CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to United States Provisional Patent Application Number

61/678,957, filed on August 2, 2012 and to United States Provisional Patent Application Number 61/711,483, filed on October 09, 2012. Both of these applications are incorporated by reference herein in their entireties.

FIELD

[0001] The subject disclosure generally relates to formation evaluation. More particularly, the subject disclosure relates to formation evaluation in gas shale and oil-bearing shale reservoirs.

BACKGROUND

[0002] Traditionally, people have tried several methods to quantify and identify characteristics of kerogen. An effective, timely method is elusive because of the nature of kerogen. Material with the same name and similar behavior within a rock may have different crystalline structure, composition, and surface effects. A method for the simultaneous estimation of mineralogy, kerogen content, and maturity using spectroscopy (scattering) is needed.

[0003] Raman spectroscopy on earth materials has been described in academic papers. For example, one reference describes the use of Raman on metamorphic rocks (metamorphic rocks have been exposed to higher pressure and temperature than the sedimentary rocks of interest to the petroleum industry). The Raman data are used to estimate mineralogy, organic content (conceptually similar to kerogen content), and a "metamorphic temperature," which is conceptually similar to maturity (maximum temperature experienced by the organic matter). Another reference describes the use of Raman on gas shales, again deriving a metamorphic temperature. None of these publications mentions potential application to cuttings analysis. Nor do they mention how that information would be used to estimate reservoir quality or completion quality or to selectively stage hydraulic fractures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] Figure 1 is a plot of absorbance as a function of wavenumbers for artificially matured kerogen samples collected via FTIR spectroscopy.

[0005] Figure 2 is a plot of fluorescence and Raman intensity as a function of wavelength for four formation samples with varied maturity.

SUMMARY

[0006] This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

[0007] Embodiments herein relate to apparatus for and a method of analyzing a sample of a subterranean formation including preparing the sample, measuring the sample using vibrational spectroscopy, and calculating a mineralogy, kerogen content, and kerogen maturity using the spectroscopy results. In some embodiments, the vibrational spectroscopy is infrared and Raman spectroscopy.

[0008] Further features and advantages of the subject disclosure will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION

[0009] The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the subject disclosure only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the subject disclosure. In this regard, no attempt is made to show structural details in more detail than is necessary for the fundamental understanding of the subject disclosure, the description taken with the drawings making apparent to those skilled in the art how the several forms of the subject disclosure may be embodied in practice. Furthermore, like reference numbers and designations in the various drawings indicate like elements.

[0010] Formation evaluation in gas shale and oil-bearing shale reservoirs involves estimation of quantities such as mineralogy, kerogen content (kerogen is solid, insoluble organic matter in sedimentary rocks), and thermal maturity (reflecting the extent of alteration of the kerogen due to thermal processes). These quantities are important for estimating the reservoir quality and completion quality of the formation, and measurement of these quantities as a function of depth is desirable in nearly every well in shale plays. Herein, we describe methods for estimating mineralogy, kerogen content, and maturity of a gas shale or oil-bearing shale from vibrational spectroscopy that includes infrared absorption and Raman scattering spectroscopies. In some embodiments, the infrared spectroscopy and Raman spectroscopy are performed simultaneously or within 30 minutes of each other. The information may be used to estimate reservoir quality and/or completion quality. Some embodiments may then use the information to selectively stage hydraulic fractures.

[0011] Some embodiments benefit from vibrational spectroscopy including comparing the Raman and IR results. If IR results indicate a mature kerogen and low kerogen content while Raman shows a mature kerogen and high kerogen content, then this sample data likely suffers from low IR sensitivity at high maturity so the IR should be discarded and Raman retained. If Raman results indicate an immature kerogen and low kerogen content while IR shows immature and high content, then this probably suffers from low Raman sensitivity at low maturities so only the IR should be retained. If they both show moderate maturity and the same kerogen content, both sets of results indicate a likely valid result with low probability of error. If both spectroscopy methods show low kerogen content, then the region is probably not economical. [0012] Also, if the mineralogies from IR and Raman agree, that gives confidence in the result. In some embodiments, the sample may be appropriate for analysis by both infrared absorption and Raman scattering. In this situation, the kerogen content and maturity from infrared absorption and Raman scattering are expected to agree. Disagreement may indicate potential low data quality. Regardless of the maturity, the mineralogy estimates from infrared absorption and Raman scattering are likely to agree. Similarly, mineralogy disagreement may indicate low data quality.

[0013] In some embodiments, the infrared spectroscopy results and Raman spectroscopy are evaluated in real time, for accuracy, and the more accurate spectroscopy measurement is used to calculate kerogen content and maturity and the other spectroscopy measurement is used to calculate mineralogy retained while the less accurate measurement omitted.

[0014] In some embodiments, calculating the mineralogy, kerogen content, and kerogen maturity includes Raman spectroscopy results and the calculating does not use infrared spectroscopy results when both infrared spectroscopy and Raman spectroscopy indicate high maturity and the kerogen content estimated from infrared spectroscopy is lower than the kerogen content estimated from Raman spectroscopy.

