Ann: Hydrocarbon gas recovered & evaluation program forward plan, page-403

When you increase the mudweight you're increasing the mudweight of the entire mud column and the hydrostatic pressure being applied by the column on the hole by the mud column at all points in the open hole section.

If a section in the upper part of the open hole has a lower fracture gradient than lower section or is more permeable.... then increasing mud weight to handle increasing formation pressure in lower part of hole will lead to fracking or severe invasion of the upper section.

The solution is to to either case off and isolate the upper section of the hole or in our case not drill past the depth where formation pressure requires us to bump up our mud weight.