What Does Thief Zone Mean?
A thief zone is a formation that has fractures or conduits with high permeability into which circulating fluids can be lost during drilling.
Loss of circulation happens when the mud flows uncontrollably into a formation. It is not possible to completely prevent lost circulation, as it is unavoidable to encounter fractured and high-permeability zones on the way to the target zone.
The loss of circulation to these thief zones can, however, be limited by taking appropriate precautions.
Trenchlesspedia Explains Thief Zone
Lost circulation into thief zones can be either partial or total. In partial lost circulation, mud continues to flow to the surface with some loss to the formation. Total lost circulation occurs when all the mud flows into the formation with nothing returning to the surface. If the thief zone is stable, blind drilling can continue during total lost circulation.
Lost circulation can occur for different reasons, such as:
- Encountering thief zones.
- Fractured, cavernous or high-permeability zones.
- Improper drilling conditions.
- Induced fractures caused by excessive downhole pressure.
- Setting intermediate casing too high.