All drilling challenges relate to the fundamental objective of maintaining a workable wellbore throughout the well-construction process. >> Oil & Gas Engineering Books <<
A workable wellbore can be drilled, logged, cased, cemented, and completed with minimal nonproductive time. The design of the drilling-fluid system is central to achieving this objective. With a poorly designed system there are some challenges that will occur. Some of the major problems include:
- Loss of Circulation
- Stuck Pipe
- Shale Instability
- Hole Cleaning
- Barite Sag
- Salt Formations and Rubble Zones
Most operational problems are interrelated, making them more difficult to resolve. For example, loss of circulation into a depleted zone causes a drop in hydrostatic pressure in the wellbore. When the hydrostatic pressure falls too low to hold back formation fluids, the loss incident can be compounded by an influx of gas or water, known as a flow or (when more severe) a kick. In these circumstances, the operator should increase the fluid density to stop the kick, yet avoid exacerbating the lost-circulation problem. Furthermore, the pressure differential created at the loss zone can cause the drillstring to become embedded in the wall cake, a situation called differential sticking. The drillstring should be freed quickly by mechanical or chemical methods because the longer it remains stuck, the lower the likelihood of freeing it. Failure to free the pipe can require an expensive fishing job that cannot be undertaken until the well is under control.
Another example of interrelated problems occurs when the directional-drilling operation requires an interval of sliding, in which the drillstring is not rotated for a period of time but drilling continues by means of a downhole motor. Sliding allows better directional control, but the lack of pipe rotation can impair hole cleaning. Good hole cleaning is important in all wells, but it is critical in high-angle wells, in which cuttings might fall to the low side of the wellbore and form deep cuttings beds. Failure to remove the cuttings can lead to packing off of the drillstring—another version of stuck pipe. If the drillstring is severely packed off, attempts to circulate drilling fluid might lead to excessive pressure on the wellbore, which can cause the formation below the packoff to break down.
Incidents like these are not uncommon. The drilling fluid alone cannot correct all problems, but skillful management of the drilling-fluid system by a specialist can prevent conditions that lead to wellbore instability, thereby helping the operator to achieve a workable wellbore.