Abstract
Drilling a hole into rock results in stress concentration and redistribution close to the hole. When induced stresses exceed the rock strength, wellbore breakouts will happen. Research on wellbore breakout is the fundamental of wellbore stability. A wellbore breakout is a sequence of stress concentrations, rock falling, and stress redistributions, which involve initiation, propagation, and stabilization sequences. Therefore, simulating the process of a breakout is very challenging. Thermoporoelastoplastic models for wellbore breakout analysis are rare due to the high complexity of the problem. In this paper, a fully coupled thermoporoelastoplastic finite element model is built to study the mechanism of wellbore breakouts. The process of wellbore breakouts, the influence of temperature and the comparison between thermoporoelastic and thermoporoelastoplastic models are studied in the paper. For the finite element modeling, the D-P criterion is used to determine whether rock starts to yield or not, and the maximum tensile strain criterion is used to determine whether breakouts have happened.
Highlights
When a hole is drilled into the crust, the rock mass removed from the hole will not support the surrounding rocks, which leads to the stress concentration and redistribution.If the induced stresses exceed the rock strength, parts of rock fall from the wellbore wall, which is called a wellbore breakout [1,2]
Wellbore breakouts result in problematic wellbore stability and affects drilling efficiency, so the research of wellbore breakouts is important in petroleum engineering [3,4]
The studies of wellbore breakouts show that the wellbore breakouts occur by a series of successive spalls that result from shear failure subparallel to the direction of the local minimum principal stress [3,5,6,7,8,9]
Summary
When a hole is drilled into the crust, the rock mass removed from the hole will not support the surrounding rocks, which leads to the stress concentration and redistribution. Numerous studies have been made to explain the mechanism of wellbore breakouts. Most of these models do not consider the influence of plastic damage of the rock. Akl and Whittle [23] analyzed horizontal wellbore stability in clay shale based on MIT-E3 soil model. Zhang and Yin [24] made a wellbore study using the tangent stiffness matrix method based on the Drucker-Prager criterion. The breakout initiation and its propagation are the result of stress concentration on the wellbore wall, further stress concentration at the tip of breakout, and the formation of a plastic zone around the tip [26]. A fully coupled thermoporoelastoplastic finite element model is built to study the mechanism of wellbore breakouts. The Drucker-Prager criterion is used to determine whether rock starts to yield or not, and the maximum tensile strain criterion is used to determine whether breakouts happened
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