Question 7 of 7: Borehole Drilling Method Selection; Rotary Drilling

Question 7: Borehole Drilling Method Selection; Rotary Drilling (Choose 1 of Questions 5–7 — 20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

a) Factors governing choice of drilling method

1. Nature and hardness of the material to be drilled. Soft, unconsolidated overburden calls for a very different method than hard, competent bedrock; a method that is efficient in one is often unworkable or destructive of sample quality in the other.
2. Required sample type and quality. Resource-definition drilling for assay purposes needs oriented, intact core showing true structural relationships and un-mixed intervals, whereas reconnaissance or geotechnical drilling may only require representative cuttings.
3. Required depth and hole diameter. Deep holes and large-diameter holes favour different rig capacities, rod/casing strengths and drilling-fluid circulation systems than shallow, small-diameter holes.
4. Ground conditions. Water table depth, caving or fractured ground, artesian pressure and permafrost all constrain which methods can maintain a stable, uncontaminated borehole.
5. Site accessibility and water/fluid supply. Remote sites with no available water restrict methods that rely on large volumes of circulating drilling fluid; steep or roadless terrain constrains rig size and mobility.
6. Cost and required production rate. Faster, lower-cost methods (e.g. rotary) are preferred where sample quality requirements permit them; higher cost, slower core methods are justified only when the resulting sample quality is actually needed.

b) Rotary (direct) drilling

Method. A drill bit – typically a tricone roller-cone bit for medium-to-hard formations or a drag bit for soft formations – is mounted on the bottom of a rotating drill string and turned continuously at the surface (or by a downhole motor) to cut and grind the formation. Drilling fluid (water, mud, or compressed air) is pumped down the inside of the hollow drill string, exits through nozzles in the bit, and returns up the annulus between the drill string and the borehole wall, carrying the rock cuttings to surface, cooling and lubricating the bit, and providing hydrostatic pressure to stabilize the borehole wall and control formation fluids.

Conditions of most effective use. Rotary drilling performs best in soft-to-medium, relatively unconsolidated or moderately consolidated formations – sedimentary sequences, overburden, and blast-hole/production drilling in open-pit mines – where fast penetration and hole-cleaning are valued more than obtaining an intact, structurally oriented rock sample. It is the standard method for water wells, oil and gas wells, and large-diameter production/blast holes.

Advantages. High penetration rate and relatively low cost per metre drilled; can drill large-diameter holes economically; effective in unconsolidated or poorly consolidated ground where a diamond bit would wear excessively or a core barrel would not recover sample at all; the circulating fluid continuously stabilizes the hole and removes cuttings, allowing continuous drilling without frequent tripping.

Disadvantages. Recovers only disturbed rock cuttings unless a core barrel is substituted for the bit, so it provides much poorer geological, structural and precise-interval assay information than diamond core drilling; not well suited to very hard, highly fractured, or geologically variable hard-rock terrain, where accurate lithological contacts, structural orientation and vein-width measurement require intact, oriented core rather than cuttings alone.