As a well was drilled, damage occurred in the zones around the well, even though the wellbore was not instability. An analysis model of investigating the plastic damage of boreholes in hard brittle shale is established. And the tempo-spatial variation laws of the seepage characteristics and pore pressure for drilling fluids in the strata around the wells were analyzed. The damage zones were mainly found in the zones nearby the well in the direction along minimum horizontal in-situ stress; the permeability of the shale within the damage zones was increased significantly. This made a rapid rise of pore pressure for the strata in this direction. The pore pressure on the strata around the borehole wall was shown to rise rapidly, which approximated to the pressure of the well. Without the considering of stress damage, the pressure drop gradient nearby the well was large, while the pore pressure only showed an unobvious increase with long time. The influence of the stress damage on the permeability of the strata nearby the well needed to be considered when analyzing the issues concerning the borehole of hard brittle shale, so as to avoid large error.

1. Introduction

Damage may occur due to occurrence and extension of various drawbacks such as micro-crack under influence of external factors. The damage is irreversible and material failure is a result of development and accumulation of the degradation. Dougil (1976) first introduced damage mechanics into geotechnical engineering in 1976. Krajcionovic et al. (1989) analyzed damage mechanism of rocks and established the corresponding damage model and theory to further improve the research and application of damage mechanics to rock. Shao et al. (2005, 2006) systematically studied the damages to brittle rocks and analyzed damage evolution process, extension of micro-cracks and changes of rock permeability due to extension of micro-cracks through macro- and meso-mechanics. Also, he built the constitutive model about elastic-plastic damages to brittle rocks under unsaturated condition. Salari et al. (2005, 2006) established the elastic-plastic damage model of rock materials by applying continuous thermodynamics theory, and introduced tensile-stress damage into the model. Moreover, he regarded the damage and Drucker-Prager strength criterion as function of volumetric strains and plastic potential function of materials, respectively.

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