This is a valid question. And the answer is: “It depends on the circumstances.” That’s not being evasive. It’s a sign of just how the geology of South Africa varies, ranging from the sedimentary rocks of the Karoo basin to the metamorphosed rocks of the Northern Cape, to the granite intrusions of the Cape Granite Suite.
In addition, SA’s groundwater produces varying yields and quality.Therefore, it’s to be expected that boreholes will differ in depth, in their construction, and in how they are drilled.
Here are the main cost contributors:
Depth is invariably the main determining factor when it comes to cost. Most drill quotes are based on metres to be drilled. So whether you are drilling 10m, 100m or 300m will have a significant bearing on the price.The drill depth will depend on the depth of the aquifer you are targeting. So a good knowledge of the area’s hydrogeology is essential. A sure way to waste money is to drill on and on without knowing exactly what you are targeting. Different drilling methods are required for different formations, and this will affect the cost. In some settings, it’s possible to case off the top 6m of unconsolidated material. That is, using a pipe to protect the open hole – and then drill down using air percussion.
Air percussion is where the drill hammer breaks the rock, and the chips are blown up out of the borehole using compressed air. In alluvial aquifers, mud rotary drilling uses a drilling mud to keep the sand from collapsing and a rotating bit to bore through the sand.
Water Costs: The construction of a borehole contributes requires large volumes of water adding to the borehole drilling cost.
Drilling Method: Where there is deep weathering, or changes in formation competency, or lots of clays, Odex/SYMMETRIX drilling is necessary. This method involves pulling down borehole casing during the drilling process. This keeps the borehole from collapsing, and can also enable you to drill through hard formations.
Borehole Construction is a large contributor to cost. It is very important that a borehole is constructed appropriately. With an alluvial aquifer, you will need to choose your gravel pack and slot size correctly to avoid it silting up, while in a competent rock formation you will need to decide carefully whether there is a risk of borehole collapse.
‘Competent’ is relatively brittle, solid strata that deform by faulting, fracturing or folding, rather than flowing under stress, and are more likely to collapse, as noted.
‘Incompetent’ beds deform more readily under stress. As you can imagine, a 100m-deep borehole with just 6m of steel casing at the top will cost far less than a borehole that is fully encased; and steel casing will cost more than PVC.
If the driller needs to bring their drill rig some distance to get to you, the establishment cost may be higher.
Site access can also sometimes pose a problem, further complicating pricing the process.