[0015] Calculating the mineralogy, kerogen content, and kerogen maturity includes infrared spectroscopy results and the calculating does not use Raman spectroscopy results when the infrared spectroscopy and Raman spectroscopy indicate low maturity and the kerogen content estimated from infrared spectroscopy is higher than the kerogen content estimated from Raman spectroscopy in some embodiments.

[0016] Some embodiments may use Raman spectroscopy results for the kerogen content and maturity are taken from Raman spectroscopy and use infrared spectroscopy results for the mineralogy. Conversely, some embodiments may use infrared spectroscopy results for the kerogen content and maturity and use Raman spectroscopy for the mineralogy.

[0017] In detail, for low maturity formations, this procedure can be completed using infrared absorption spectroscopy, such as Fourier Transform Infrared Spectroscopy (FTIR), Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), or attenuated total reflectance (ATR). However, at relatively high maturities, the kerogen signal near 2,900 cm ^"1 becomes weak, preventing accurate estimation of kerogen content and maturity. For high maturity formations, this procedure can be completed using Raman scattering spectroscopy alone. However, at relatively low maturities, the kerogen signal at Raman shift near 1,600 cm ^"1 becomes weak and independent of maturity, preventing accurate estimation of kerogen maturity. The combination of infrared absorption and Raman scattering results in a technique appropriate for both low and high maturity formations including gas shale or oil-bearing shale.

[0018] In some embodiments, the method comprises collecting a sample of a formation including a rock which may be core or cuttings. If the sample is cuttings from a well drilled with an oil- based mud, the cuttings are cleaned using similar techniques to the techniques disclosed in United States Patent Application Serial No. 13/446,985 filed April 13, 2012, entitled, "Method and apparatus to prepare drill cuttings for petrophysical analysis by infrared spectroscopy and gas sorption," which is herein incorporated by reference. Some cleaning embodiments expose the sample to a cleaning fluid. In some embodiments, the cleaning fluid comprises a surfactant. Some embodiments may benefit from crushing the sample to a diameter of about 10-100 micron. Some embodiments will not include exposing the sample to acid. Some embodiments may benefit when demineralizing does not occur.

[0019] Also, measurements can be made on the cuttings samples or core samples in the field or in the laboratory. Cuttings samples may include a solid collected from a drilling operation. The measurements and characterization of a sample may occur within 1 mile of a wellbore from which the sample is collected. Some embodiments may occur in less than 24 hours. The calculation of the sample properties may occur before recovering hydrocarbons begins, after producing hydrocarbons begins, during reservoir characterization during production, while drilling the formation or at multiple times during the life of a reservoir traversed by a wellbore.

[0020] Embodiments of the method also include measuring the infrared absorption spectrum of the rock using techniques disclosed in co-owned, United States Patent Application Serial No. 13/446975 filed April 13, 2012, entitled, "Method and apparatus for simultaneous estimation of quantitative mineralogy, kerogen content and maturity in gas shale and oil-bearing shale", the contents of which are herein incorporated by reference.

[0021] In the infrared absorption spectrum, the kerogen content is estimated from the area under the 2,800 - 3,000 cm ^"1 peak and the kerogen maturity is estimated from the ratio of the area under CH ₂ peaks (centered near 2849 and 2923 cm ¹ ) to the area under the CH ₃ peaks (centered near 2864 and 2956 cm ^"1 ). Also, there is a small carbonate signal in the 2,900 region. That overlaps the kerogen signal and can complicate the interpretation. In some embodiments, we measure the carbonate using an isolated signal near 2,500 cm ^"1 whose intensity is proportional to the intensity of the carbonate signal near 2,900 cm-1 , then subtracting off the carbonate signal at 2,900 cm ^"1 to get an accurate kerogen signal. In some embodiments, using the peak height provides similar information as using the area under the peaks. Figure 1 illustrates the FTIR spectra for artificially matured kerogen. This shows that as the kerogen maturity increases, FTIR may not provide an accurate estimate.

[0022] In the Raman scattering spectrum, the kerogen content is estimated from the area under the 1 , 100-1 ,700 cm ^"1 peak. The kerogen maturity is determined using the vitrinite reflectance. Some embodiments may select a Raman shift of about 100 - 1200 cm ^"1 for calculating mineralogy.

[0023] Herein, the fitting procedure (LBWF) includes two components: a Lorentzian (L) profile for the D-band and weak substructures, if any, and a Breit-Wigner-Fano (BWF) profile for the G- band. Also, spectral parameters include the peak position COD,G, the peak intensity I _D ,G, the integrated intensity AD,G, ^an d the full width at half maximum FWHM-D,G. The spectral parameters derived from the LBWF fit (FWHM-D, G, COD, COG, ID/IG and A _D /[A _D +AG]) are plotted versus VR. The spectral parameters that correlate with VR include the following.

