The mud weight at which lost circulation occurs depends critically on hole deviation. Similarly, collapse of a high angle well may occur with a mud weight which would provide adequate support in a vertical hole. For this reason, a Mechanical Stability Log (MSL) has been developed which, for a given well plan, calculates the range of mud weights which could be used for drilling that well without causing lost circulation or hole instability problems (eg sloughing or caving). It must be noted that chemical effects are not considered: that is, lost circulation or hole instability problems are assumed to have purely mechanical origins.
The MSL uses wireline logs obtained in wells drilled in the vicinity of the planned well as input data. Ideally, density, gamma ray, compressional sonic and shear sonic logs are used. If all of these are not available estimates of "safe" mud weight can still be obtained although with less reliability.
When a borehole is drilled, the process may be thought of as one of replacing the rock which was originally in the hole with drilling mud. This causes a disturbance to the stresses local to the hole because a column of rock which supported three, probably different, principal stresses is replaced by fluid in which the three principal stresses are equal and, typically, lower than any of the stresses in the original rock column. This stress change causes deformation of the rock surrounding the hole and may lead to failure.
In 1968, Fairhurst calculated the stress change that would occur local to a borehole drilled in a formation which deformed according to a linear elastic constitutive equation. This solution is given, for reference, in the appendix. It has been used by Bradley, amongst others, in considering wellbore stability issues.
Figure 1 is calculated using Fairhurst's solution. The solid line defines the mud weight which if exceeded with result in tensile failure of the borehole. The two dotted lines define mud weights outside which (that is, above the upper line and below the lower line) Coulomb shear failure of the hole wall occurs. it should be noted that the figure is not in any sense a type curve: the graph shown is for a specific depth, with specific assumed in-situ stresses, and specific rock strength and failure properties.
It will be assumed in what follows that if tensile failure occurs lost circulation may result and that, therefore, to be "safe", the mud weight must lie below the tensile failure curve. Furthermore, it will be assumed that shear failure may result in sloughing and hole collapse: to be "safe". The mud weight must he between the two shear failure curves. So in figure 1, "safe" mud weights for the particular lithology at the particular depth considered, are those in The shaded region. It is of particular interest to note from this figure that no "safe" mud weight exists for deviations greater than about 65deg.