Study On Influence Of Shale Hydration On Shaft Stability

The problem of borehole stability of mud shale is a complex problem in the word and it will restrict the efficiency of the exploration and exploitation of oil field in our country. Hydration will cause a variety problem of mud shale, such as pore pressure rise, rock strength reduction, etc. These change will cause borehole unstability and make the borehole easy to be shearing collapse. Therefore, study the influence of shale hydration to wellbore has important meaning to solve this problem.The main work of this dissertation is as follows:(1) This paper study the influence of shale hydration to the stability of wellbore. The physical and Chemical properties of shale is described. The law of water absorption of shale is studied based on the experiment of mud shale spontaneous dialysis device and the law of water absorption has also been analyzed.(2) This paper analyze the change rule of elastic modulus、Poisson’s ratio、cohesion、 internal friction angle and expansion stress.(3) The new model of shale borehole stability has been established, which is based on mechanic-chemistry coupling model and the elastic stress model of permeable borehole. The expansive stress and pore pressure is considered in the constitutive equation. The new model has considered the influence of shale hydration to rock mechanical parameters and pore pressure. The additive stress which is caused by the change of pore pressure is also considered in the model. The stress of borehole surrounding rock is composed of three parts:the hydration expansive stress when formation is considered to be isotropic; the non-uniform stress when formation is considered to be anisotropy and the additive stress which is caused by the change of pore pressure.(4) The drilling filed data has been used to analyze borehole stability of shale based on the new model. The change rule of elastic modulus、Poisson’s ratio、cohesion、internal friction angle and expansion stress has been got in shale stratum. Then the change rule of radial stress and tangential stress can be got by the new model. According to the strength criterion, the stability of shale stratum in different distance away from borehole axis can be got. Rock strength will decrease and pore pressure will increase because of shale hydration, this will cause the change of stress around the borehole. In this case, the borehole stability of shale will be changed. The shear collapse place will be deeper with the increase of time.