Fremgangsmåte og anordning for avviksboring

A method for causing deviation of borehole being drilled by rotary drilling uses a bit having at least one nozzle 42 through which a drilling fluid is caused to flow such that in use, the flow of drilling fluid through the nozzle allows one sector of the bit to cut more effectively than the remainder of the bit, the method comprising modulating the flow of the drilling fluid through the nozzle such that the cutting action of the one sector is optimised when in the desired direction of deviation and such that the cutting action of the one sector is reduced at other locations in the borehole as the bit rotates. Apparatus for controlling the direction of drilling of a borehole comprises a rotary drill bit having at least one nozzle for a drilling fluid to flow therethrough, the flow of drilling fluid through the nozzle over cutting structures on the bit causing one section of the bit to have a comparatively improved cutting action with respect to the remainder of the bit; means being provided to modulate the flow of drilling fluid through the nozzle so as to modulate the cutting action of the one section such that the comparatively improved cutting action occurs in a desired direction of deviation of the hole.