Estimating Maximum Horizontal Stress Magnitude Based on Borehole Breakout Geometry-A Semi-analytical Poroelastic Model

Knowledge of the state of in situ stresses is critically important in analyzing wellbore and underground structure stability. While the vertical and minimum horizontal stresses may be derived relatively straightforward with the well-established methodologies, the magnitude of maximum horizontal stress cannot be measured directly and must be indirectly estimated. In this paper, a new method for estimating the maximum horizontal stress magnitude is proposed based on borehole breakout geometry, including the width and depth. A semi-analytical stationary poroelastic solution is firstly developed to evaluate the stress and pore pressure distributions around the breakouts. The strength of the breakout tip is evaluated by considering several mechanisms responsible for breakout stabilization. The maximum horizontal stress magnitude is then estimated from the stability condition of stress and rock strength at the breakout tip which attracts the highest stress concentration. The newly derived maximum horizontal stress estimation model is validated based on several sets of laboratory borehole breakout experiments conducted under three-dimensional stress conditions. Comparison between the model prediction and the experimental results showed that the estimated maximum horizontal stress has a good approximation to the maximum horizontal stress magnitudes applied and showed a significant improvement over the existing method based on breakout width only. A fully coupled semi-analytical stationary poroelastic solution is developed for evaluating stress and pore pressure distribution around breakouts.Breakout tip strength is assumed to be equal to the thick-walled hollow cylinder wall strength with its hole radius equal to the equivalent radius at the breakout tip.Maximum horizontal stress magnitude is evaluated based on the stability condition of stress and strength at the breakout tip.The maximum horizontal stress magnitude model is validated based on laboratory borehole breakout experiments.