Wellbore stability is critical for drilling. A stable open-hole requires the surrounding sediment and rock to bear the stresses that amplify around the wellbore cavity. The surrounding rock must hold stresses until casing is set or for undetermined time if left uncased.
Wellbore stability depends on two set of variables (Figure 6.1): one set which is out of our control and another set of variables that we can control.
The pressure in the wellbore is one of the main variables to maintain wellbore stability. Mud (mass) density and vertical depth (TVD) determine the mud hydrostatic pressure (in the absence of additional pressure controls at the surface - such as in managed pressure drilling):
The pressure gradient within the wellbore is proportional to mud density (Fig. 6.2). This quantity is usually measured and reported in p.p.g. (pounds-force per gallon). For example the pressure gradient for fresh water is 9,800 N/m (= 9.8 MPa/km = 0.44 psi/ft), about 8.3 ppg. The lithostatic gradient of 1 psi/ft is equivalent to 1 (psi/ft) (8.3 ppg/0.44 (psi/ft)) = 18.9 ppg. The “equivalent circulation density” is also reported in ppg and take into account pressure drops in the annulus.
Over-balanced drilling implies . Over-balanced drilling favors the formation of a “mud-cake” or “filter-cake” on the wall of the wellbore which permits adding stress support on the wellbore wall approximately equal to (Fig. 6.3). The resulting effect is similar to an impermeable and elastic membrane applying a stress on the wellbore wall (similar to the membranes used in triaxial tests). Under-balanced drilling may be preferred in some specific instances.