Study On The Effects Of Hydration On Borehole Stability In Shale

Shale usually contains a significant amount of water-sensitivity clays. When in contact with drilling fluid, shale and drilling fluid interact with each other, which leads to swelling due to hydration. This not only alters the stress distribution near the borehole, but also reduces the strength of the shale. So, the problem of the borehole instability in shale becomes very serious. From the point of view of mechanics and chemistry, specialists all over the world have done a lot of research and achieved great achievements, but the problem has not been solved successfully. With the development of the petroleum industry and the extension of the exploring field, the problem becomes more and more serious..In this paper, experiments were carried out on the mechanical properties of the shale while under hydration and determined the relationship between them and water content of the shale. Due to hydration, the shale becomes a complex media, the water content, the elastic modulus and the strength of which are all changeable. Under the assumption that the shale is linear elastic, a model was presented to calculate the stress distribution near the borehole under hydration. The model was solved using differential method and a Windows program was deleloped. With the program, we can calculate the distribution of the water content, the stress distribution and the collapse pressure that is necessary for keeping the borehole stable. Finally, an engineering case is used and the result is analyzed.The study methods and the conclusions in this paper are of great significance for solving the borehole instability in shale.