The mud window

As seen in previous sections, low mud pressure (small mud wall support $\sigma _3$ for hoop stress $\sigma _1$) encourages shear failure while too much pressure encourages tensile fractures (well pressure adds tensile hoop stresses). Hence there is range of mud pressure for which the wellbore will remain stable during drilling. This is called the mud window and has a lower bound (LB) and an upper bound (UB) which depend on wellbore mechanical stability as well as in other various technical requirements (Fig. 6.17).

Figure 6.17: Mud window based on mechanical wellbore stability, pore pressure and minimum principal stress. HF: hydraulic fracture.

Small breakouts $w_{BO} \leqslant 60^{\circ}$ may not compromise wellbore stability and permit setting a more flexible lower bound for the mud window. Large breakouts $w_{BO} \geqslant 120^{\circ}$ can lead to uncontrolled shear failure and breakout growth leading to stuck borehole assemblies and even wellbore collapse. Likewise, small drilling-induced tensile fractures with wellbore pressure $P_W$ lower than the minimum principal stress $S_3$ can be safe and extend the upper bound for the mud window. However, wellbore pressures above $S_3$ can result into uncontrolled mud-driven fracture propagation. Eq. 6.17 may suggest a safe breakdown pressure value above the minimum principal stress $P_b > S_3$. However, this calculation assumes tensile strength everywhere in the the borehole wall. Any rock flaw or fracture ( $T_S = 0 MPa$) may reduce drastically $P_b$.

There are other factors to take into account in the determination of the mud window in addition to mechanical wellbore stability. First, a mud pressure below the pore pressure will induce formation fluid flow into the wellbore. The fluid flow rate will depend on the permeability of the formation. Tight formations may be drilled underbalanced with negligible formation fluid production. High mud pressure respect to the pore pressure will promote mud losses (by leak-off) and damage reservoir permeability. Second, a mud pressure above the far field minimum principal stress $S_3$ may cause uncontrolled hydraulic fracture propagation and lost circulation events.

Maximizing the mud window -taking advantage of geomechanical understanding among other variables- is extremely important to minimize the number of casing setting points and minimize drilling times (Fig. 6.18). The mud pressure gradient in a wellbore $dP_W/dz$ is a constant and depends solely on the mud density. Therefore, drilling designs are based on the mud pressure respect to a datum (usually the rotary Kelly bushing - RKB) expressed on terms of “equivalent density”. In any open-hole section the value of pressure $P_W$ in a plot equivalent density v.s. depth is a straight vertical line (Fig. 6.1). The depth of casing setting results from a selection of mud densities that cover the range between of the mud window as a function of depth. Wider mud windows reduce the number of casing setting points.

Figure 6.18: Drilling mud density and wellbore design. The figure on the right is an improved design of the one on the left, based on careful geomechanical analysis of wellbore stability.