METHOD AND SYSTEM FOR DIRECTIONAL DRILLING

The invention relates to variation of concentrations of abrasive particles in a stream of drilling fluid mixed with abrasive particles, passed as an abrasive jet through abrasive nozzle(s) of a drill bit along rotations thereof, to vary the erosive power of the stream along angular sections of the borehole for directional drilling. During subsequent time periods majorities of abrasive particles are alternately deflected into two parallel channels with a different flow resistance. A resulting velocity difference between said majorities makes that the subsequently deflected majorities recombine downstream of the channels, so that high concentration stream portions of the combined majorities are formed which alternate low concentration stream portions. Synchronising the frequency of the stream portions with the rotational velocity of the drill bit results in a consistently higher erosive power of the abrasive jet within a selected angular section of the borehole than outside thereof.