Deflection of boreholes

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The methods used for deflection of boreholes are:

1) Casing deflection

2) Use of wedges, e.g., the Hall-Rowe deflecting wedge.

3) Sonic orienting assembly

4) Arc cutting.

Fig. 3.12. A deflected borehole and 3 vertical boreholes for underground gasification of coal

Deflection techniques give good results when the deflection is in the same direction as the natural deviation. As a matter of fact, deflection can be very inconvenient and ineffective if it works against the natural deviation. Deflection of boreholes resulting in a few branch holes off the parent hole eliminates the need for a number of holes from the surface during prospecting (See Fig. 3.11).

Deflection techniques are usually adopted in deep oil well holes. They are very rarely used in the mining industry in India so far. In United Kingdom the deflecting technique has been used in a project for underground gasification of coal at a site near Newark, Nottinghamshire. Mining Magazine, May 1986, narated the following information about the project:

Underground gasification requires at least two boreholes drilled from the surface and connected together in the coal seam. Air or some other reactant is pumped down one hole to create combustion and gas is removed from the other.

The coal seam for gasification in question is 2 m thick. 600 m below surface. N.C.B.'s Engineers in United Kingdon will drill four boreholes from the surface. Of these the first one, D, though commencing as a vertical borehole, would be deflected and curved 'through 90° to enter the coal seam horizontally and remain in the seam for a distance of more than 300 m (Fig. 3.12). Special instruments would have to be used in order to keep the drill in the' coal seam. After completion of such deflected borehole, N.C.B.'s Engineers will drill three vertical surface boreholes to join with the in-seam hole. Steam and oxygen would be forced down one vertical hole to stimulate the production of gas to be extracted through one or both of the other vertical holes. The gas would be cleaned before being burned on site unless some local use could be found for it. It is envisaged that the full trial could take up to six years.