Thirteen at one go! A High Precision Survey Recorded By GyroMaster
The remit was to sink shafts as part of the project to develop a mineral deposit.
The shaft construction team faced substantial engineering challenges because of the depth of the target area and the presence of running sand, clay, and thick beds of deep-lying aquifers down to about 815 m.
To overcome these issues, the mining engineers decided to create a large frozen cylinder in the overlying rock around the final circumference of the shaft to ensure the temporary stability and water-tightness of the exposed shaft walls prior to the installation of the lining system.
For this purpose, a series of directional, vertical freeze holes were drilled in a circular pattern to a depth of 820 m at each of the three shaft construction sites. A cooling agent was then pumped in a closed circuit through the freeze pipes.
The coolant temperature of -38 °C draws so much heat from the surrounding strata that any groundwater in this area eventually freezes solid. The resulting freeze wall acts as a hydrological barrier against the adjoining groundwater.
This serves to improve the mechanical properties of the rock and, consequently, enhance its structural stability during the shaft sinking operation.
However, during sinking, a geological anomaly was encountered in one of the three shafts just below the 815 m level.
The rock beds being worked through proved to be much less uniform than the selective geological investigations had indicated.
With groundwater also found to be present down to the 825 m level it was decided to suspend the shaft sinking work.
If the sinking operation was to continue safely and efficiently, it was clear that the anomaly in question would have to undergo separate stabilisation and sealing.
Meaning the critical zone in question, which was located between 815 m and 840 m depth, should be frozen using thirteen additional freeze holes.
Core samples were also to be recovered from the lower segment of the new drill holes to obtain further information on the anomaly.
Precision in Drilling
As a specialist company with experienced technical personnel and drilling engineers, International Drilling & Mining GmbH (IDMG) was therefore commissioned to develop a planning solution and then to carry out the precise directional drilling operation needed for the on-site freeze holes.
One of the main challenges facing the team responsible for drilling the thirteen additional freeze holes lay in the fact that as the construction and shaft sinking work had already commenced, and indeed was fairly well advanced, there was only a limited amount of space left at the surface for new drilling points due to the presence of the sinking tower, shaft house and other structures.
Any drilling points still available were offset horizontally by as much as 15 m from the target zones at 848 m depth. This effectively ruled out any all-vertical drilling of the thirteen extra freeze holes needed.
The new freeze holes therefore had to be drilled from ‘further out’ using an S-shaped path.
Furthermore, the new drilling points were in places only about 2 m apart and were directly adjacent to existing structures, which meant that the drilling team had to drill these holes with high precision within a very confined space.
The existing freeze wall created by the initial drilling operation also meant that a large area of the overburden was already frozen to below -20 °C.
The directional drilling work and drill-fluid management process, therefore, the trajectory of the new holes had to be adjusted to this situation.
The drilling plan specified that the first section would be drilled vertically to a depth of 450 m, and then the driller had to kick off from vertical to create 15 m of departure before reaching the 700 m depth.
The hole below 700 m would then return to the vertical and continue to the target depth of 848 m.
The maximum allowable deviation from the planned drilling path was 60 cm to prevent any collisions and intersections between the existing freeze holes and the ten new holes.
The freeze was penetrated at a depth of 500 m. You can see the shaft in a longitudinal section and the existing freeze wall, and one of the thirteen new deviated freeze holes in the figure 3.
The directional drilling section was completed using a downhole motor with a directional drilling diameter of 216 mm (8½ inches) operating in conjunction with a GyroMaster™ unit, which is the most accurate and reliable survey tool from SPT (Stockholm Precision Tools).
Despite the difficult downhole conditions and continuously changing borehole temperature and pressure, the GyroMaster™ provided the required precision survey for this project.
All thirteen S-shaped directional freeze holes were completed on time and to a high level of precision within the required radial deviation tolerance of 60 cm. All the casing pipes were then installed as planned.
Drilling directional freeze holes of this kind not only takes years of practical drilling experience and the ability to master difficult geological and hydrogeological conditions but also requires high-precision directional survey equipment, such as the SPT GyroMaster™.
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