Abstract

A three-dimensional fracture model developed in the context of the combined finite-discrete element method is incorporated into a two-way fluid-solid coupling model. The fracture model is capable of simulating the whole fracturing process. It includes pre-peak hardening deformation, post-peak strain softening, transition from continuum to discontinuum, and the explicit interaction between discrete fracture surfaces, for both tensile and shear fracture initiation and propagation. The fluid-solid coupling model can simulate the interactions between moving fluids and multi-body solids. By incorporating the fracture model into the coupling model, a methodology of using the new coupling model to capture fracturing behaviour of solids in fluid-solid coupling simulations is proposed. To solve the problem in the coupling model of having adaptive continuous meshes being used by the fluid code and discontinuous meshes in the solid code, a scheme to convert different meshes is developed. A single fracture propagation driven by fluid pressures is simulated and the results show that the modelling obtains the correct critical load and propagation direction for fluid-driven fracturing. Several important phenomena, such as stress concentration ahead of the fracture tip, adaptive refinement of fluid mesh as a response to the fracture propagation and fluids flowing into fractures, are properly captured.

1. INTRODUCTION

Hydraulic fracturing has been used in the oil and gas industry for more than half a century. It is particularly important for the extraction from unconventional reservoirs, which otherwise would be considered as uneconomical. In the research field of hydraulic fracturing, an increasing amount of effort has been put into the development of novel numerical models to simulate realistic scenarios. However, the numerical modelling of hydraulic fractures still remains a challenge for computational mechanics because of the complexities in fluid-solid coupling, fracture characterisation, and particularly in three dimensions, the complicated geometry and topology update when fractures propagate. In recently years, many attempts have been made and significant progress achieved in three-dimensional hydraulic fracturing simulations. Carter et al. (2000) [1] proposed a fully threedimensional hydraulic fracture model, but they neglected the fluid continuity equation in the area around the fracture. Secchi and Schrefler (2012) [2] developed a method to simulate three-dimensional hydraulic fractures in porous media and presented an example of a concrete dam. In this paper, a three-dimensional fracture model is incorporated into a two-way fluid-solid coupling model for hydraulic fracture simulations. The three-dimensional fracture model is capable of simulating the whole fracturing process. It includes prepeak hardening deformation, post-peak strain softening, transition from continuum to discontinuum, and the explicit interaction between discrete fracture surfaces, for both tensile and shear fracture initiation and propagation. After incorporating it into the fluid-solid coupling model, where the interaction between fluids and solids can be explicitly simulated, the initiation and propagation of fractures can be driven by forces generated both from solids loading and transferred from fluids loading, e.g. fluid pressure, which significantly extends the application of this fracture model into some very important areas that would not be possible with only solids modelling, e.g. the simulation of hydraulic fractures.

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