Borehole survey and factors

BORE HOLE DEVIATION, BOREHOLE SURVEY & DEFLECTION

The departure of a bore hole, whether vertical or inclined, from its set course on its own is known as deviation of the bore hole. Such deviation is often very significant, usually 2° per 30 m length. The deviation is both in azimuth and off the vertical i.e., the bore hole tends to wander away from the proposed plane and also changes its angle with the horizontal.

Measurement of the deviation of the borehole is called borehole survey. 

In the drillers' terminology deflection of a borehole is the deliberate and intentional deviation of a borehole brought about by the driller with a view to correct the borehole's course or to force it into a curve which is sharper than, or in another direction to, that caused by deviation.

Fig. 3.11 Deviation of borehole and directional drilling.

Borehole al D, because of deviation, reports "No orebody". One parent hole at A covers a wide area for prospecting with the help of deflected branch holes and eliminates the need for a number of prospecting boreholes from the surface.

Major factors affecting deviation, in general, are depth and angle of the borehole and the nature of the strata drilled. Certain drilling techniques also influence the deviation and with judicious application of the same, deviation can be controlled and kept at a minimum.

Experienced drillers have observed from long experience that borehole deviation follows certain rules which apply generally but to which there are some exceptions. These rules may briefly be stated as follows:

(1)  Deviation may occur in holes of as small a depth as 100 m.

(2)  Deviation increases with depth.

(3)  Borehole tends to follow the bedding plane between a hard rock and a soft rock. Contact zones of dykes, veins, lenses, especially hard and soft act as natural deflecting planes. In soft ground and shear zones deviation is common.

(4) Some drilling techniques have a major primary influence on deviation, such as hole diameter, bit pressure, core barrel and casing.

Diameter of hole: — Large diameter holes tend to deviate less than the small diameter holes. It has been observed that in 'BX' size the deviation of borehole is 2° to 3° per 30 m and with other conditions being similar it may become double of this in ‘AX’ size and triple in 'EX' size.

Bit pressure: High bit pressure accelerates deviation; thus, a sharp bit drilling at a moderate penetration will require less bit pressure (and consequently deviate less) than a blunt bit or a bit that is fed faster.

Core barrel: A diamond drill hole tends to stay straight. One of the factors against deviation is the constraining of the barrel by the hole. This is affected by the stiffness of the barrel and its clearance in the hole and such long, heavy walled core barrels close to hole side will reduce deviation. Worn core barrels aggravate tendency to deviate; this is also the case if light bore rods are used.

Deveiation can be suitably controlled and reduced by

(a)  paying attention to causes stated at (4) above.

(b)  using full diameter drill rods for the entire length of the borehole and using proper couplings etc., with good threads to avoid , angularity and lack of concentricity.

(c) correcting the hole's course by deflection methods, described shortly, where necessary.