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Borehole Drilling – Drilling and Casing

After siting approval (Once your ZINWA Authority to Drill Permit has been granted), the drilling team mobilizes equipment and materials to the site, setting up the drilling rig at the identified point. The drilling rig creates a borehole using rotary or percussion methods, reaching the desired depth as recommended in the siting report. Once the target depth is reached, casings (e.g., Class 10 casing) are installed to stabilize the borehole and prevent contamination.

Skylake Borehole Drilling has a fleet of Drilling Rigs that are suitable for Down the Hole (DTH) air percussion drilling, which is by far the best and most effective method. DTH drilling can be performed in most types of rock, hard or soft. It’s a commonly used method in the mining industry (blast hole drilling), water well drilling, construction, and in the oil and gas industry.

DTH Drilling

In DTH drilling, the drill string rotates while the drilling hammer continuously strikes down into the rock. Inside the hammer, a piston powered by compressed air gives the drill bit its striking power. This combined with the rotational movement means the rock is crushed in a very efficient manner. The drilling team will record rate of penetration of the drill bit, the size and type of casing and well screen used in the as-built construction of the borehole. The final depth of the borehole reported as a depth in metres below ground level will not be less than the agreed depth.

The depth(s) reported at which water is reported to be encountered during drilling (water strikes) below ground level, water strike yields, will be recorded in litres/sec, for each of the water strike encountered. The individual blow-out yield for successive water strikes is determined by subtracting the latest measured yield from the previous value. The final blow-out yield of the borehole will be made after the borehole has reached its final depth. In order to obtain as accurate a measurement as possible, the method of determination will be appropriate to the magnitude of the yield.

The other information that will be recorded include;

  • The date of start/completion of the borehole.
  • A detailed description (provided by the professional person) of the geological formations encountered in the borehole.
  • Type of sanitary seal and the depth in metres of installation.
  • Type and size (mm) of gravel pack, depth(s) of installation (m) and volume (m3).
  • Backfill material and depth of installation in metres.

Samples

Geological Formation Samples will be collected, one sample per five metres drilled and where the lithology changes/contact zones. The samples will be logged on site in a consistent and orderly fashion away from drilling operations, i.e., out of the way of general site activity and normal pedestrian and vehicular traffic on or across the area of drilling operations.

Casings

uPVC casings, Class 9/ 10, will be installed and lowered to the bottom of the borehole with machine slotted casing in areas of water breaks(i.e., lining the whole borehole depth) and the final internal diameter of the plastic casing will be 140mm.

Temporary casing/ Construction casing of diameter 180mm uPVC PN10 may be installed in the borehole when drilling under conditions where there is possibility of collapse of loose, broken or weathered formations. Intermediate casing will be installed when necessary to support any broken or fractured zones within the hole at any depth and when it is intended to reduce the hole size.

The minimum wall thickness will be for production casings. Production casing will be installed, under its own weight, from surface through unstable, unconsolidated, weathered or fractured rock to the bottom of the borehole, making the borehole a double cased borehole.

Borehole Development

The borehole will be developed to maximize its yield after completion of the drilling. Development entails the removal of fine and clogging material and might include activities such as flushing, hydro-jetting or surging. This will be done until the free flow of clear water is devoid of any suspended material, and if drilling muds are used, to be certain that any residual material is exhausted. The minimum development time will be 1 hr.

On completion of the drilling and construction of the borehole, we will fit a temporary end cap to the top of the casing. This cap will be removed during pump installation. All the information on borehole drilling will be recorded for sharing with the client.

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