Issue 27

X. Ran et alii, Frattura ed Integrità Strutturale, 27 (2014) 74-82; DOI: 10.3221/IGF-ESIS.27.09 75 degrees. At present, there are several models applied to analyze borehole stabilization, such as liner elastic model, elastic- plastic model [5] and fluid-solid coupling model of porous media [4]. However, most of studies pay little attention to rock damage. Practically,the stress redistribution after excavation will cause the produce of damage (Fig.1). In the process of rock damage, permeability will change greatly with the fracture producing, extending and connecting [6]. Meanwhile, rock strength and elastic modulus will decrease obviously. All of this can effects the result of force analysis, so it is necessary to involve the damage in seepage-stress coupling calculation of wall rock. Figure 1 : Damage zone of wall rock in the uniform ground stress. Under the change of external load or environment, the macro-mechanical properties deterioration of materials or structure caused by the initiation and propagation of microstructure defects (such as micro-cracks, micro-porosity, etc.) or other irreversible changes is called damage. In the study of seepage in fractured rock mass of damage mechanics, Yi [7] established a seepage-damage coupling model, and a damage evolution equation in the compression shear or tensile shear state. Zheng [8] proposed a seepage-damage coupling theory and a permeability tensor expression which considered the effect of damage of fractured rock. Jiang Tao [9] deduced a damage constitutive model and a seepage-damage constitutive model of brittle rock based on mesomechanics. Yang [10] and Zhao [11] studied the seepage-damage-fracture coupling theory and its application. Based on poroelastic theory, Selvadurai [12] studied the damage and permeability variation of geological materials which relate to stress. For the rock, the meaning of plasticity mainly refers to the sliding between inner fissuring. But the damage refers to the producing and extending of inner fissuring. The damage means a decrease of net loaded area in geometry. Using the plastic-damage coupling constitutive model, the mechanical behaviour could be represented exactly. Based on the theory of continuum damage mechanics, the damage evolution model according to equivalent plastic strain is established in this paper. Additionally, the permeability evolution equation is established in relation to the evolution of plastic damage. Aiming at a practical drilling process in an oilfield in western China, the physical model of borehole rock is built. Using this model to simulate the disturbance damage and the permeability evolution in drilling, the distribution of stress of wall rock and the relationship between borehole instability and drilling fluid density is obtained. This study will improve the theory of mechanical mechanism of borehole collapse and fracture. S EEPAGE - STRESS - DAMAGE COUPLING MODEL onlinear behaviour of rock caused mainly by micro cracks. If the micro cracks become a macro-fracture by accumulating, extending and connecting, then the degradation of material property which includes the strength, rigidity and ductility will arise. For describing it, the damage variable is needed according to irreversible thermodynamics. The damage variable can be chosen from the characteristics of micro-structure (the quantity, length, area and volume of micro cracks) or the experiment date (the elastic modulus, strain, yield stress, density and wave velocity). To apply the model in practical engineering, the damage variable and its evolution law is need to define. In this paper, the damage variable is supposed as follows: 1) Rock damage occurs and increases with plastic deformation simultaneously. 2) The rate of damage rise is gradually diminishing with the development of plastic deformation. N