Borehole drilling simply means boring a hole to a particular depth to hit the water bed. It is the bed that supplies the water to the structure and residents on surface. However, before the bed can be reached, a survey by name Geophysics needs to be carried out to determine its point. The first decision is to determine where to drill.
SITING OF THE BOREHOLE WELL
There are numerous ways in which we can go about deciding where to place the borehole on your property. GEDGhana is stuffed with hydro-geologists who make use of various geographical methods to probe beneath the surface. By employing these geophysical methods, we are able to infer the subsurface geology of an area. The geological features of hydrogeological importance including faults, fractures and lithological contact zones are targeted for drilling. There is a 60 to 85% success rate in ideal conditions. It is even more important for urban clients to use the services of legitimate borehole drilling companies such as GEDGhana since urban infrastructure such as electrical cables, phone lines and pipelines can prove an even greater challenge during the drill siting phase.
THE DRILLING AND CONSTRUCTION OF THE BOREHOLE
Once the drilling target is determined, the next phase is the actual drilling of the borehole. Many people are under the mistaken impression that when you simply drill, there is an immediate endless supply of water. There is a lot that needs to happen before this occurs but the general drilling method used in domestic borehole drilling is known as “down-the-hole” air flush rotary percussion. Domestic boreholes are initially drilled through topsoil and weathered overburden rock. This facilitates the installation of steel casings. Casing is usually only required through the unstable overburden. However, high yielding fracture zones may require that the borehole is cased from top to bottom.
ARTIFICIAL GRAVEL PACK INSTALLATION
After the casing and screen string have been inserted, an artificial gravel pack is installed at the walls of the borehole into the annular space, forming a natural backfill or ‘gravel pack’ that helps to filter incoming water. The screen slot sizes are such that only the finer content of this backfill is allowed into the borehole.
At this stage clean, potable water would emerge from the mess being pumped out of a new borehole. After the installation of the permanent casing, screens, and gravel packs, dirty water, mud, crushed rock, oil (from the drilling machinery), and perhaps other debris will be left in the hole. The Development repairs damage done to the adjacent aquifer by the drilling process, develop the aquifer (increase transmissivity), and enhance the performance of the borehole. Development also encourages a gravel filter pack to settle properly, eliminating voids, which may necessitate topping up the gravel pack a little. We continue with the process until the water being discharged from the borehole is in the judgment of the owner or supervisor, as clean as possible. The development method used by GEDhana is the blowing yield.
After drilling has been completed and the sanitary seal put in place, borehole test pumping is carried out. It has the following objectives:
• to measure the performance of the borehole
• to determine the efficiency of the borehole, or variation of its performance under different rates of discharge.
• To quantify aquifer characteristics, such as transmissivity, hydraulic conductivity, and storativity.
Test pumping generally requires that the pumping rate be controlled fairly easily and accurately. The commonest method we use is an electric submersible pump, with valve control of the discharge pipe at the surface. This is lowered into the borehole at the end of a rising main to the intended pumping level in the hole. For water levels to be easily measured during the pumping and recovery phases of the test, a flexible plastic pipe open at the lower end is attached (e.g. by cable ties, adhesive tape, or binding wire) to the rising main as it is lowered into the hole. A dip meter cable is introduced down this pipe to record changes in water level. Discharge during the test is measured by means of bucket timing or with a flow meter built into the discharge pipe. Water pumped out is directed downslope and well away (more than 100 metres) from the pumped borehole.
PUMPING AND PIPING OF THE BOREHOLE (MECHANIZATION)
The final stage of borehole drilling requires the pumping and piping of water from the successful borehole. The end use of the water will largely determine the type of pumping and reticulation system that will be installed.