Method of drilling and jet drilling system

Abstract

A method of drilling comprises providing a drill string in a borehole, the drill string comprising an abrasive jet drill head including a jet nozzle and providing a passageway for fluid to the nozzle; supplying a supply fluid comprising a supply concentration of abrasive particles to the jet drill head; generating an jet comprising a jetting concentration of abrasive particles at the nozzle, wherein the jetting concentration depends on the supply concentration; and modulating the jetting concentration; wherein the supply concentration is modulated in order to modulate the jetting concentration. Further, an drilling assembly comprising an drill head with a jet nozzle, a passageway for fluid from a drill string to the jet nozzle, and a modulation means arranged along the fluid passageway for modulating the concentration of abrasive particles in fluid flowing through the fluid passageway towards the jet nozzle.

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 to the jet nozzle;   supplying a supply fluid mixture via the drill string to the abrasive jet drill head, wherein the supply fluid mixture received at the abrasive jet drill head comprises a supply concentration of abrasive particles;   generating an abrasive jet of a jetting fluid mixture comprising a jetting concentration of abrasive particles at the jet nozzle, so as to blast with an erosive power on an impingement area of the borehole, wherein the jetting concentration depends on the supply concentration;   moving the impingement area of the abrasive jet along a selected trajectory in the borehole; and   modulating the jetting concentration of the abrasive particles while the impingement area is being moved;   wherein the supply concentration of abrasive particles in the supply fluid mixture is modulated in order to modulate the jetting concentration in the abrasive jet.   
     
     
         2 . 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. 
     
     
         3 . The method according to  claim 1  wherein the jetting concentration is controlled so as to be modulated in relation with the position of the impingement area on the selected trajectory. 
     
     
         4 . The method according to  claim 3 , wherein the position of the jet nozzle along the trajectory is determined, and used in order to control the supply concentration. 
     
     
         5 . The method according to  claim 1  wherein moving the impingement area comprises rotating the jet nozzle. 
     
     
         6 . The method according to  claim 5 , wherein the jet nozzle is rotated at a selected rotational frequency, and wherein the supply concentration is modulated at a modulation frequency that is the same or an integer fraction of the rotational frequency. 
     
     
         7 . The method according to  claim 1  wherein the abrasive particles comprise magnetic abrasive particles, and wherein the supply concentration is modulated by modulating a magnetic field in the passageway, wherein modulating the supply concentration of magnetic particles in the supply fluid mixture comprises modulating a magnetic field at a collection surface arranged along the passageway between a first value, 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. 
     
     
         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 from a connected tubular drill string part to the jet nozzle, and a modulation means for modulating the concentration of abrasive particles in fluid flowing, during operation, from the tubular drill string part through the fluid passageway towards the jet nozzle of the abrasive jet drilling bit, wherein the modulation means is arranged along the fluid passageway, wherein, if 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, wherein the modulation means is arranged along the passageway upstream of the mixing location. 
     
     
         9 . The abrasive jet drilling assembly according to  claim 8 , wherein the modulation means is integrated with the abrasive jet drill head. 
     
     
         10 . The abrasive jet drilling assembly according to  claim 8 , wherein the modulation means is arranged to form part of a drill string above the abrasive jet drill head. 
     
     
         11 . The abrasive jet drilling assembly according to  claim 8  wherein the modulation means comprise
 a collection surface, 
 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. 
 
     
     
         12 . The abrasive jet drilling assembly according to  claim 11 , further comprising a measurement device for detecting the angular orientation of the jet nozzle in the borehole, and wherein the modulation control means is arranged to control the magnetic holdup device in dependence on the detected angular orientation. 
     
     
         13 . The abrasive jet drilling assembly according to  claim 11  wherein the magnet arrangement comprises at least one electromagnet. 
     
     
         14 . The abrasive jet drilling assembly according to  claim 11  wherein the magnet arrangement comprises at least one permanent magnet. 
     
     
         15 . The abrasive jet drilling assembly according to  claim 8 , further comprising a control unit for controlling the jetting concentration in relation with a position of an impingement area of the abrasive jet drill head during operation.