Study on Mechanism Wellbore Instability

摘要

The shales, fine ― grained sedimentary rocks, play an important role in wellbore instability. The annular total loss due to wellbore instability is as high as 500 million dollars all over the world. There are various types of shales which will cause wellbore instability in different mechanisms. There are many factors which will cause wellbore instability, so it is difficult to determine the exact point at which wellbore instability may become so sever that wellbore failure occur. After intensive studying the wellbore instability mechanism, all factors which have much to do with wellbore instability are compared and a new approach used for model ― based assessments is proposed to avoid wellbore failure. Conventional methods of improving wellbore stability rely on the concept of a support pressure at the wellbore wall which can be achieved, for instance, by increasing the density of the drilling fluid. The effectiveness wellbore wall coating materials or capillary threshold pressures to prevent fluid pressure penetration into drilled shales is critical for maintaining the support pressure effect.