• FWHM-D and A _D /[A _D +A _G ] for the maturity range 3<VR<7%, along with Id/Ig for 5<VR<7%

• FWHM-G ID/IG and ro _D for the maturity range 1 <VR<5 %;

• Possibly COG. [0024] In some embodiments, the kerogen maturity is estimated from the lineshape in the region 1,100 - 1,700cm ^"1 . The region contains multiple peaks including the graphitic band (G) at 1,600 cm ^"1 , and defect bands D1-D5 occurring near 1350, 1600, 1500, 1175, and 1250cm ^"1 respectively. The maturity of the kerogen can be estimated from the G/Dl ratio, the D1/(G+D1+D2) ratio, or the D5/G ratio. Figure 2 illustrates that most mature kerogen has a relatively reliable lineshape for estimating kerogen properties. The kerogen content is then estimated from both the estimated kerogen content from the infrared absorption spectrum and the estimated kerogen content from the Raman scattering spectrum. If both estimates of kerogen content are low (less than one percent), the sample is unlikely to represent an economically viable prospect.

[0025] In some embodiments, if the kerogen content estimate from infrared absorption is low (less than one weight percent) and the kerogen content estimate from Raman scattering is high (more than one weight percent), one may calculate a maturity estimate from infrared absorption. If the maturity estimate from infrared absorption is high (CH ₂ :CH ₃ ratio <2), the sample is likely to mature for accurate analysis by infrared absorption. In this situation, the kerogen content and maturity is discarded from infrared absorption and the kerogen content and maturity from Raman scattering is retained.

[0026] Conversely, when the maturity estimate from Raman scattering is low by using the defect band and graphitic band ratio analysis such as D5/G > 1.3, or other Raman analysis methods, the sample is likely too immature for accurate analysis by Raman scattering. In this situation, the maturity from Raman scattering is discarded and the maturity from infrared absorption is retained.

[0027] The mineralogy, kerogen content, and maturity determined may be used to estimate reservoir quality and completion quality and therefore to selectively stage hydraulic fractures, as disclosed in United States Patent Application Serial No. 13/447109 filed April 13, 2012, and entitled, "Reservoir and completion quality assessment in unconventional (shale gas) wells without logs or core," the contents of which are herein incorporated by reference. [0028] The workflow for simultaneously and accurately estimating mineralogy, kerogen content, and maturity in a gas shale or oil-bearing shale from Raman spectroscopy is as follows:

[0029] 1. Collect a sample of rock. The sample can be either core or cuttings. If the sample is cuttings from a well drilled with oil-based muds, the cuttings need to be cleaned similar to United States Patent Application Serial Number 13/446,985.

[0030] 2. Measure the Raman spectrum of the rock. The specifics of the spectral acquisition are known in the references. Briefly, the sample is excited (for example by laser) and the Raman scattered radiation is dispersed (for example with a grating) and detected (for example with a charge coupled device). The Raman shift range should be approximately 100 - 2,000 cm ¹ . This measurement can be performed in the laboratory or at the well-site.

[0031] 3. Optionally focus the excitation light to a small spot (approximately 1 micron), and raster to collect an image using known techniques.

[0032] 4. Optionally correct for fluorescence background using known techniques.

[0033] 5. Fit the spectra region in the range 100 - 1,200 cm-1 to obtain the mineralogy. This fitting could include any of the techniques know in the art, including partial least squares, wavelets, and linear combination (Beer's law). Pure mineral standard spectra are given in references.

[0034] 6. Take the area under the region 1,100 - 1,700 cm-1 to represent the kerogen content. An advantage of Raman spectroscopy is that this region is free of signals from minerals, permitting accurate separation of the kerogen and mineral peaks.

[0035] 7. Use the lineshape in the region 1,100 - 1,700 cm-1 to estimate the maturity. The region contains multiple peaks including the graphitic band (G) at 1 ,600 cm-1 , and defect bands Dl - D5 occurring near 1350, 1600, 1500, 1175, and 1250 cm-1 respectively. The intensity of these peaks can be used in the following equations: _ 4.4G

Dl

Γ = -455 — + 641

G + DI + D2 where L is the size of aromatic units, T is the highest temperature the kerogen has been exposed to, and G and D# are the intensities of the labeled peaks. These two quantities are maturities metrics (both L and T increase with maturity). Optionally these quantities can be related to more familiar maturity metrics such as vitrinite reflectance and/or T _max (from Rock-Eval). That is, alternatively or in parallel, some embodiments may also benefit from using the vitrinite reflectance methods described in more detail above.

[0036] 8. Use the mineralogy (step 5), kerogen content (step 6), and maturity (step 7) determined here to estimate reservoir quality / completion quality and therefore to selectively stage hydraulic fractures, as described in United States Patent Application Serial Number 13/447109, filed April 13, 2012, which is incorporated by reference herein. Some additional patent applications relate to this technology. United States Patent Application Serial Number 13/446975, filed April 13, 2012, United States Provisional Patent Application Serial Number 61/523650, filed August 15, 201 1 , United States Patent Application Serial Number 13/446995, filed April 13, 2012, and United States Patent Application Serial Number 13/359121 , filed January 27, 2012. All four of these applications are incorporated by reference herein.

[0037] Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from this invention. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. § 112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words 'means for' together with an associated function.