COMPRESSIVE AND TENSILE FAILURE OF INCLINED WELL BORES AND DETERMINATION OF IN-SITU STRESS AND ROCK STRENGTH

In this paper we investigate the occurrence of compressive and tensile failures of arbitrarily inclined well bores under a wide variety of s tress conditions. The principal assumptions in this analysis are that the rock is isotropic and that it deforms elastically to the point of failure. As has been shown by previous investigators, for a given stre ss state and well bore orientation, it is straightforward to predict t he orientation of the failures around the well bore as well as whether failure is likely to occur depending on such parameters as rock stren gth and borehole fluid pressure. However, as the stress state is almos t never known in situ, we demonstrate how observations of compressive and tensile wall failures in inclined holes can be used to constrain i n situ stress orientations and magnitudes if there are independent dat a on the magnitude of the least principal stress from either leak-off or microfrac tests and on the formation pore pressure. We further demo nstrate how once the stress state is determined, it is possible to ass ess both an upper bound on the effective in situ rock strength and the degree to which increasing the borehole fluid pressure (or mud weight ) can reduce the likelihood of borehole failure. Through application o f this methodology to an inclined well bore in an area of complex faul ting in the Gulf of Mexico, we illustrate how it is possible to utiliz e observations of borehole failures to determine the magnitude and ori entation of the stress tenser in areas such as offshore sedimentary ba sins where drilling inclined well bores is quite common.