METHOD AND APPARATUS FOR ACOUSTIC TELEMETRY IN A BOREHOLE

摘要

1. A borehole communication system for telemetry through a compressible drilling fluid comprising: - a drilling fluid source configured to supply drilling fluid under pressure through a conduit towards a drill bit; - a gas inlet in fluid communication with the conduit configured to supply gas into the drilling fluid thereby rendering the drilling fluid downstream of the inlet compressible; - a pulser in the borehole configured to generate pressure pulses in the compressible drilling fluid corresponding to a predetermined pattern; - a reflector positioned downstream from the gas inlet dimensioned so as to cause in response to an incident pressure wave traveling from the pulser towards the surface, a reflected pressure wave having the same pressure polarity as the incident pressure wave; and - a pressure sensor positioned downstream of the reflector adapted to sense pressure in the compressible drilling fluid and generate electrical signals corresponding to the sensed pressure. 2. The system according to claim 1 wherein the conduit includes a drill string and surface conduits and the gas inlet is located on one of the surface conduits. 3. The system according to claim 2 wherein the pulser is located in a bottom hole assembly in the vicinity of the drill bit. 4. The system according to claim 1 further comprising a processor in electrical communication with the pressure sensor adapted to demodulate the electrical signals generated by the pressure sensor. 5. The system according to claim 1 wherein the energy of an incident pressure wave absorbed by the reflector is greater than 20%. 6. The system according to claim 5 wherein the energy of an incident pressure wave absorbed by the reflector is greater than 30%. 7. The system according to claim 6 wherein the energy of an incident pressure wave absorbed by the reflector is greater than 40%. 8. The system according to claim 1 wherein the reflector has a value of λ of greater than about 0.25. 9. The system according to claim 8 wherein the reflector has a value of λ of greater than about 0.5. 10. The system according to claim 9 wherein the reflector has a value of λ of greater than about 1. 11. The system according to claim 1 wherein the reflector is a fixed orifice plate. 12. The system according to claim 1 wherein the reflector comprises an adjustable aperture. 13. The system according to claim 1 wherein the compressible drilling fluid is highly compressible. 14. The system according to claim 1 wherein the pressure sensor is positioned on the conduit downstream of the reflector at a distance of more than about 12 times the diameter of the conduit from the reflector. 15. The system according to claim 14 wherein the pressure sensor is positioned more than about 60 times the diameter of the conduit from the reflector. 16. The system according to claim 1 further comprising: - a gas supply in fluid communication with the gas inlet via a gas conduit; and - first and second pressure sensors positioned on either side of a flow restriction located in the gas conduit. 17. A method for detecting telemetry signals traveling from a downhole source towards the surface through a compressible drilling fluid comprising the steps of: - reflecting incident pressure waves in the compressible drilling fluid traveling towards the surface, thereby generating reflected pressure waves having the same pressure polarity as the incident pressure waves; and - sensing the pressure of the compressible drilling fluid at a location downstream of where the reflections are generated. 18. The method of claim 17 wherein the pressure is sensed using a pressure sensor, and further comprising the step of demodulating electrical signals generated by the pressure sensor using a processor in electrical communication with the pressure sensor. 19. The method of claim 17 wherein the energy of an incident pressure wave absorbed during reflection is greater than 20%. 20. The method of claim 19 wherein the energy of an incident pressure wave absorbed during reflection is greater than 40%. 21. The method of claim 17 wherein a reflector is used to generate the reflections, the reflector having a value of λ of greater than about 0.25. 22. The method of claim 21 wherein the reflector has a value of λ of greater than about 1. 23. The method of claim 17 wherein an adjustable aperture is used to generate the reflections. 24. The method of claim 17 wherein a reflector is used to generate the reflections, and the pressure is sensed at a location in a conduit located downstream at a distance of more than about 12 times the diameter of the conduit from the reflector. 25. The method of claim 24 wherein the pressure is sensed at a position more than about 60 times the diameter of the conduit from the reflector.