Method and system for directional drilling

The invention relates to variation of concentrations of abrasive particles (92) in a stream (90) of drilling fluid mixed with abrasive particles, passed as an abrasive jet through abrasive 5 nozzle(s) of a drill bit (10) along rotations thereof, to vary the erosive power of the stream (90) along angular sections of the borehole (4a') for directional drilling. During subsequent time periods majorities (92m1, 92m2) of abrasive particles are alternately deflected into two parallel channels (21, 22) with a different flow resistance. A resulting velocity difference between said majorities makes that the subsequently deflected majorities recombine 10 downstream of the channels, so that high concentration stream portions (90h) of the combined majorities are formed which alternate low concentration stream portions (90l). 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 (90) within a selected angular section (4a") of the borehole (4a) than outside thereof. 15