Drilling in The Highlands of Papua New Guinea is very challenging. The surface topography is extremely mountainous, with deep gorges and razoiback ridges. In addition, the Darai Limestone outcrops at surface and is karstified to a depth often in excess of 1,500 m, with the presence of underground rivers and caverns being common. The subsurface geology is even more complex with steeply dipping formations (at times near vertical), faults, and inverted sequences where the geological strata have been overturned by folding.
Well pads are constructed by flattening off the top of a mountain ridge or peak. Drilling from these elevated locations provides additional hydrostatic head to reduce the mud weight required in high-pressure formations encountered deeper in the well. Well pad construction is extremely costly but is more economic from peaks than from valley bottoms. Valley bottom locations often do not provide sufficient hydrostatic pressure head to maintain well control in deep, high-pressure formations without having to use excessively high mud weights.
The Darai Limestone is drilled with an air-foam mixture, as 100% lost circulation is typical. The karstification of this formation makes it unstable and prone to rubble collapse and mobile sediment packoffs.