It was late June when we, the water club members, headed to Base Titanium for a field trip on borehole drilling. Not only did we learn about borehole drilling but also about fire extinguishing. Geoffrey Wekesa (our club patron), Willy Sasaka (hydrogeologist), and Geoffrey Mwania (our guide from Base Titanium) helped us on this visit.
We learned that a borehole is often drilled to tap water from underground. Different types of ground material moves out when the hole is being drilled. We learned about two types of borehole drilling:
- Air drilling – this type uses an air compressor: This is where air from an air truck is put in a drilling rig so that air can be compressed to the hole so that it can be drilled.
- Mud-drilling – this type uses water when drilling: This is where two water holders are dug. An amount of polymer is added to one of the water holders, a truck is connected to it so that it can transfer the polymerised water to the drilling rig. The drilling rig is than connected to a drill bit and the polymerised water is transferred to the hole being drilled. The polymerised water that will have been used will then be transferred to the second water holder. It will contain different types of ground materials which will have been emitted from the drilled borehole. The water will settle out the material and then be transferred to the first water holder again. The polymer that is added to the water is slippery, so as to prevent collapse of the walls of the hole being drilled. It is non-biodegradable; that is, it cannot be changed to a harmless natural state by the action of bacteria.
Note: Some drilling polymers are biodegradable (often made from guar gum), but they are prone to rotting during the drilling process, especially in the ambient temperatures that are common in Kwale County. So drilling contractors here use non-biodegradable polymers instead. They use two types, commercially known as Poly-Bore and EZ-MUD. Both are manufactured by Baroid Industrial Drilling Products and are not ecotoxic. Although they do not biodegrade, they can break down naturally by UV radiation if left exposed to sunlight for long enough. They can also be chemically broken down. After drilling is complete, the drillers add granular Chlorine 60 (calcium hypochlorite) mixed with gravel to the annular space to disinfect the borehole and, in the process, break down (denature) the polymer. The annular space is the gap between the outside of the pipe that is inserted into the borehole (also referred to as casing) and the sides of the hole (called the wellbore). The borehole is then ‘developed’ – a process that clears out fine drill cuttings and dirty water (including the denatured polymer). Water is pumped out of the borehole and monitored for physical and chemical parameters until only clean water is coming out – meaning it is free from fine suspended materials and has stable pH and electrical conductivity. At this point the borehole is no longer impacted by the polymer or other effects of drilling and the water can be used.