Potential cavings/wellbore instability
Situation
Traditionally, selection of Oil Based Mud (OBM) can be a challenge (source: SPE 184661 and SPE 150714).
Case: During drilling, large amount of cavings resulted in severe overpull and almost stuck pipe. Due to high concentration of KCL exposed to the formation over long time.
Result: Poor wellbore state was plugged back. Entire new 17 ½” section was drilled with OBM. Actual time used on the well was 132 days compared to planned 98 days (the total length 5200 m Measured Depth), which is an extra 34 days compared to planned.
Learning: Some overburden claystone, siltstone and interbedded sandstones can have cavings/unstable wellbore if wrong KCL when wellbore is exposed to the drilling fluid for several days.
Corrective action: Rock mechanical analysis could identify optimal KCL concentration in the drilling fluid changes with depth and sensitivity on the borehole inclination.
Solution
Cost savings: MudSim could have predicted that the formation had a potential for unstable wellbore/cavings. Potential cost reduction could have been achieved by eliminating 34 extra days of drilling. Anticipating a daily rig-rate of MNOK 4,8 /day, the result is a total saving of NOK 163 mill.
Environmental impact: The drilling of the section was decided to continue with OBM due to the unexpected problems with instability. Pre-drilling analysis identified WBM, with the right KCl concentration, could have been used. A MudSim simulation can identify WBM as an alternative instead of OBM in the 17 ½” section. No dangerous goods, less use of support vessel, less fee for final disposal of environmental hazardous waste.
