Breakout analysis for deviated wellbores

Consider a place subjected to strike-slip stress regime with $S_{Hmax}$ oriented at an azimuth of 070$^{\circ }$ with known values of principal stresses (Fig 6.23). The maximum stress anisotropy lies in a plane that contains $S_1 = S_{Hmax}$ and $S_3 = S_{hmin}$, a plane perpendicular to the axis of a vertical wellbore. Hence, maximum stress amplification at the wellbore wall will happen for a vertical wellbore. The minimum stress anisotropy lies in a plane that contains $S_2 = S_{v}$ and $S_3 = S_{hmin}$, perpendicular to a horizontal wellbore drilled in direction of $S_{Hmax}$. Given a mud pressure and a fixed friction angle, we can calculate the maximum required $UCS$ for a given wellbore orientation using a shear failure criterion $\sigma_1 = UCS + q \: \sigma_3$, and the principal stresses from equations in Fig. 6.22. The plots in Fig. 6.23 are examples of this calculation. The maximum required $UCS$ corresponds to the wellbore direction with maximum stress anisotropy (vertical wellbore - red region), and the minimum required $UCS$ corresponds to the wellbore direction with minimum stress anisotropy (horizontal wellbore with $\delta = 070^{\circ}$ - blue region). Following the same concepts discussed before, we would expect breakouts at 160$^{\circ }$ and 340$^{\circ }$ of azimuth on the sides of a vertical wellbore. A horizontal wellbore drilled in the direction of $S_{hmin}$ would tend to develop breakouts on the top and bottom of the wellbore.

Figure 6.23: Stereonet plots to verify the rock strength required to avoid breakouts and the wellbore breakout angle for a given rock strength and wellbore pressure.

PROBLEM 6.5: Consider a place with principal stresses $S_v = 70$ MPa, $S_{Hmax} = 67$ MPa (at 070$^{\circ }$), $S_{hmin} = 45$ MPa, $P_p = 32$ MPa, and $P_W = 32$ MPa. Calculate the required UCS using the Coulomb failure criterion (with $\mu_i = 0.8$) for all possible wellbore orientations. Plot results in a stereonet projection.

SOLUTION

PROBLEM 6.6: Consider a place with principal stresses $S_v = 70$ MPa, $S_{Hmax} = 105$ MPa (at 070$^{\circ }$), $S_{hmin} = 85$ MPa, $P_p = 32$ MPa, and $P_W = 32$ MPa. Calculate the required UCS using the Coulomb failure criterion (with $\mu_i = 0.8$) for all possible wellbore orientations. Plot results in a stereonet projection.

SOLUTION