Methods of Borehole survey

Bore hole survey

Measurement of a drill hole deviation is called surveying a bore hole and all methods of bore hole survey are aimed to locate any point along the hole-course with respect to three dimensional coordinates attaining varying degrees of accuracy. Some methods are used to measure the deviation in angle only; others measure both angle and bearing deviation but at a predecided point only and the most sophisticated instruments can measure both angle and bearing, deviation in a continuous manner all along the bore hole course.

Some of the common methods are given below:

Etch method using Hydro fluoric acid

This method can be adopted to measure the deviation in angle only (off the vertical). HF has the property of corroding glass. The HF solution partly filling a glass test tube, corrodes the inner wall of the test tube up to the column of 'h, liquid and thins its wall up to the plane of separation of air. The upper part of the tube not in contact of liquid will remain unchanged. This plane of separation can thus be perceived, later on, on the same empty test tube, by a line, known as 'etch line’. When the test tube stands vertical, the etch line will be normal to the longer axis of the tube. In any other tilted position the etch line will make corresponding angle. This angle, known as 'etch angle’ can be measured and tilt angle can be found. This principle is applied in HF method of bore hole survey. To measure the inclination of diamond drill holes a small quantity of dilute HF solution is poured in a glass tube, lowered into the drill hole in a proper container and leaving it for sufficient time for the acid to etch a line on the inside of the tube and then measuring the angle of etch. HF with concentration generally 5°/o, 8% and 10% are preferred.

Hydrofluoric acid is properly mixed by shaking the container well. By adding distilled water to HF proportionately (by volume) the desired concentration can be obtained. About 50 ml of prepared acid is poured into the test tube and thoroughly shaken. After inserting the stopper in the tube, it is put into a container (suitable for 'AX' 'BX' or "NX' size depending on the size of the hole) made up of either brass or mild steel. The container should have water tight joints even under heavy pressure. The container coupled with the drill rods, is lower into the bore hole at desired depths. Inclination readings are normally taken at every 50 to 60 m and four containers are lowered at a time at desired intervals. For shorter bore holes 10% acid with about 40 to 45 minutes etch time is sufficient.

When the drill rods and container are withdrawn from the drill hole the etch tube should be removed as quickly as possible, emptied immediately and washed to stop further etching.

The line of etch is marked on the tube with fine dots using Indian ink, care being taken to mark the high and lower points carefully. The angle of etch or the apparent dip angle is measured on a protractor or on a graph paper. The angle of this etched line with the long axis of the tube gives the inclination of the drill hole at the depth tested.

The acid tube etch method is commonly adopted and is considered reasonably reliable. It can be done even for a cased borehole and can be carried out by the drilling crew. It is the oldest method of bore hole surveying.

Some instruments used for measuring deviation of boreholes off the vertical as well as off its intended bearing are the Carlson compass and Tropari instrument. A Borehole Camera developed by M/s Eastman Kodak Co. of Germany gives continuous record of borehole deviation from top to bottom.

For use in mining areas where deep holes are not required such photographic instruments suffer from the following disadvantages:

1.  They are expensive.

2.  They require facilities to process the film.

3.  They are equipped with very small magnetic compass needles and are subject to disturbance by strong fields around rods.

In Kolihan, Madhan Kudhan, Akwali and other sections of the Khetri copper belt, Rajasthan, drill holes have been surveyed by HF (etch) method and the data obtained indicate that if this survey is conducted accurately it can furnish reliable data regarding the inclination of bore holes. While variations may be noted in some of the individual readings, the overall result is comparable with the inclination as measured by using a more sophisticated instrument like 'Tropari' except that this method gives the reduced level of any point along the hole course but its position in two dimensional coordinates is uncertain.

Directional Drilling

The term "Directional Drilling'' means controlling the course of a borehole so as to follow a pre-determined path and complete the bore hole at the desired sub-surface location. The main objectives of directional drilling are:

1) To control deviation in a drill hole requiring accuracy during its course.

2) To return crooked holes to their normal course.

3) To sidetrack obstructions like lost tools, broken bits, etc.,

4) To alter the course of the drill hole to enable a zone intersection at stipulated angle.

5) To provide multiple intersections by drilling branch holes from previously drilled parent hole, and

6) For many such drilling problems.

However, the range of directional drilling is limited to a circle, having the drilling machine as its centre, with a diameter equal to the depth of the borehole.