Unexpected Challenges: Sand, Soil, Clay And Rock On 4600-Foot Bore

June 2011 Vol. 66 No. 6

The DrillTech crew completed the first bore in relatively smooth fashion after eight days, realizing production rates that averaged 250 to 300 feet per hour after making some initial minor adjustments. After reaming the bore in stages using a variety of incremental size reamers — ranging from 18 to 36 inches in diameter — the hole was swabbed and 969 feet of the 40-foot sections of 24-inch pipe, already welded together and strung out on stretched rollers, was pulled back through the bore. The equipment was then repositioned for tackling the second, longer 4,628-foot bore.

Quick change
Soil samples through the environmentally sensitive area had been taken from the projected entry and exit points and lacked any detailed information about what would be encountered in between. The bore began with some clay, followed by areas of sand; nothing of critical significance or that was particularly challenging at the onset. Suddenly, however, things began to change as the operator sensed a dramatic shift in conditions.

“Having previously drilled in the area before on different jobs, we were suspicious there could be several changes in the terrain over the course of the 4,600-foot shot,” Davidson says. “As expected, we did encounter a lot of variation in formations that the soil samples didn’t show, many of which surprised us.”

"These conditions were something that we didn't anticipate, so we had to go to a rock reamer to cut through the formation," Davidson explains. “We didn't realize from the soil samples there would be this much rock and didn't have the appropriate tooling to get through it on site, so we had to order a split bit hole opener. Because we discovered this when drilling the pilot bore we got that rolling and the bit arrived in time and never caused production to be halted.”

With the split bit hole opener installed, drilling commenced as the drill rig was outfitted with the tooling required to tackle the rock. The pesky rock and unpredictable abrupt changes to alternating ground conditions consisting of sand and varying densities of clay made the second bore tough to navigate. It also reduced production rates and extended the duration of the second bore to more than five weeks. But all things considered, Davidson was satisfied with the overall results.

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