Shale Borehole Wall Stability Research Based On Damage Mechanics

Shale borehole wall stability is one of the great problems that affect the development of drilling technology. Borehole wall instability sharpened as a problem as a result of water going into shale strata and deteriorating shale rock mechanics property factors, and weakening the supporting ability of rocks that surrounds borehole. Based on damage mechanics theory, the thesis deals with borehole wall instability mechanism and its evolution mechanism when the water is seen in the strata. The following work is mainly discussed in the thesis.(1) Through analyzing the mineral composition, structure feature and electrochemistry properties of shale, borehole wall instability model is set up, and hydrate deterioration and damage mechanisms in the rock surrounding borehole are also discussed. Damage variables under complex stress, damage constitutive and damage evolution equations are confirmed with combination to damage mechanics.(2) Through shale experiment research, hydrating diffusion equation is set up, and how shale mechanics factors deteriorates with water cut and time is analyzed. Shale damage constitutive function is deduced in combination of the strength and deformation theory when water is met in shale,which helps to get the evolution rhythm between rock property factors and damage constitutive, and to confirm the damage limit of shale.(3) Stress distribution equilibrium equation with damage variables in rocks surrounding borehole wall is set up combining with statics, geometry and physics. In combination of linear and elastic mechanics, stress in straight and horizontal wellbore walls. And shale hydration stress distribution is deduced based on equivalent energy principle, and the overall stress distributions in rocks surrounding borehole wall are set up with the help of them.(4) Through consideration of damage variable, shale shear and tension damage failure criteria, as well as strata collapse and breakdown pressure calculating model is set up. Then examples are given to determine the shale borehole wall hydration,damage,collapse and breakdown period. Thus a theory of predicting the shale borehole wall instability has been come up in the thesis.