A frequently encountered problem during drilling of a wellbore is sticking of the drill string in the wellbore.

Such sticking can be, for example, due to differential sticking of drill string joints to the wellbore wall as a result of a pressure difference between the drilling fluid pressure in the wellbore and the pressure of earth formation fluid present in the pores of the formation.

The problem is more frequently encountered in depleted oil/gas reservoirs where the pore pressure has decreased significantly.

It is an objective of the invention to provide an improved method of drilling whereby the problem of sticking has been solved.

It is another objective of the invention to provide a drill string for use in such method.

In accordance with the invention there is provided a method of drilling a wellbore in an earth formation using a drill string connected at the upper end thereof to a rotary drive and being provided with a drill bit, the drill string comprising a plurality of couplers arranged at selected drill string intervals, each coupler inter- connecting two adjacent sections of the drill string and being operable in an engaged mode in which the coupler transmits torque between the adjacent drill string sections and a disengaged mode in which the adjacent drill string sections are free to rotate relative to each

other about the longitudinal axis of the drill string, each coupler being controllable by control means for moving the coupler between said engaged mode and said disengaged mode, the method comprising a) rotating the drill string so as to drill a portion of said wellbore while each coupler is in the engaged mode; b) when the drill string has become stuck in the wellbore, disengaging a selected one of said couplers which thereby becomes the disengaged coupler; c) rotating the part of the drill string between the rotary drive and the disengaged coupler ; d) engaging the disengaged coupler, and disengaging the next lower coupler which thereby becomes the disengaged coupler; e) repeating steps c)-d) until the disengaged coupler is the lowermost coupler.

In accordance with another aspect of the invention there is provided a drill string for drilling a wellbore in an earth formation, the drill string comprising a drill bit, means for connecting the upper end of the drill string to a rotary drive, and a plurality of couplers arranged at selected drill string intervals, each coupler interconnecting two adjacent sections of the drill string and being operable in an engaged mode in which the coupler transmits torque between the adjacent drill string sections and a disengaged mode in which the adjacent drill string sections are free to rotate relative to each other about the longitudinal axis of the drill string, each coupler being controllable by control means for moving the coupler between said engaged mode and said disengaged mode.

The invention will be described hereinafter in more detail and by way of example with reference to the accompanying drawing in which

Fig. 1 schematically shows a drill string for use in the method according to the invention.

In Fig. 1 is shown a drill string 1 extending into a wellbore 3 formed in an earth formation 5. The lower end of the drill string 1 is provided with a drill bit 6 and the upper end of the drill string 1 is connected to a rotary table 7 at surface for rotating the drill string, which rotary table 7 forms part of a conventional drilling rig 9. The drill string 1 is provided with four couplers 10,12,14,16 arranged at selected intervals along the length of the drill string 1, thus dividing the drill string in drill string sections 18,20,22,24,26.

Sections 18,20 are interconnected by coupler 10, sections 20,22 are interconnected by coupler 12, sections 22,24 are interconnected by coupler 14, and sections 24,26 are interconnected by coupler 16. A fluid passage (not shown) for drilling fluid extends in longitudinal direction through the drill string 1.

Each coupler is operable in an engaged mode in which the coupler transmits rotation between the two drill string sections interconnected by the coupler, and a disengaged mode in which the couplers allows free rotation of the two drill string sections interconnected by the coupler relative to each other about the longitudinal axis of the drill string. Thus, in the engaged mode coupler 10 transmits rotation from section 18 to section 20, coupler 12 transmits rotation from section 20 to section 22, coupler 14 transmits rotation from section 22 to section 24, and coupler 16 transmits rotation from section 24 to section 26.

Conversely, in the disengaged mode coupler 10 allows free rotation of section 18 relative to section 20, coupler 12 allows free rotation of section 20 relative to section 22, coupler 14 allows free rotation of section 22

relative to section 24, and coupler 16 allows free rotation of section 24 relative to section 26.

Each coupler 10,12,14,16 is provided with an electronic control system (not shown) for selectively switching the coupler from the engaged mode to the disengaged mode and for selectively switching the coupler from the disengaged mode to the engaged mode. Each control system is in communication with the fluid passage of the drill string 1 and is uniquely responsive to a selected fluid pressure variation in the fluid passage.

Thus, the control system of coupler 10 is uniquely responsive to a first pressure variation, i. e. the other couplers are not responsive to the first pressure variation. Similarly, the control system of coupler 12 is uniquely responsive to a second pressure variation, the control system of coupler 14 is uniquely responsive to a third pressure variation, and the control system of coupler 16 is uniquely responsive to a fourth pressure variation.

The first, second, third and fourth pressure variations are in the form of mutually different series of pressure pulses induced to a column of drilling fluid present in the fluid passage by a fluid pump (not shown) at the drilling rig for pumping drilling fluid through the drill string.

During normal operation the drill string is rotated by the rotary table 7 in order to drill a portion of the wellbore 3, whereby each coupler 10,12,14,16 is in the engaged mode. When the drill string becomes stuck, for example due to differential sticking of the string 1 to the wellbore wall during a period of time in which an additional drill string part is added to the string 1, the fluid pump is operated to induce the first pressure variation to the column of fluid. As a result the control system of coupler 10 responses so as to switch the

coupler 10 to the disengaged mode, while the other couplers remain in the engaged mode. The rotary table 7 is then rotated, and as a result drill string section 18 is rotated due to the fact that only a limited amount of torque is needed to free drill string section 18 when compared to the torque which would be needed to free the entire drill string 1.

In a next step the pump is operated to produce again the first pressure variation to the fluid column thereby inducing the control system of coupler 10 to switch the coupler 10 back to the engaged mode. Subsequently the pump is operated to produce the second pressure variation in the fluid column. As a result the control system of coupler 12 responses so as to switch the coupler 12 to the disengaged mode, while the other couplers remain in the engaged mode. The rotary table 7 is then rotated, and as a result drill string sections 18 and 20 are simultaneously rotated. It is thereby relevant to note that no differential sticking of drill string section 18 has occurred in the short period of time between engaging coupler 10 and disengaging coupler 12.

Next the pump is operated to produce again the second pressure variation to the fluid column thereby inducing the control system of coupler 12 to switch the coupler 12 back to the engaged mode. Subsequently the pump is operated to produce the third pressure variation in the fluid column. As a result the control system of coupler 14 responses so as to switch the coupler 14 to the disengaged mode, while the other couplers remain in the engaged mode. The rotary table 7 is then rotated, and as a result drill string sections 18,20 and 22 are simultaneously rotated. No differential sticking of drill string sections 18 and 20 has occurred in the short period of time between engaging coupler 12 and disengaging coupler 14.

Next the pump is operated to produce again the third pressure variation to the fluid column thereby inducing the control system of coupler 14 to switch the coupler 14 back to the engaged mode. Subsequently the pump is operated to produce the fourth pressure variation in the fluid column. As a result the control system of coupler 16 responses so as to switch the coupler 16 to the disengaged mode, while the other couplers remain in the engaged mode. The rotary table 7 is then rotated, and as a result drill string sections 18,20,22 and 24 are simultaneously rotated. No differential sticking of drill string sections 18,20 and 22 has occurred in the short period of time between engaging coupler 14 and disengaging coupler 16.

Finally the pump is operated to produce again the fourth pressure variation to the fluid column thereby inducing the control system of coupler 16 to switch the coupler 16 back to the engaged mode. The rotary table 7 is then rotated, and as a result the entire drill string 1 is rotated. In this manner the drill string 1 has been freed from differential sticking in a stepwise manner.