OVERVIEW
Discovery Drilling Botswana encountered severe issues with stuck pipe at two different depths whilst using a PHPA mud system. Clear Solutions recommended the use of a Pure-Bore® mud system which prevented further instances of stuck pipe as well as improving the quality of the core samples recovered and dramatically increasing the drilling rate by three times what had previously been seen.
Challenge
Two wireline coring rigs were utilised for the drilling of mineral exploration boreholes at Boteti Diamond Mine in Botswana. Discovery Drilling Botswana were drilling the boreholes at angles of approximately 55 degrees to a target depth of between 700 to 850 m. They encountered problems with stuck pipe at +/- 160 m and 250 m while drilling with HQ rods, oversize bits and stabilisers, and a conventional PHPA mud system. After examining the cores generated during this phase of the project, it became apparent that the depths at which they experienced sticking problems occurred when the formation changed from a compact grey mudstone to a more fragile red sandstone. Lack of fluid loss control in the PHPA mud system combined with poor rheological properties was washing out the sandstone below the mudstone creating a “ledge” upon which the stabilisers and bit were hanging up because of the high hole angles.
Further analysis of the drilling data also indicated that the lack of good fluid loss control across the high permeability zones meant they were also getting “differentially” stuck. This was further compounded by poor hole cleaning and cuttings settlement through the PHPA mud system in the high hole angle sections.
Solution
Clear Solutions suggested a Pure-Bore® based mud system to help improve drilling rates, hole cleaning, cuttings suspension, core recovery and help prevent further stuck pipe issues.
On use of the Pure-Bore® mud system, drilling progressed rapidly with drilling rates increasing dramatically by three times that previously seen. As a result of the excellent rheological and fluid loss control properties created by the Pure-Bore® polymer, core quality at the problematic depths also improved significantly with no discernible erosion or crumbling.
Further to this, the high ROP’s with low torque and drag readings also indicated that the “ledges” were no longer being formed down hole, resulting in no further instances of stuck pipe.
Conclusion
By moving to a Pure-Bore® based mud system the operators were able to complete their mineral exploration boreholes in a third of the time without any further instances of stuck pipe and with significant improvements to the quality of the core samples obtained.
