Methods of Drilling a Borehole

Rotary Drilling

Rotary drilling stands as the most widely used technique for boreholes, employing a rotating drill bit powered by a rig to cut through soil and rock. Drilling fluid, such as mud or compressed air, circulates down the pipe to cool the bit, stabilize walls, and flush cuttings to the surface.

This method splits into air rotary and mud rotary variants. Air rotary excels in dry, hard rock by using compressed air for faster penetration and minimal fluid waste, while mud rotary handles unstable formations better through viscous fluid that supports borehole walls.

Operators select bits like PDC for abrasive soft-to-medium rock or DTH hammers for hard formations, achieving depths over 300 meters reliably.

Percussion and Cable-Tool Drilling

Percussion drilling, also called cable-tool, relies on repeated impacts from a heavy chisel bit suspended on a cable. The bit lifts and drops to shatter rock, with water added to form slurry that removes debris.

Suited for shallow wells in unconsolidated soils, this low-tech method requires no rotation, making it portable for remote Kenyan sites. However, it progresses slowly—about 5-10 meters per day—limiting use to depths under 100 meters.

Modern rigs enhance efficiency, but rotary methods have largely replaced it due to speed advantages in deeper projects.​

Auger Drilling

Auger drilling uses a large helical screw, or auger, rotated into soft, cohesive soils to lift material directly to the surface without fluids. Bucket augers handle larger diameters, ideal for geotechnical surveys or shallow water boreholes up to 30 meters.

This economical approach thrives in clays and sands but fails in rocks or gravels, where the auger binds. In Nairobi’s variable soils, it’s common for initial site investigations before advancing to rotary.​

No circulation system simplifies setup, reducing costs for community projects.​

Advanced Techniques

Reverse Circulation (RC) drilling employs dual-wall pipes to return cuttings up the center, minimizing contamination for high-quality samples in mineral exploration or precise water yield tests. It boosts efficiency in consolidated formations.​

Sonic drilling applies high-frequency vibrations to liquefy soil around the bit, enabling undisturbed core recovery in sensitive environmental sampling. Though costly, it’s gaining traction for contamination-prone aquifers.

Down-the-Hole (DTH) hammers combine rotation with percussive blows at the bit tip, excelling in hard rock for depths exceeding 500 meters.

Site Preparation and Process

Successful drilling begins with hydrogeological surveys using resistivity or seismic methods to pinpoint aquifers, avoiding dry holes. Permits from Kenya’s Water Resources Authority follow, ensuring compliance.​

Mobilization involves rig setup, often truck-mounted for rural access. Drilling progresses in stages: pilot hole, reaming if needed, casing installation with PVC or steel pipes, and gravel packing for filtration. Geophysical logging verifies yield and quality post-drilling.

Development flushes debris via airlifting or surging, followed by pump testing to confirm sustainable output, typically 5-50 cubic meters per hour in Kenyan aquifers.

Casing and Completion

Casing prevents collapse, with slotted sections in aquifers allowing water entry. Cement grout seals the annulus against surface contamination, vital for potable supplies.​

Screen installation matches slot size to aquifer sand, preventing clogging. Headworks include sanitary seals and vents, protecting against pollutants in flood-prone areas.​

Considerations for Kenya

In Nairobi County, fractured volcanic rocks favor rotary or DTH, while sedimentary basins suit augers. Costs range KSh 2,000-5,000 per meter, influenced by depth (50-200m) and method.​

Sustainability demands community management, regular yield tests, and rehab every 5-10 years via surging or chemicals. Climate variability underscores boreholes’ role in resilience.