When we are drilling through a thick shale zone we can pick up certain indications of over pressure from the drill cuttings, rate of penetration, mud logs (gas/oil in the mud), friction drag on the drill pipe (both vertical movement and rotational drag), measurements while drilling (MWD) and wire line logs.

In the case of Wahoo-1 the shale may be over pressured due to being caught up in the faulting and folding zone that is occurring as the Australian plate is pushing against the Pacific plate. These various fault blocks may be sealed between faults and there is no place for the water in the shale to go when it gets compressed by the tectonic forces and overburden. This leaves us a shale containing excessive (above normal) amounts of water to drill through. I believe it is common to see increasing amounts of gas in the shale as we drill deeper if the shale is overlying a gas zone. At Wahoo I think they were seeing increasing amounts of gas in the shale with increasing depth. This gas is seen in the mud and is just the gas that is contained in the shale that is being drilled. They eventually drilled into some permeable streaks that were actually capable of flowing gas into the hole. The gas in the shale will not give you a gas kick but the permeable zones will give you a gas kick if the mud weight is less than the formation pressure. From the mud weight required to contain the gas in the formation they have determined an approximation of the formation pressure in the permeable streaks above the limestone. So that is the point where they decided to suspend the well temporarily for safety reasons. This suspension is based on the equipment on location, the casing program that has been followed so far and the unknown items that we might find as we drill deeper i.e. depth and pressure of the limestone reservoir if present.

In the case of Raptor-1 and Bobcat-1 we have not been given any information that would indicate excessive pressure caused by tectonic forces. These shale zones seem to be much thicker than we see at Wahoo-1 so most of the over pressuring of the shale may be caused by the overburden and being sealed between faults. We also do not have any indication of any permeable zones above the carbonate at these locations.

Here is a Schlumberger article that tells you more than you ever wanted to know about drilling through over pressured shale. http://tinyurl.com/k5vrsyq