Method and steering assembly for drilling a borehole in an earth formation

Abstract

A method of drilling a borehole in an earth formation using a drill string including a control system for controlling the borehole trajectory. The control system is operable to receive a selected signal, to determine the borehole depth from at least one length increment and the number of times the selected signal is received, and to use the borehole depth as input parameter for controlling the borehole trajectory in correspondence with a desired borehole trajectory. While drilling, for each length increment that the drill string is lowered into the borehole, transmitting the selected signal to the control system; inducing the control system to determine the borehole depth from said length increment and the number of times the selected signal is received; and operating the control system to use the borehole depth as input parameter for controlling the borehole trajectory in correspondence with the desired borehole trajectory.

1 . A method of drilling a borehole in an earth formation using a drill string including a control system for controlling the borehole trajectory, wherein the control system is operable to receive a selected signal, to determine the borehole depth from at least one length increment and the number of times the selected signal is received, and to use the borehole depth as input parameter for controlling the borehole trajectory in correspondence with a desired borehole trajectory, the method comprising:
 a) operating the drill string to drill the borehole, thereby lowering the drill string into the borehole;   b) for each length increment that the drill string is lowered into the borehole in step (a), transmitting the selected signal to the control system;   c) inducing the control system to determine the borehole depth from said at least one length increment and the number of times the selected signal is received by the control system;   d) operating the control system to use the borehole depth as input parameter for controlling the borehole trajectory in correspondence with the desired borehole trajectory.   
     
     
         2 . The method of  claim 1 , wherein the signal is transmitted to the control system each time the drill string is lowered into the borehole a respective one of said at least one length increment. 
     
     
         3 . The method of  claim 1  wherein the drill string is assembled from a plurality of drill string sections, and wherein each length increment corresponds to the length of a respective drill string section. 
     
     
         4 . The method of  claim 1  wherein the selected signal includes a variation of a drilling parameter detectable by the control system. 
     
     
         5 . The method of  claim 4 , wherein a stream of fluid is pumped through the drill string during drilling, and wherein said drilling parameter comprises a flow rate of the stream of fluid. 
     
     
         6 . The method of  claim 4  wherein the drill string is rotated during drilling, and wherein said drilling parameter comprises a rotary speed of the drill string. 
     
     
         7 . The method of  claim 1  wherein the drill string rests on the borehole bottom during drilling, and wherein said drilling parameter comprises a compressive force exerted by the drill string to the borehole bottom. 
     
     
         8 . The method of  claim 7 , wherein the drill string is lifted from the borehole bottom to induce a variation of said compressive force exerted by the drill string to the borehole bottom. 
     
     
         9 . The method of  claim 1  further comprising operating the control system to use at least one of borehole azimuth and borehole inclination as input parameter for controlling the borehole trajectory in correspondence with the desired borehole trajectory. 
     
     
         10 . The method of  claim 1  wherein the control system forms part of a bottom hole assembly of the drill string, which bottom hole assembly is lowered into the borehole. 
     
     
         11 . A steering assembly for steering the direction of drilling into an earth formation, the steering assembly being adapted to form part of a drill string extending into a borehole into the earth formation, the steering assembly comprising a steering system for setting the direction of drilling in response to a control signal, and a control system for providing the control signal to the steering system, the control system comprising
 memory means for storing a desired borehole trajectory;   one or more directional sensors for determining azimuth and inclination;   means for obtaining a borehole depth;   means for determining an actual borehole trajectory using the borehole depth, azimuth and inclination as input parameters;   means for determining a deviation between the desired borehole trajectory and the actual borehole trajectory, and for determining the control signal from the deviation, wherein the means for obtaining a borehole depth comprises   receiver means for receiving a selected signal characteristic for a length increment that the drill string is lowered into the borehole;   a counter for determining the number of times the selected signal was received; and   calculation means adapted to determine the borehole depth from at least one length increment and the number of times the selected signal is received.   
     
     
         12 . The steering assembly according to  claim 11 , wherein the memory means is adapted to store the at least one length increment. 
     
     
         13 . The steering assembly according to  claim 11  further comprising a power supply. 
     
     
         14 . The steering assembly according to  claim 1  wherein the steering assembly forms part of a drill string including a drill bit. 
     
     
         15 . The steering assembly according to  claim 14 , wherein the steering assembly is arranged at a distance of 100 m or less from the drill bit.