Abstract
One of the wellbore instability problems in vertical wells are breakouts in Zubair oilfield. Breakouts, if exceeds its critical limits will produce problems such as loss circulation which will add to the non-productive time (NPT) thus increasing loss in costs and in total revenues. In this paper, three of the available rock failure criteria (Mohr-Coulomb, Mogi-Coulomb and Modified-Lade) are used to study and predict the occurrence of the breakouts. It is found that there is an increase over the allowable breakout limit in breakout width in Tanuma shaly formation and it was predicted using Mohr-Coulomb criterion. An increase in the pore pressure was predicted in Tanuma shaly formation, thus; a new mud weight and casing programs are proposed to overcome such problems in the drilling operations in field developments plans.
Highlights
Breakouts are one of the common wellbore stability problems in vertical wells
Breakout occurs in three stages, namely; breakout initiation, breakout propagation and breakout stabilization (Olcott, et al, 2013); (Moore, et al, 2012); (Cheatham, 1993); (Zheng, et al, 1989)
Flowing of drilling fluid adjacent to the wellbore wall region is the cause of breakout propagation and stabilization (Cheatham, 1993)
Summary
Breakouts are one of the common wellbore stability problems in vertical wells. Breakout is spalling of the rocks from the circumference of the wellbore (Gough and Bell, 2017); (Zoback, et al, 2003). Initiation of breakouts occurs due to concentration of stress around part of the wellbore circumference. Flowing of drilling fluid adjacent to the wellbore wall region is the cause of breakout propagation and stabilization (Cheatham, 1993). Breakout starts to develop in the direction of the maximum stress concentration which is parallel and the same direction and orientation as the minimum horizontal stress (Gough and Bell, 2017); (Zoback, et al, 2003). Breakouts usually analyzed depending on the linear elasticity assumption (Gough and Bell, 2017); (Zoback, et al, 2003); (Zhou, 1994). This study comes to the conclusion that Mohr-Coulomb criteria gave good results in vertical wells
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