Method of drilling and abrasive jet drilling assembly

Abstract

A method of drilling comprises providing a drill string in a borehole, the drill siring comprising an abrasive jet drill head including a jet nozzle, and providing a fluid passageway between the object's surface and the jet nozzle; supplying a fluid mixture comprising magnetic particles to the abrasive jet drill head and blasting from the nozzle an abrasive jet into impingement with the object; and modulating a jetting concentration of magnetic particles in the abrasive jet by modulating a magnetic field a first value, at which magnetic particles are collected from the fluid mixture, and a second value, at which magnetic particles are released into the fluid mixture. Further, an abrasive jet drilling assembly comprising a modulation means for modulating the concentration of the magnetic abrasive particles in the fluid jet a modulation control means for selectively changing a magnetic field between first and second values.

1 . A method of drilling into an object, the method comprising
 providing a drill string in a borehole in the object, the drill string comprising an abrasive jet drill head at its lower end, the drill head including a jet nozzle, the drill string providing a passageway for fluid from the object's surface to the jet nozzle;   supplying a fluid mixture comprising magnetic particles via the drill string towards the abrasive jet drill head and blasting from the jet nozzle an abrasive jet with an erosive power into impingement with the object;   moving an impingement area of the abrasive jet along a selected trajectory in the borehole; and   modulating a jetting concentration of magnetic particles in the abrasive jet while the impingement area is being moved;   wherein modulating the jetting concentration of magnetic particles in the abrasive jet comprises modulating a magnetic field at a collection surface arranged along a flow path of fluid towards the jet nozzle between a first value of the magnetic field, at which magnetic particles are collected from the fluid mixture at the collection surface, and a second value, at which magnetic particles are released into the fluid mixture from the collection surface.   
     
     
         2 . The method according to  claim 1 , wherein the magnetic field is modulated by moving a magnet relative to the collection surface. 
     
     
         3 . The method according to  claim 1 , wherein the magnetic field is modulated by modulating a drive current of an electromagnet. 
     
     
         4 . The method according to  claim 1 , wherein modulating the magnetic field comprises moving a magnetic connector for magnetically connecting a magnet and the collection surface between a connecting position and a non-connecting position. 
     
     
         5 . The method according to  claim 1 , wherein the particle concentration is controlled so as to be modulated in relation with the position of the impingement area on the selected trajectory. 
     
     
         6 . The method according to  claim 1 , wherein a supply concentration of abrasive particles in the fluid mixture is modulated in order to modulate the jetting concentration in the abrasive jet. 
     
     
         7 . The method according to  claim 1 , wherein a downhole recirculation system is provided, to recirculate abrasive particles after their discharge from the jet nozzle to a mixing location with the supply fluid mixture along the passageway, and wherein the supply concentration is modulated upstream of the mixing location. 
     
     
         8 . An abrasive jet drilling assembly connectable to a tubular drill string part, and comprising:
 an abrasive jet drill head with a jet nozzle,   a passageway for fluid comprising magnetic abrasive towards the jet nozzle,   and a modulation means for modulating the concentration of the magnetic abrasive particles in fluid flowing, during operation, through the jet nozzle;   the modulation means comprising
 a collection surface, and 
 a magnetic holdup device for exerting a magnetic field at the collection surface, comprising a magnet arrangement and a modulation control means for selectively changing the magnetic field at the collection surface between a first value, at which magnetic particles are collected at the collection surface, and a second value, at which magnetic particles are released from the collection surface. 
   
     
     
         9 . The abrasive jet drilling assembly according to  claim 8 , wherein the magnet arrangement comprises at least one electromagnet, preferably wherein the modulation control means is arranged to modulate a drive current of the electromagnet. 
     
     
         10 . The abrasive jet drilling assembly according to  claim 8 , wherein the magnet arrangement comprises at least one permanent magnet, preferably wherein the at least one permanent magnet is movable with respect to the collection surface. 
     
     
         11 . The abrasive jet drilling assembly according to  claim 8 , wherein the magnetic holdup device comprises an actuator for changing the relative position of the magnet arrangement and the collection surface. 
     
     
         12 . The abrasive jet drilling assembly according to  claim 8 , wherein the magnet arrangement further comprises a selectively movable magnetic connector. 
     
     
         13 . The abrasive jet drilling assembly according to  claim 8 , wherein the passageway is at least partially an annular passageway having inner and outer annular walls, and wherein the collection surface is at least partially arranged on the inner annular wall. 
     
     
         14 . The abrasive jet drilling assembly according to  claim 8 , wherein the magnet arrangement comprises at least two magnets that are positioned at different angular positions with respect to flow direction along the passageway. 
     
     
         15 . The abrasive jet drilling assembly according to  claim 8 , wherein the abrasive jet drilling assembly further comprises a downhole recirculation system arranged to recirculate abrasive particles during operation after their discharge from the jet nozzle to a mixing location along the passageway, and wherein the collection surface is arranged along the passageway upstream of the mixing location.