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A method of monitoring gas in a downhole environment is discussed which provides downhole a mid-infrared light emitting diode, operates the diode to transmit respective infrared signals on a first optical path extending from the diode through a downhole gas sample and a second optical path extending

A method of monitoring gas in a downhole environment is discussed which provides downhole a mid-infrared light emitting diode, operates the diode to transmit respective infrared signals on a first optical path extending from the diode through a downhole gas sample and a second optical path extending from the diode through a reference gas sample, detects the transmitted infrared signals, and determines the concentration of a component of the downhole gas sample from the detected signals.

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What is claimed is: 1. A method of monitoring gas in a downhole environment, comprising: providing downhole a mid-infrared light emitting diode; operating the diode to transmit respective infrared signals on a first optical path extending from the diode through a downhole gas sample and a second o

What is claimed is: 1. A method of monitoring gas in a downhole environment, comprising: providing downhole a mid-infrared light emitting diode; operating the diode to transmit respective infrared signals on a first optical path extending from the diode through a downhole gas sample and a second optical path extending from the diode through a reference gas sample; detecting the transmitted infrared signals; determining the concentration of a component of the downhole gas sample from the detected signals; wherein the first optical path is substantially free of water and hydrocarbon in their liquid states. 2. A method according to claim 1, wherein the light emitting diode is operated in forward bias. 3. A method according to claim 1, wherein the light emitting diode is operated in reverse bias. 4. A method according to claim 1, wherein the identified component is CO2. 5. A method according to claim 1, wherein the identified component is CH4. 6. A method according to claim 1, wherein the identified component is H2S. 7. A method according to claim 1, wherein the light emitting diode has a room temperature peak emission wavelength in the range from 3 to 5 μm. 8. A method according to claim 1, wherein the reference gas sample comprises a predetermined concentration of said component. 9. A method according to claim 1, wherein the light emitting diode is operated to transmit a further infrared signal on a third optical path extending from the diode, the third optical path having a defined absorbance at the emission wavelength of the diode. 10. A method according to claim 9, wherein the defined absorbance is zero. 11. A method according to claim 1, wherein the length of the first optical path is adjusted or selected according to the expected concentration of said component. 12. A method according to claim 1, further comprising filtering the downhole gas sample to substantially remove liquids therefrom. 13. A method according to claim 1, wherein a plurality of mid-infrared light emitting diodes are provided, each diode being adapted for use in a respective temperature range, and the diodes are selectively operated according to the downhole temperature. 14. A method according to claim 1, wherein the first optical path comprises a waveguide which passes through the downhole gas sample, the infrared signal on the first optical path being transmitted along the waveguide by internal reflection. 15. A method according to claim 1, wherein a plurality of respective photodiode detectors are provided to detect the transmitted infrared signals. 16. A method according to claim 1, further comprising extracting the downhole gas sample through a gas extraction membrane to substantially remove liquids therefrom, the membrane allowing the transport of gases across it but preventing the transport of liquid water and hydrocarbon. 17. A sensor for monitoring gas in a downhole environment, comprising: a mid-infrared light emitting diode; a compartment for containing a reference gas sample; detection means for detecting respective infrared signals transmitted on first and second optical paths extending from the diode, the apparatus being arranged such that in use the first optical path traverses a downhole gas sample and the second optical path traverses said compartment; a processor for determining the concentration of a component of the downhole gas sample from the detected signals; wherein the sensor is arranged such that, in use, the first optical path is substantially free of water and hydrocarbon in their liquid states. 18. A sensor according to claim 17, wherein the light emitting diode has a room temperature peak emission wavelength in the range from 3 to 5 μm. 19. A sensor according to claim 17, wherein the detection means is arranged to detect a further infrared signal transmitted on a third optical path extending from the light emitting diode, the third optical path having a defined mid-infrared absorbance at the emission wavelength of the diode. 20. A sensor according to claim 19, wherein the defined absorbance is zero. 21. A sensor according to claim 17, wherein the length of the first optical path is adjustable or selectable according to the expected concentration of said component. 22. A sensor according to claim 17, further comprising a filter for filtering the downhole gas sample to substantially remove liquids therefrom. 23. A sensor according to claim 17, comprising a plurality of mid-infrared light emitting diodes, each diode being adapted for use in a respective temperature range, and the diodes being selectively operable according to the downhole temperature. 24. A sensor according to claim 17, further comprising: at least one optical window on the first optical path, the window defining a boundary of the downhole gas sample; and an ultrasonic cleaner for removing liquid from the surface of the or each window. 25. A sensor according to claim 17, further comprising a waveguide which passes through the downhole gas sample, the infrared signal on the first optical path being transmitted along the waveguide by internal reflection. 26. A sensor according to claim 25, further comprising an ultrasonic cleaner for removing liquid from the surface of the waveguide. 27. A sensor according to claim 17, wherein the detection means comprises a plurality of respective photodiode detectors for detecting the transmitted infrared signals. 28. A sensor according to claim 17 which is located downhole. 29. A well tool comprising the sensor of claim 17. 30. A well tool according to claim 29 which is a production logging tool. 31. A well tool according to claim 29 which is a wireline sampling tool. 32. A sensor according to claim 17, further comprising a gas extraction membrane for extracting the downhole gas sample and substantially removing liquids therefrom, the membrane allowing the transport of gases across it but preventing the transport of liquid water and hydrocarbon.