Method for performing Raman spectroscopy within a logging while drilling instrument
A downhole tool has a tool body with an outer diameter equal to a borehole diameter, at least one cavity formed in and opening to an outer surface defining the outer diameter of the tool body, a light source, a filter, and a light detector mounted in the at least one cavity, and a window disposed at the opening of the at least one cavity, wherein the window encloses the cavity.
1. A method, comprising:
cutting a fresh surface in a wall of a borehole extending through a formation with one or more blades of a downhole tool;
taking a Raman spectroscopy measurement of the fresh surface from within the borehole extending through the formation before the fresh surface is exposed to borehole fluids with a Raman spectroscopy measurement system, the Raman spectroscopy measurement system including:
a light source disposed in a first cavity in an outer surface of a blade of the one or more blades, wherein the first cavity is enclosed by a first window flush with the outer surface of the blade;
a filter disposed in a second cavity in the outer surface of the blade, wherein the second cavity is enclosed by a second window flush with the outer surface of the blade; and
a light detector disposed in the second cavity;
generating a Raman spectrum from the Raman spectroscopy measurement; and
determining kerogen maturity in the formation from the Raman spectrum.
2. The method of claim 1 , further comprising:
determining a Raman band separation between a defect peak and a graphite peak on the Raman spectrum; and
correlating the Raman band separation with a vitrinite reflectance equivalent to determine the kerogen maturity in the formation.
3. The method of claim 1 , wherein the Raman spectroscopy measurement is a back-scatter measurement.
4. A method, comprising:
drilling a borehole through a formation;
cutting a fresh surface in a wall of the borehole with one or more blades of a downhole tool;
taking a Raman spectroscopy measurement of the fresh surface within the borehole during the drilling to obtain a Raman spectrum before the fresh surface is exposed to borehole fluids with a Raman spectroscopy measurement system, the Raman spectroscopy measurement system including:
a light source disposed in a first cavity in an outer surface of a blade of the one or more blades, wherein the first cavity is enclosed by a first window flush with the outer surface of the blade;
a filter disposed in a second cavity in the outer surface of the blade, wherein the second cavity is enclosed by a second window flush with the outer surface of the blade; and
a light detector disposed in the second cavity;
determining kerogen maturity in the formation from the Raman spectrum; and
geo-steering the drilling of the borehole based on the kerogen maturity.
5. The method of claim 4 , wherein the Raman spectroscopy measurement is sent from within the borehole to a surface of the formation via a telemetry system.
6. The method of claim 4 , wherein the Raman spectroscopy measurement system is a back-scatter Raman spectroscopy measurement system.
7. The method of claim 4 , wherein determining the kerogen maturity comprises:
determining a Raman band separation between a graphite peak at approximately 1600 cm −1 and a defect peak at approximately 1350 cm −1 on the Raman spectrum; and
correlating the Raman band separation with a known maturity indicator.
8. The method of claim 4 , wherein determining the kerogen maturity comprises:
determining a band area ratio of the area under one or more defect peaks on the Raman spectrum to the area under a graphite peak on the Raman spectrum; and
correlating the band area ratio with a known maturity indicator.
9. The method of claim 8 , wherein the band area ratio is the sum of the areas under a primary defect peak and one or more secondary defect peaks over the area under the graphite peak on the Raman spectrum.
10. A method, comprising:
deploying a downhole tool into a borehole extending through a formation, wherein the downhole tool comprises:
a tool body one or more blades extending radially from the tool body;
a first cavity formed in an outer surface of a blade of the one or more blades, wherein a first window encloses the first cavity and is flush with the outer surface of the blade;
a light source mounted in the first cavity;
a filter mounted in the at least one a second cavity formed in the outer surface of the blade, wherein a second window encloses the second cavity and is flush with the outer surface of the blade; and
a light detector mounted in the second cavity, wherein the light source and the light detector are configured to rake back-scatter measurements;
cutting a fresh surface in a wall of the borehole with the downhole tool with the one or more blades;
taking a Raman spectroscopy measurement of the fresh surface from within the borehole with the downhole tool to obtain a Raman spectrum before the fresh surface is exposed to borehole fluids;
determining kerogen maturity in the formation from the Raman spectrum; and
geo-steering drilling of the borehole based on the kerogen maturity.
11. The method of claim 10 , wherein the downhole tool is a Logging While Drilling instrument for use in oil and gas wells.
12. The method of claim 10 , wherein at least one of the first cavity or the second cavity comprises a cooling device enclosed therein.
13. The method of claim 10 , wherein the downhole tool is part of a bottom hole assembly.
14. The method of claim 10 , wherein the one or more blades have a larger outer diameter than a diameter of the borehole.
15. The method of claim 10 , wherein the light source is a solid-state laser.
16. The method of claim 10 , wherein the filter is a prism or diffraction grating.
17. The method of claim 10 , wherein the light detector is a charge coupled device.