Drilling a borehole and hybrid drill string

Abstract

Method of drilling a borehole into an object, the method comprising providing a first borehole section ( 5 ) extending into the object from its surface; running a first drill string part ( 3 a ) including a fluid jet drill head ( 10 ) in the first borehole section, wherein the first drill string part comprises a length of flexible tubing; generating fluid jet so as to blast with an erosive power on an impingement area of the borehole, thereby deepening the borehole to provide a second borehole section ( 5 a ), and assembling a plurality of jointed pipe elements ( 11 ) forming a second drill string part ( 11 a ) to the top of the length of flexible tubing in the course of providing the second borehole section; and hybrid drill string ( 16 ), comprising a length of flexible ( 3 ) tubing as well as a plurality of jointed pipe elements ( 11 ), and further comprising a bottomhole assembly comprising a fluid jet drill head ( 10 ).

1 . A method of drilling a borehole into an object, the method comprising:
 providing a first borehole section extending into the object from its surface;   running a first drill string part including a fluid jet drill head in the first borehole section, wherein the first drill string part comprises a length of flexible tubing;   generating fluid jet so as to blast with an erosive power on an impingement area of the borehole, thereby deepening the borehole to provide a second borehole section, and   assembling a plurality of jointed pipe elements forming a second drill string part to the top of the length of flexible tubing in the course of providing the second borehole section.   
     
     
         2 . The method according to  claim 1 , wherein the fluid jet is an abrasive fluid jet and the fluid jet drill head is an abrasive fluid jet drill head. 
     
     
         3 . The method according to  claim 1 , wherein the drill string is rotated while deepening the borehole. 
     
     
         4 . The method according to  claim 1 , wherein the second drill string part in the first borehole section has a maximum diameter larger than the diameter of the second borehole section. 
     
     
         5 . The method according to  claim 1 , wherein the length of flexible tubing is connected to an auxiliary flexible member when the first drill string part is run into the first borehole section. 
     
     
         6 . The method according to  claim 1 , wherein the length of flexible tubing comprises a coupling at its upper end. 
     
     
         7 . The method according to  claim 6 , wherein the coupling is used for connecting one of the second drill string part and the auxiliary flexible member according to  claim 6  to the first drill string part. 
     
     
         8 . The method according to  claim 1 , wherein flexible tubing guidance equipment is provided at surface, and wherein the coupling and the flexible tubing guidance equipment are dimensioned such that the coupling can pass through the flexible tubing guidance equipment. 
     
     
         9 . The method according to  claim 1 , wherein the method further comprises
 raising the assembled first and second drill string parts in the borehole,   disassembling the jointed pipe elements from the length of flexible tubing,   providing a longer length of flexible tubing as first drill string part and repeating the steps of the method of  claim 1 .   
     
     
         10 . The hybrid drill string, comprising a length of flexible tubing as well as a plurality of jointed pipe elements, and further comprising a bottomhole assembly comprising a fluid jet drill head. 
     
     
         11 . The hybrid drill string according to  claim 10 , wherein the length of flexible tubing comprises a single piece of tubing having a length of at least 30 m. 
     
     
         12 . The hybrid drill string according to  claim 10 , wherein the length of flexible tubing comprises a single piece of tubing having a length of at least 50 m. 
     
     
         13 . The hybrid drill string according to  claim 10 , wherein the outer diameter of the flexible tubing is at least 12 mm smaller than the outer diameter of the jointed pipe elements as measured at their joints. 
     
     
         14 . The hybrid drill string according to  claim 10 , wherein the outer diameter of the flexible tubing is at least 3 mm smaller than the outer diameter of the jointed pipe elements as measured outside their joints. 
     
     
         15 . The hybrid drill string according to  claim 10 , wherein the length of flexible tubing has an outer diameter in the range of from 4.5 cm to 12.7 cm.