Compressive and tensile failure of inclined well bores and determination of in situ stress and rock strength

TLDR

The analysis assumes isotropic, elastically deforming rock, and prior work shows that for a given stress state and borehole orientation, failure orientation and likelihood can be predicted based on rock strength and fluid pressure. The study investigates how observations of compressive and tensile failures in inclined well bores can constrain in situ stress orientations and magnitudes, and how, once the stress state is known, upper bounds on effective rock strength and the effect of increased borehole fluid pressure on failure likelihood can be assessed. The authors use observations of compressive and tensile wall failures in inclined holes, combined with independent measurements of the least principal stress and pore pressure, to constrain in situ stress orientations and magnitudes. Applying the method to an inclined well bore in the Gulf of Mexico demonstrates that borehole failure observations can determine stress tensor magnitude and orientation in offshore sedimentary basins with complex faulting.

Abstract

In this paper we investigate the occurrence of compressive and tensile failures of arbitrarily inclined well bores under a wide variety of stress 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 stress state and well bore orientation, it is straightforward to predict the orientation of the failures around the well bore as well as whether failure is likely to occur depending on such parameters as rock strength and borehole fluid pressure. However, as the stress state is almost never known in situ, we demonstrate how observations of compressive and tensile wall failures in inclined holes can be used to constrain in situ stress orientations and magnitudes if there are independent data on the magnitude of the least principal stress from either leak‐off or microfrac tests and on the formation pore pressure. We further demonstrate how once the stress state is determined, it is possible to assess 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 of this methodology to an inclined well bore in an area of complex faulting in the Gulf of Mexico, we illustrate how it is possible to utilize observations of borehole failures to determine the magnitude and orientation of the stress tensor in areas such as offshore sedimentary basins where drilling inclined well bores is quite common.