Electromagnetic surveying for hydrocarbon reservoirs
View Patent ↗An electromagnetic survey method for surveying an area that potentially contains a subterranean hydrocarbon reservoir. The method comprises detecting a detector signal in response to a source electromagnetic signal, resolving the detector signal along at least two orthogonal directions, and comparing phase measurements of the detector signal resolved along these directions to look for a phase separation anomaly indicative of the presence of a buried hydrocarbon layer. The invention also relates to planning a survey using this method, and to analysis of survey data taken using this survey method. The first and second data sets may be obtained concurrently with a single horizontal electric dipole source antenna. The method is also largely independent of a source-detector pair's relative orientation and so provides for good spatial coverage and easy-to-perform surveying.
1. An electromagnetic survey method for surveying an area that is thought or is known to contain a subterranean hydrocarbon reservoir, comprising:
transmitting a source electromagnetic signal from a source location;
detecting a detector signal at a detector location in response thereto;
obtaining survey data indicative of phase difference between first and second components of the detector signal resolved along different first and second directions respectively; and
forming the phase difference between the first and second components to determine the presence or absence of a subterranean hydrocarbon formation.
2. The survey method of claim 1 , wherein the first and second components are radial and azimuthal with reference to the source location-receiver location geometry.
3. The survey method of claim 1 , wherein the first and second components are vertical and azimuthal with reference to the source location-receiver location geometry.
4. The survey method of claim 1 , wherein the first and second components are vertical and radial with reference to the source location-receiver location geometry.
5. The survey method of claim 1 , further comprising obtaining survey data indicative of phase of a third component of the detector signal resolved along a third direction orthogonal to the first and second directions.
6. The survey method of claim 5 , wherein the first and second and third components are vertical, radial and azimuthal with reference to the source location-receiver location geometry.
7. The survey method of claim 1 , wherein the first and second directions are orthogonal.
8. The survey method of claim 1 , wherein the source electromagnetic signal is broadcast from an antenna mounted on a submersible vehicle which is towed over the survey area to move the source location.
9. The survey method of claim 1 , wherein the source location is fixed.
10. The survey method of claim 1 , wherein the source electromagnetic signal is emitted at different frequencies to obtain survey data at a plurality of different frequencies.
11. The survey method of claim 1 , wherein the source electromagnetic signal is emitted at a frequency of between 0.01 Hz and 10 Hz.
12. A method of analysing results from an electromagnetic survey of an area that is thought or known to contain a subterranean hydrocarbon reservoir, comprising:
providing survey data indicative of phase difference between first and second components of a detector signal resolved along different first and second directions respectively;
extracting the phase differences from the survey data; and
determining a metric from the phase differences to determine the presence or absence of hydrocarbon.
13. The analysis method of claim 12 , wherein the first and second components are radial and azimuthal with reference to the source location-receiver location geometry.
14. The analysis method of claim 12 , wherein the first and second components are vertical and azimuthal with reference to the source location-receiver location geometry.
15. The analysis method of claim 12 , wherein the first and second components are vertical and radial with reference to the source location-receiver location geometry.
16. The analysis method of claim 12 , further comprising obtaining survey data indicative of phase of a third component of the detector signal resolved along a third direction orthogonal to the first and second directions.
17. The analysis method of claim 16 , wherein the first and second and third components are vertical, radial and azimuthal with reference to the source location-receiver location geometry.
18. The analysis method of claim 12 , wherein the first and second directions are orthogonal.
19. The analysis method of claim 18 , wherein the phase differences are extracted by rotationally transforming the survey data from an instrument frame to a source frame.
20. A computer program product comprising a machine readable medium bearing machine-executable instructions for implementing the method of claim 12 .
21. A method of planning an electromagnetic survey of an area that is thought or known to contain a subterranean hydrocarbon reservoir, comprising:
creating a model of the area to be surveyed including a seafloor, a rock formation containing a postulated hydrocarbon reservoir beneath the seafloor, and a body of water above the seafloor;
setting values for depth below the seafloor of the postulated hydrocarbon reservoir and resistivity structure of the rock formation;
performing a simulation of an electromagnetic survey in the model; and obtaining from the model phase differences between first and second components of a detector signal resolved along different first and second directions respectively.
22. The planning method of claim 21 , wherein the first and second components are two of radial, vertical and azimuthal with reference to the source location-receiver location geometry.
23. The planning method of claim 21 , further comprising: repeating the simulation for a number of distances between a source and a detector and frequencies in order to select optimum surveying conditions in terms of source-to-detector distance for probing the hydrocarbon reservoir.
24. A computer program product comprising a machine readable medium bearing machine-executable instructions for implementing the planning method of claim 21 .
25. A method for obtaining hydrocarbon from an area that contains a subterranean hydrocarbon reservoir; comprising:
performing an electromagnetic survey of the area to obtain survey data indicative of phase differences between first and second components of a detector signal resolved along different first and second directions respectively;
determining a metric from the phase differences that is predictive of the presence or absence of hydrocarbon;
identifying the subterranean hydrocarbon reservoir using the metric;
penetrating the subterranean hydrocarbon reservoir with a hydrocarbon-producing well;
extracting hydrocarbon from the subterranean reservoir using the hydrocarbon-producing well.
26. A method for obtaining hydrocarbon from an area that contains a subterranean hydrocarbon reservoir, comprising:
extracting hydrocarbon from the subterranean hydrocarbon reservoir, the subterranean hydrocarbon reservoir having been determined to contain hydrocarbon by means of an electromagnetic survey comprising the steps of:
performing an electromagnetic survey of the area to obtain survey, data indicative of the phase differences between first and second components of a detector signal resolved along different first and second directions respectively;
determining a metric from the phase differences that is predictive of the presence or absence of hydrocarbon; and
identifying the subterranean hydrocarbon reservoir using the metric.
27. A method according to claim 26 , wherein the extracting step includes penetrating the subterranean hydrocarbon reservoir with a hydrocarbon-producing well.