Drilling mud weights are typically selected based on the requirement that hole quality is sufficient to successfully reach target depth. Unfortunately, acceptable hole enlargements while drilling may exceed safe limits for completion performance, e.g., to achieve effective zonal isolation or to avoid damage to expandable screens. Breakout analyses to select optimal mud weight to maintain wellbore stability typically define hole quality in terms of the width of failed zones and do not provide quantitative information about breakout depth. Although laboratory experiments have been run and theoretical models have been developed to predict enlargement depth as a function of breakout width, relationships derived from these studies have never been validated using field data. We present here a subset of analyses of detailed measurements of breakout widths and depths from image logs and caliper data within the reservoir sections of more than 20 wells. In sands, breakout depth increases with breakout width, in agreement with models and laboratory results. Although in some cases shales have more severe enlargements, breakouts with similar widths are in most cases deeper in sands than in shales. One explanation for this observation is that in these wells the sands have a lower residual strength, whereas in shales which are less brittle the nominally failed material provides sufficient support that further breakout growth is inhibited. There is some evidence that in extremely weak sands this relationship may be reversed (i.e., shales are more brittle). While models of stable breakout shape in materials with residual strength support this hypothesis, other factors may be important, including initial strength and internal friction.
Wellbore breakouts are zones of enlargement due to failure of the rock at the wellbore wall at orientations where the greatest stress concentrations around the well exceed the strength of the rock.