Well monitoring by means of distributed sensing means
Methods and apparatus for downhole monitoring are provided by interrogating an unmodified optic fiber ( 102 ) arranged along the path of a well bore ( 106 ) to provide a distributed acoustic sensor and sampling data gathered from a plurality of contiguous sensing portions of the fiber. The sampled data is processed to determine one or more well bore parameters. The well bore parameters may comprise a well condition profile, and may be obtained in response to an acoustic stimulus.
1. A method for downhole monitoring comprising:
interrogating an unmodified optic fibre arranged along the path of a well bore to provide distributed acoustic sensing;
simultaneously sampling data gathered from a plurality of contiguous portions of said fibre; and
processing said data to determine one or more well bore parameters;
wherein processing said data includes detecting the response to an acoustic stimulus, and
wherein said parameter is a well condition profile.
2. A method according to claim 1 , wherein said acoustic stimulus comprises a perforation event, and the step of determining said well condition profile comprises determining the speed of an acoustic pulse in the well bore casing.
3. A method according to claim 1 , wherein processing said data includes detecting at least one fracture event, and said parameters include at least one of: depth of fracture, rate of fracturing, range of fracture, and fracture density.
4. A method according to claim 1 , wherein processing said data includes detecting well fluid flow, and said parameters include flow rate and depth of flow.
5. A method according to claim 1 , wherein processing comprises detecting multiple event types.
6. A method according to claim 1 , wherein the sampling rate is greater than or equal to 1 kHz.
7. A method according to claim 1 , wherein the sampling rate is greater than or equal to 5 kHz.
8. A method according to claim 1 , wherein at least 100 channels are sampled simultaneously.
9. A method according to claim 1 , wherein at least 250 channels are sampled simultaneously.
10. A method according to claim 1 , wherein said channels correspond to contiguous portions of fibre less than or equal to 10 m in length.
11. A method according to claim 1 , wherein processing said data comprises performing transient detection.
12. A method according to claim 11 , wherein transient detection includes maintaining a measure of mean absolute deviation.
13. A method according to claim 12 , wherein said measure of mean absolute deviation is updated by exponential averaging using a decay term, N.
14. A method according to claim 13 , wherein said decay term N is adaptively varied.
15. A method according to claim 1 , wherein processing said data comprises frequency analysis.
16. A method according to claim 1 , wherein said data is processed in real time.
17. A method according to claim 1 , further comprising adjusting interrogation parameters to vary the contiguous portions of fibre from which data is sampled.
18. A method according to claim 17 , wherein said interrogation parameters are adjusted adaptively in response to detected events.
19. A system for downhole monitoring, said system comprising:
a fibre optic interrogator adapted to provide distributed acoustic sensing on an unmodified fibre arranged along the path of a well bore;
a sampler arranged to sample a plurality of channels output from said interrogator simultaneously to provide acoustic data from a plurality of contiguous portions of said fibre at each of a plurality of times; and
a data analyser adapted to process said sampled data to detect well events and output parameters associated with detected events;
wherein said data analyser is configured to detect the response to an acoustic stimulus, and determine a well condition profile.
20. A system according to claim 19 adapted to detect multiple event types.
21. A system according to claim 19 , adapted to detect both fracture events and fluid inflow.