When a well is drilled in a formation, stressed solid material is removed and replaced with a fluid under pressure. Whilst rock can support shear and normal stresses, the mud is unable to support shear stress, which leads to a redistribution of the in situ stresses and an alteration in the stress state of the formation around the well. Elastic theory is used to evaluate the well pressure at which shear failure (collapse/breakout) will initiate at the borehole wall. Initial shear failure within a layer occurs when a low mud pressure creates a large enough stress differential between the maximum principal stress and the minimum principal stress to exceed the failure criterion for that layer.
A simplified approach to wellbore stability involves using poro-elastic analysis to compute stress states around the wellbore. These stresses are then compared with a failure criterion. In many instances (but not all) results are found to be somewhat pessimistic in predicting the onset of shear failure (collapse/breakout) around a well, particularly when considering stability and minimum mud weights during overbalanced drilling. Under such situations, the presence of a mud cake can maintain hole stability, so that although the borehole wall may have undergone limited yield there may not be any significant or observable instability. Similarly, the non-linear nature of rocks means that the stresses at the wellbore, computed using linear elasticity, may slightly exceed those which occur in reality.
The concept of the “safe” mud window is illustrated below:
The lower limit of the mud weight window is defined by the pore pressure gradient. The mud weight must be higher than the pore pressure to avoid a “kick”. When the mud weight is too low to balance the effective stress acting on the formation, shear failure will occur, which manifests as wellbore breakout. If the mud weight is raised too high, mud “losses” will occur. In intact formations the upper limit of the mud weight window is defined by the fracture gradient.
The fracture gradient is calculated by evaluating the stress concentration at the wellbore wall and evaluating when it will overcome the formation tensile strength. It will generally be significantly higher than the minimum in-situ stress. Should the mud weight be raised above the fracture gradient, fractures will be initiated and the formation will take losses. The safe mud weight window for a particular formation will depend on the rock mechanical properties, the stress, pore pressure and the well trajectory.
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