Over the past 30 years Umvoto has pioneered and remained central to exploring and successfully developing deep groundwater resources for agricultural, rural and bulk municipal supply. The Table Mountain Group (TMG), occurring extensively across the Western and Eastern Cape provinces of South Africa has proven to be a challenging but highly rewarding target to establish and hone best practices for high yielding (>50 litres/second [l/s]), deep borehole drilling (300-1 000 metres [m]).
Drilling deep boreholes (especially in the quartzitic lithologies typical of the TMG) poses numerous challenges related to isolating the deep target aquifer from overlying aquifers/aquitards, the competence of the aquifer material as well as the artesian pressures encountered at depth. In overcoming these challenges Umvoto, together with competent drilling contractors, have applied an adaptive approach using unique combinations of industry-leading drilling technologies incorporated into multi-technique drilling plans.
Some of the drilling techniques utilised to successfully achieve target depths are explored below.
Down-the-hole (DTH) hammer rotary air percussion
The “traditional” down-the-hole (DTH) rotary air percussion technique has often been utilised in the upper ~300 m of deep borehole drilling through competent rock.
This technique comprises of a pneumatic hammer and bit combination that hammers and rotates to penetrate consolidated rock. Compressed air drives the rotating percussion hammer, with decompressed air carrying drill cuttings and water back up to surface. This method has, on occasion, been adjusted to include a surface diverter to act as a conduit through which blowout material is diverted directly into drilling sumps and containers to avoid spills into the sensitive surrounding environment (see Figure 1 and Figure 2 below).
Air percussion reverse circulation (RC)
The air percussion reverse circulation (RC) method is based on the same bit-hammer combination as the conventional pneumatic DTH percussion method while utilising various subs and a dual tube rod system to direct compressed air flushed down an outer rod tube, out of the bit face and the return of decompressed air, drill cuttings and water back up via an inner rod tube. The contained, circulated system and lack of annular discharge makes this a more environmentally compliant alternative.
Drilling advance and productivity, as with the traditional DTH pneumatic-based method, is largely limited by the up-hole velocities required to overcome the hydraulic head, which increases with depth and high-yielding waterstrikes. The DTH bits used in both techniques additionally require constant cumbersome inwelling and outwelling for bit management.
Flooded RC
Once sufficient depth (>250 m) is attained to offer adequate Weight On Bit (WOB), the system is converted to a flood RC system (see Figure 3 for an example of a site setup). Although this method also uses a circulation system, it overcomes the major limitations of the pneumatic-based DTH and RC air percussion methods’ productivity as water, instead of air, is circulated (or ‘flooded’) through the dual tube rods. A rotating tri-cone bit is also used, with advance driven by weight. During drilling compressed air is directly injected into the flooded inner tube of the dual tube rods where it subsequently decompresses and expands, lowering the density of the inner tube’s water column. A mixture of water, decompressed air and drill cuttings are then forced up the drill string by the denser outer fluid column.
It should be noted that prior to its application in the City of Cape Town New Water Programme (CCT NWP), the use of the flooded RC technique and tri-cone bits to the extremely abrasive TMG quartzites was novel. The closed circuit, flooded/stable hydrostatic column and use of a container system simplified environmental mitigations and allowed advance through deep, unstable (fractured, faulted and/or heterogenous) lithologies. Penetration rates slow significantly during the application of this technique; however, they generally increase with depth (as the WOB increases) and the drilling diameter is reduced (higher point load pressure). The consistency of the technique additionally increases drilling efficiencies and production— with less frequent inwelling and outwelling trips necessary for bit management.
Water hammer
The water hammer technique is utilised in the last ~800 to >1000 m portion of the borehole where significant hydrostatic water columns, reduced diameters (<200 mm) and artesian flows and pressures are encountered. Umvoto’s proven successes in applying the water hammer (Wassara) method in drilling the >600 m deep, high artesian flow (e.g., ~50 l/s at 8 bar pressure) 2008 DAGEOS project and current CCT NWP TMG production boreholes makes this an ideal method for targeting ultra-deep production depths.
The water hammer technique works under the same principle as the pneumatic DTH, however in the place of air, highly pressurised water drives the Wassara water hammer at high frequencies and delivery pressures (110-180 bar; see Figure 4). The high frequency hammering, consistent hydrostatic column, and simplified piston and valve movement result in reduced energy consumption, minimal hole deviation, more stable sidewall conditions, extremely quick penetration rates (3-6 min/m) and a high-power output ratio necessary for deeper drilling. The constant need to outwell and do bit management between runs and the need for a consistent high-volume supply of freshwater tend to be the primary constraints on production. The low velocity flushing water also means environmental mitigations are minor and easily controlled during drilling, making this an environmentally sensitive drilling option.
Multiple intervals of successive casing strings were installed throughout the drilling process to aid with borehole stability, isolate the target aquifer and in preparation for expected artesian conditions. Casing and annular seal material specs (including, but not limited to wall thicknesses, density, collapse strength, burst pressure, diameters, and screen types) are planned with the intended drilling diameters, depths and geological/hydrogeological conditions taken into account. Waterstrikes, associated blowyields and artesian flows (rates, strike depths and pressures) are meticulously recorded throughout the entire drilling process, in combination with a modified 60 to 90 minute ‘stepped’ airlift yield test and/or quick informal calibration and stepped pump test during and after drilling is completed.
Ultimately each technique has their pros and cons and when utilised as a combined multi-technique drilling method, targeting the ideal drilling conditions for each technique, a tapered advance to depths in excess of
1000 m can be achieved.
Challenges encountered
The combination of ultra-deep, wide-diameter production borehole drilling projects undertaken by Umvoto host a range of challenges:
- High volume (combination of wide diameters and deep depths), low drilling operation pressures and reduced penetration rates during drilling;
- Accounting for the pristine setting of the projects wellfields which meant that all contractor and borehole construction activity had to be carefully planned, managed and monitored to ensure limited impact to rare and endangered flora and fauna;
- Socio-economic factors (theft, vandalism, stakeholder engagements and land-use); and
- Complex geological, hydrogeological and structural settings resulted in challenging drilling conditions, including:
- The extremely competent and abrasive TMG quartz arenites;
- High artesian pressures and yields;
- Structural overthickening (increased confined aquifer target depths);
- Highly unstable fractured, brecciated and gouge fault zones;
- Large pseudokarstic cavities; and
- Intense competency heterogeneity in intersected lithologies (especially the TMG Goudini Fm.).
These challenges were overcome through combined planning, continual adaptation and mitigation measures initiated between key project role-players.
The way forward
The truly innovative and integrated approach taken by Umvoto in collaboration with various key role-players has set a standard for large-scale, deep-target, South African municipal supply wellfield developments. These innovative developments also boast with producing the deepest wide-diameter municipal water supply boreholes in the Western Cape (if not South Africa). Umvoto’s foundational Monitor, Model and Manage principles means that the sustainability of developed wellfields remains paramount and Umvoto additionally ensures that robust and diverse monitoring schemes covering a range of operational parameters, hydrogeological parameters (water-level, water-quality and isotope) and ecological parameters are implemented prior, during and after development. The extensive knowledge gained ultimately stands to offer quicker, more precise deep production drilling in future contracts.
