Method and apparatus for operating a downhole tool

1. A method of drilling a wellbore, comprising:

running a drilling assembly into the wellbore through a casing string, the drilling assembly comprising a tubular string, upper and lower underreamers, and a drill bit;

injecting drilling fluid through the tubular string and rotating the drill bit, wherein at least one of the underreamers remain locked in the retracted position;

sending a first instruction signal to the underreamers to extend one of the underreamers;

drilling and reaming the wellbore using the drill bit and the extended underreamer;

sending a trigger portion and a command portion of a second instruction signal to the underreamers via modulation of a rotational speed of the drilling assembly or modulation of a drilling fluid flow rate, thereby extending the other of the underreamers, wherein sending the second instruction signal includes:

sending the trigger portion to a control module to monitor for the command portion; and

sending the command portion to instruct the other of the underreamers to extend; and

reaming the wellbore using the extended other underreamer.

2. The method of claim 1 , wherein the upper underreamer is extended first.

3. The method of claim 1 , wherein the first instruction signal is sent via a RFID tag.

4. The method of claim 1 , wherein sending the command portion via modulation occurs after sending the trigger portion.

5. A method of drilling a wellbore, comprising:

running a drilling assembly into the wellbore through a casing string, the drilling assembly having a tubular string, a MWD tool or LWD tool, an underreamer, and a drill bit;

injecting drilling fluid through the tubular string and rotating the drill bit, wherein the underreamer remains locked in a retracted position;

sending an instruction signal having a trigger portion and a command portion to the underreamer, wherein sending the instruction signal includes:

sending the trigger portion to trigger a control module of the underreamer to monitor for the command portion, and

sending the command portion to instruct the control module to extend the underreamer; and

reaming the wellbore using the extended underreamer.

6. The method of claim 5 , wherein the instruction signal is sent via modulation of a rotational speed of the drilling assembly or modulation of a drilling fluid flow rate.

7. The method of claim 6 , wherein modulation of the rotational speed or fluid flow rate is time based.

8. The method of claim 6 , wherein modulation of the rotational speed or fluid flow rate is not time based.

9. The method of claim 5 , wherein the trigger portion is identified by measuring a rate of change of pressure over time and comparing the rate of change to a predetermined rate of change of pressure over time value.

10. The method of claim 9 , wherein the command portion includes one or more bits for specifying a target tool.

11. A method of drilling a wellbore, comprising:

running a drilling assembly into the wellbore through a casing string, the drilling assembly having a tubular string, a drill bit, and a remotely operable, two-position downhole tool;

sending a first instruction signal to the downhole tool, thereby causing the tool to move from a first position to a second position;

performing a downhole operation using the downhole tool in the second position;

sending a second instruction signal to the downhole tool, thereby causing the downhole tool to return to the first position; and

wherein at least one of the first and second instruction signals includes:

a trigger portion for triggering a control module of the downhole tool to monitor for a command portion, and

the command portion for instructing the downhole tool to move to the second position in response to the first instruction signal or return to the first position in response to the second instruction signal.

12. The method of claim 11 , wherein at least one of the trigger portion and the command portion is produced by modulating a fluid flow rate pattern of a drilling fluid, or modulating an angular speed of the tubular string, or by pressure pulses in the drilling fluid.

13. The method of claim 12 , wherein the flow rate pattern includes flowing the fluid at or above a first flow rate and then at or below a second, lower flow rate for the same period of time for at least two cycles.

14. The method of claim 11 , wherein the downhole tool is an underreamer.

15. A method of drilling a wellbore, comprising:

running a drilling assembly into the wellbore through a casing string, the drilling assembly having a tubular string, a first underreamer, a second underreamer, and a drill bit;

injecting a drilling fluid through the tubular string and rotating the drill bit, wherein at least one of the first and second underreamers remain in a retracted position; and

sending a first instruction signal to the first underreamer and a second instruction signal to the second underreamer, wherein at least one of the first and second instruction signals includes:

a trigger portion for triggering a control module of the first or second underreamers to monitor for a command portion, and

the command portion instructs the first or second underreamers to move to the second position in response to the first instruction signal or return to the first position in response to the second instruction signal.

16. The method of claim 15 , wherein at least one of the trigger portion and the command portion is produced by modulating a fluid flow rate pattern of the drilling fluid, or modulating an angular speed of the tubular string, or by pressure pulses in the drilling fluid.

17. The method of claim 15 , wherein the first underreamer is located above the second underreamer, the first instruction signal to the first underreamer is sent using an RFID tag, and the second instruction signal includes the trigger portion and the command portion.

18. The method of claim 17 , wherein the second underreamer is located below a MWD tool or LWD tool.

19. The method of claim 18 , wherein the RFID tag flows past the MWD tool or LWD tool and is received by the second underreamer.