Percussive test drilling with out-of hole and down-the-hole hammers

Rotopercussive techniques have played a dominant part in boreholes drilled by Cogema for uranium ore exploration. In the case of objectives of under 200 m depth, Cogema prefers to use the Hydrofore with the Montabert hydraulic hammer out of the hole. To improve the performances of this method, water injection was been adopted with piston pumps which are customarily used in core drilling. In inclined as in vertical operations, the average speed including stoppages varies from 10 to 15 m/h depending on depth and difficulties. The rate of advance drops with increasing depth. In the case of objectives beyond 200 m and up to 500 m depth, down-the-hole Mission hammers are used; the drill rig is a type RCG 2500 designed and built by Cogema. Beyond these depths, the hydrostatic pressures and the flow rates of the water influx in connection with the hydrogeological characteristics will progressively affect the proper functioning of the down-the-hole hammer. Directional drilling is under full control with the out-of-hole hammer but not with the down-the-hole hammer in an inclined borehole