ABSTRACT

In order to investigate the fracture development and the associated fluid flow in rocks, an improved flow-stress-damage (FSD) model, implemented with Rock Failure Process Analysis code (RFPA3D), is presented. The numerical code is based on linear elastic damage mechanics on mesoscopic scale and FEM. For simulating the complete progressive 3-D failure and macroscopic mechanical behaviors of rock materials, rock properties such as elastic constants, peak strength, and Poisson ratio are randomly distributed to reflect the initial random distributed weakness in mesoscopic scale. The improved FSD model is used to represent the permeability variation at the two stages (stress-dependent permeability for pre-failure and deformation-dependent permeability for post-peak stage) of rock matrix at the elemental scale. The fracture initiation, propagation, and coalescence in the rock sample and the seepage field evolution with stress and damage variation are represented visually during the whole failure process. The simulation results compare well with reported experimental results which indicate that RFPA3D incorporated with the improved FSD model is a valid tool in understanding the physical essence of the evolutionary nature of fracture phenomena as well as the fluid flow in rocks.

1 INTRODUCTION

Within rock or civil engineering structures, predicting the behavior of fluid flow through fractured and fracturing rocks, especially in highly stressed rocks, is a formidable task. Based on the underlying mechanical and hydraulic natures of the basic components of a rock mass, a large number of numerical model and computer codes for flow-stress or flow-strain coupling analysis have been developed in recent years to investigate the coupling action between fluid and solid (Noorishad 1971, Bruno 1994, Zhang et al 1996, Rutqvist et al 2001). However, when attempting with these approaches to model mechanically induced progressive failure of rock and the associated changes in hydraulic conditions, it is difficult to track the dynamic evolution of fractures if wholesale redefinition of the model is to be avoided. To overcome this, many scholars have proposed several kind of numerical model and conducted corresponding hydro-mechanical modeling of rock in recent years (Li et al 2001, Jing 2001, Yuan 2005). But the methods are not yet fully capable of reproducing pressure-sensitive hydro-mechanical responses °

Investigations mentioned above are all limited to two-dimensional conditions. As we know, many rock mechanical problems, except plane strain problems and plain stress problems, such as crack propagation and crack interaction are related to many directions and cannot be simplified to two-dimension problems. Few presentations can be found to explain the progressive failure of rock in three-dimensions and related nonlinear behaviors resulting from material heterogeneities (Cundal et al 1998). The rock specimen was subjected to numerical tests to investigate some characteristic features of the complete stress-strain curves and the phenomena observed during progressive fracture°

In this paper, an improved flow-stress-damage (FSD) coupling model for saturated rocks is presented and numerical simulations are conducted to investigate the three-dimensional failure process and fluid flow in brittle and heterogeneous rocks.

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