Borehole instabilities during drilling may cause substantial problems. A borehole stability problem is an example of what drillers refer to as a “tight hole” or “stuck pipe” incident. There are a variety of reasons for getting a tight hole or stuck pipe scenario but in a large number of cases, the primary cause is the failure of the borehole. Shaly layers increase the chance of bore instability in a reservoir or in the overburden strata. These problems not only add to the drilling costs during exploration and production phases but also lead to the rig time loss and the loss of tool string in the hole in many cases. In recent years with increasing complexity of drilled wells to access more difficult reservoirs, new challenges have come up, making the stability issue more critical to handle. This warrants the need to work towards resolving this complex problem.
In the past couple of decades, the reservoir types have also changed and shifted from conventional clastic-carbonate type reservoir to tight shale, tight sand reservoir types which are challenging to drill because of its complex nature of stress, lithology and petrophysical property. Thus, there is a growing need to evaluate for an optimum mud weight to prevent possible instability issues, which requires the study of failure criteria on a case-by-case basis. Many failure criteria have been proposed to describe the rock strength under different stress conditions. These criteria are mostly intended to better utilize the laboratory measurements to analyse the field data. However, it is observed that the loading conditions common to laboratory tests are not very indicative of rock failure in cases of practical importance (such as wellbore stability). It may seem in principle to utilize relatively complex failure criteria, but it is often impractical 47 to do so because the core is rarely available for comprehensive laboratory testing, particularly in overburden rocks where many wellbore stability problems are encountered. In addition, because the stresses acting at depth are strongly concentrated around wellbores, it is usually more useful to estimate the magnitudes of in-situ stresses correctly than to have an accurate value of rock strength from detailed laboratory tests to address practical problems.
The figure shows the stress transformation system in a directional wellbore, where the azimuth angle is taken as the rotation angle around the z’-axis and the inclination angle is taken as the rotation angle around the y’-axis.