The Situation

For its Sugar Camp M-Class Mine project Frontier Kemper Constructors (FKC) planned to take the unprecedented approach of drilling a 20 ft (6.1 m) diameter blind shaft to 980 ft (298.7 m) in a single pass. The shaft would be drilled with the FKC DH350, a 350-metric ton A-frame blind shaft rig designed to drill with reverse circulation and no pilot hole.

On a similar shaft drilled earlier, FKC employed a PHPA polymer as the fluid system and encountered severe bit balling and weight problems. To meet its objectives, FKC needed to avoid the bit balling, unnecessary trips and the additional costs associated with hauling off high-density mud above 8.9 lb/gal. Compounding the fluid difficulties on this project was the location space constraints, meaning the smaller active mud pit that would only allow 65% of the required volume to be used. With conventional fluids, the smaller earthen pit would make it difficult to control mud weight and solids content. Consequently, the shortened retention time in this challenging project required a fluid system that would allow for cuttings to settle at a higher rate. In addition, FKC needed a mud that would inhibit the highly reactive bentonetic marine clays and shales to be encountered, thus eliminating bit balling.

The Solution

M-I SWACO HDD recommended FKC use its gypsum/lime-based fluid system, which was engineered specifically for the shaft drilling industry and used extensively throughout the U.S. with incredible success. HDD assured FKC the highly inhibitive mud system would eliminate bit balling and, despite the shorter retention times, encounter no problems controlling weight and solids content. Despite having no experience with gyp/lime muds and somewhat skeptical, FKC agreed to allow HDD to apply the specially formulated fluid on this extremely demanding and high-profile project.

The Results

Frontier-Kemper not only met its objectives in the high-status project, but went to establish a world blind shaft drilling record. At 980 ft (298.7 m), the contractor broke the previous record for a 20 ft (6.1 m) blind shaft by some 630 ft (192.0 m). Despite non-productive time (NPT) associated with the failure of a crossover sub and a dislodged cutter, FKC still met its targeted drilling time of 94 days, without having to make a single trip for a balled up bit, thereby saving time and costs. In addition, as the shaft was completed in one pass with a final mud weight of 8.5 lb/gal (1.0 s.g.), FKC also eliminated the costs of having to haul off and dispose of any fluid.

The contractor was so impressed that it chose HDD to provide fluids and engineering on upcoming 14 ft (4.2 m) diameter shaft in Kentucky. HDD also is bidding on numerous other shaft projects throughout the U.S. and Australia.

The Details

The gyp/lime was made up, but owing to space constraints of the earthen mud pit only 65% of the necessary volume or roughly 94,000 bbl would be available for use. Industry standard called for 2 ½ times the finished hole volume of fluid for weighting up, which in this case would have been 144,000 bbl.

Drilling proceeded to 172 ft (52.4 m) when the crossover sub between the drill pipe and the bottom hole assembly failed leaving the 522,000 lb (236,775 kg) BHA at the bottom of the hole. FKC was concerned the hole would deteriorate and stick the BHA before fishing tools large enough to retrieve the drilling assembly. However, the highly inhibitive characteristics of the gyp/lime system prevented hole sloughing, and squeezing thus retrieval was completed trouble free, allowing the sub to be repaired and returned to service.

Later, the drilled hole had to remain static for 28 days while awaiting bit repairs. FKC expected severe sloughing shale and tight hole conditions when tripping back into the shaft; however, little or zero tight hole was encountered.

Fluid parameters were maintained as programmed throughout the shaft drilling project with the calcium level, the filtrate, as well as Viscosity maintained daily within the set parameters of the fluid program. Mud density held at 8.4 lb/gal without the use of solids control equipment and fluid circulation was maintained at 5,800-6,400 gpm with 110-155 psi air pressure to insure adequate hole cleaning.

Summary

The M-I SWACO HDD gyp/lime fluid system exceeded all of the contractor’s expectations for this high-profile blind shaft drilling project.

• Established world drilling record for 20 ft (6.1 m) diameter, 980 ft (298.7 m) blind shaft
• No bit balling encountered
• Eliminated fluid-related trips
• No tight holes observed even with prolonged static periods
• Considerably reduced time and costs