This invention relates to the positioning of deflection wedges in drill holes which are horizontal or have a pronounced horizontal component.
Deflection wedges for vertical holes are well known and consist essentially of frusto-conical segments which are wedged in position in a drill hole. When next a drill bit is inserted in the hole, the wedge causes the bit to drill at an angle to the axis of the original hole. Deflection wedges are normally positioned at the end of a drill string, let down the hole, orientated to give the correct deflection and then severed from the drill string as by shearing a shear pin. The drill string is then removed leaving the wedge in position--see U.S. Pat. No. 2,950,900.
Devices suitable for deflecting vertical holes cannot be used without change in deflecting horizontal holes. Nowadays horizontal or near horizontal holes are used extensively in very deep mining for exploration.
In South Africa a device has been used on a small scale and with success in holes up to 500 m long. Beyond that length problems are encountered. Basically the mechanism relies on a weighted droparm the free end of which follows a cam. When as a result of rotation of the string the cam attains a position where there is a vertical cam path, the droparm falls and releases a trigger.
The trigger closes a valve which stops water flow through the drill string and causes a hydraulic pulse which can be detected at the drilling machine. This pulse indicates that the cam has attained the correct position, and as the wedge is orientated relatively to the cam, that the wedge is in the correct position. By pushing the drill string forward, the wedge is wedged in position and a pin is sheared to allow withdrawal of the drill string.
When the hole is near horizontal, the drill string rests on the bottom wall of the hole and is subject to a large amount of friction when one turns it to get the correct orientation. When the drill string is very long, the friction results in a wind-up of the drill string and the resulting stored energy when released causes the free end of the string, i.e. the orientation mechanism and wedge, to whip. This energy may be released as the drill string is pushed forward to secure the wedge and shear the pin with the result that the wedge loses its original correct orientation. Also if the droparm falls during the release of energy the wedge may continue to rotate after the hydraulic pulse, indicating correct orientation, has been transmitted to the drilling machine.