physics of these coupled processes. However powerful and versatile these models are, they must be verified against analytical solutions and/or physical experiments. This paper describes the current status of the FEFFLAP code, which models fluid-driven discrete fracture propagation in jointed media. We also discuss the results of a preliminary series of physical tests in which hydrofractures were driven across slanted interfaces between dissimilar materials, in blocks loaded in biaxial compression.

Introduction

Hydraulic fracturing is employed both for the stimulation of tight hydrocarbon reservoirs, and for estimating in-situ stresses in rock. It involves a complex and coupled process of fracture propagation and fluid flow. In rock masses, the interaction of induced fractures with natural joints and interfaces adds another level of complexity to the problem of predicting the behavior of the hydrofractures, i.e., predicting the behavior of the hydrofractures, i.e., extension, and containment, or lack of it. To gain insight in this problem, the Unconventional Gas Program of the Lawrence Livermore National Laboratory Program of the Lawrence Livermore National Laboratory has an active effort in the development of numerical models [this paper]. Numerical methods are emphasized because of their power and versatility in handling complex physics. Still, numerical models, however powerful they are, must be verified against analytical solutions or physical experiments before they can be fully exercized and trusted.

This paper describes the current status of development and testing of the FEFFLAP code (Finite Element Fracture and Flow Analysis Program). FEFFLAP represents the coupling of the FEFAP discrete fracture propagation code and of the JTFLO program, a propagation code and of the JTFLO program, a LLNL-enhanced version of an earlier code for analysis of fluid flow in rock fractures. We also present the results of a preliminary series of physical tests in which hydrofractures were driven across slanted interfaces between dissimilar materials, in blocks loaded in biaxial compression.

THE FRACTURE AND FLOW MODEL: FEFFLAP
Discrete Fracture Propagation: FEFAP

The foundation for the solid fracture analysis described here is the Finite Element Fracture Analysis Program (FEFAP) developed at Cornell University. FEFAP analyzes planar and axisymmetric structures for crack initiation and growth. The program combines linear and non-linear fracture program combines linear and non-linear fracture mechanics theory, the use of interactive computer graphics, and a unique, automatic remeshing capability to allow the user to initiate and propagate up to ten discrete cracks simultaneously.

FEFAP has been used for a variety of applications involving crack growth in rock, concrete, and metal structures. An example of a large scale problem is that involving the cracking of the problem is that involving the cracking of the Tennessee Valley Authority's Fontana Dam [101. Figures 1 and 2 show meshes before and after crack propagation modeling. A comparison between propagation modeling. A comparison between predicted and observed crack paths is shown in Figure 3. predicted and observed crack paths is shown in Figure 3. The salient capabilities implemented in the present version of FEFAP are:

. Complete interactive-graphical execution of the program. Each analysis step is directed by the user with alphanumerical and graphical feedback of the results of this step. After any complete crack propagation step, the analysis can be terminated and restarted from the previous step. The emphasis in the program design is on providing versatility to the analyst. One is not locked into a batch-produced result via the initial data input.

. Automatic, discrete crack nucleation at arbitrary points and angles on an edge or in the interior of a domain, as specified by the analysis.

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