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Challenges in drilling salt formations and rubble zones
Issues associated with drilling salt formations
The five major problems that typically are associated with drilling of salt formations are:
- Bit-balling and packoff because of reactive shales within the salt
- Wellbore erosion when drilling through the salt formation and/or through shales above or below the salt formation
- Excessive torque and packoffs caused by salt creep
- Well-control issues
- Excessive mud losses
The rubble zone that might lie beneath or adjacent to the salt section usually consists of a series of highly reactive shale stringers that are embedded in unconsolidated sand. The zone could be overpressured at the entry point because of a gas pocket under the salt, then underpressured for the remainder of the section.
Effect on drilling fluids
Catastrophic mud loss below the salt is the most challenging of these problems and prevents most operators from drilling rubble zones with oil-based fluids (OBFs) and synthetic-based fluids (SBFs). The decision about whether to use an SBF or a salt-saturated water-based fluid (WBF) usually is based on the known risk of lost returns. The SBF can provide increased drilling efficiency and a faster rate of penetration (ROP), but a salt-saturated WBF provides adequate control over hole enlargement and might be preferable where the potential for large losses exists. Seawater or undersaturated pills might facilitate ROPs without creating excessive washout, but extended use of undersaturated WBF might cause excessive hole enlargement where the longest exposure to the drilling fluid occurs.
Using hydraulics-modeling software and pressure-while-drilling (PWD) data, a hydraulics baseline can be established while drilling through the salt formation and before reaching the rubble zone. If it is necessary to lay down the PWD tool before drilling through the rubble, this baseline can help the operator maintain a minimal equivalent circulating density (ECD) in the rubble zone.
- Whitfill, D.L. and Hemphill, T. 2003. All Lost-Circulation Materials and Systems Are Not Created Equal. Presented at the SPE Annual Technical Conference and Exhibition, Denver, 5-8 October. SPE-84319-MS. http://dx.doi.org/10.2118/84319-MS.
Noteworthy papers in OnePetro
Infante, E.F., U. of Minnesota; Chenevert, M.E., U. of Texas: Stability of Boreholes Drilled Through Salt Formations Displaying Plastic Behavior, 15513-PA, http://dx.doi.org/10.2118/15513-PA
Crispin Chatar, Sushil Mohan, and Mark Imler, SPE, Schlumberger: Overcoming a Difficult Salt Drilling Environment in the Gulf of Mexico: A Case Study, 128192-MS, http://dx.doi.org/10.2118/128192-MS