Abstract:

Borehole breakouts, a series of complex processes that simultaneously affect the borehole wall, are characterized by initiation, propagation, and stabilization sequences. This paper presents a hydro-mechanical model incorporating plasticity strain softening by a Drucker-Prager-plastic yielding criteria and fluid seepage by Darcy's law in simulating breakout behavior. Damage-induced permeability change is also considered and modeled with a predefined relationship between the magnitude of plastic strain and permeability. A sensitivity study was performed using the proposed model to quantify the effects of in-situ stresses, mud pressure, and seepage time on breakout initiation and/or propagation. Results show that stress concentration induced initial breakout shape with low mud pressure is larger than that with high mud pressure. Fluid flow in the formation significantly affects breakout propagation and makes it a time-dependent process. For low mud pressures, the initial breakout shape without seepage is similar to the final breakout shape after seepage. For high mud pressure, the initial breakout shape is smaller than final breakout shape. The final breakout shape is also influenced by anisotropic in situ stresses and formation permeability. This proposed model provide a useful tool for evaluating the risk of wellbore breakout.

Introduction

A “borehole breakout” is a phenomenon in which portions of failing rock spalls from the wellbore wall due to high compressive stress around wellbore wall during or after drilling. Excessive breakout can lead to problematic wellbore stability and integrity issues, such as wellbore washout, wellbore collapse, and completion difficulties. Moreover, spalled cuttings resulting from borehole breakout are difficult to circulate out while drilling, which negatively affect drilling efficiency. On the other hand, field breakout events can be analyzed to obtain useful in-situ stress information. Breakouts are routinely used to determine the direction of horizontal stresses, and some authors have also suggested using it to constraint the magnitude of the maximum horizontal stress (Zheng, Kemeny, and Cook, 1989; Zoback et al., 1989; Dart, 1990; Ito, Zoback, and Peska, 2001).

For a vertical well, Bell and Gough (1979) first observed that breakouts normally occur as a cross-sectional elongation in regions of high compressive stress (usually tangential stress around wellbore) and that they are parallel to the direction of the minimum horizontal stress (J. S. Bell, 1979). For a deviated well, stress concentrations around wellbore are influenced by well azimuth, well inclination, in situ stresses, rock properties, and drilling mud pressure (Li and Gray, 2015). The breakout of a deviated well also occurs at the region of maximum compressive stress but it is no longer parallel to the direction of minimum horizontal stress (Zhou, 1994). Generally, the initial shape of breakouts induced by stress concentration for hard and brittle rocks can be estimated by a linearly elastic model with idealistic assumptions that wellbore is circular and intact and rock properties are homogeneous for both vertical and deviated wellbores.

This content is only available via PDF.
You can access this article if you purchase or spend a download.