When drilling through a formation sufficient hydrostatic mud pressure must be
1. Prevent the borehole collapsing and
2. Prevent the influx of formation fluids.
To meet these 2 requirements the mud pressure is kept slightly higher than formation pressure. This is known as overbalance. If, however, the overbalance is too great this may lead to:
1. Reduced penetration rates (due to chip hold down effect)
2. Breakdown of formation (exceeding the fracture gradient) and subsequent lost circulation (flow of mud into formation)
3. Excessive differential pressure causing stuck pipe.
The formation pressure will also influence the design of casing strings. If there is a zone of high pressure above a low pressure zone the same mud weight cannot be used to drill through both formations otherwise the lower zone may be fractured.
The upper zone must be “cased off”, allowing the mud weight to be reduced for drilling the lower zone. A common problem is where the surface casing is set too high, so that when an overpressured zone is encountered and an influx is experienced, the influx cannot be circulated out with heavier mud without breaking down the upper zone. Each casing string should be set to the maximum depth allowed by the fracture gradient of the exposed formations. If this is not done an extra string of protective casing may be required. This will not only prove expensive, but will also reduce the wellbore diameter. This may have implications when the well is to be completed since the production tubing size may have to be restricted.
Having considered some of these problems it should be clear that any abnormally pressured zone must be identified and the drilling programme designed to accommodate it.
Institute of Petroleum Engineering, Heriot-Watt University