Detection of solid organic material and fluids in a shale rock by means of low field NMR

Abstract

A method and device for detection of solid organic matter and fluids in a shale rock by means of low field Nuclear Magnetic Resonance (NMR) in a single measurement, by submitting a rock sample to a 2D NMR assay comprising applying a 2D pulse sequence with a saturation-recovery, or inversion-recovery, in an indirect dimension and an FID-CPMG in a direct dimension. The method can be used as an analytical technique for rock samples from unconventional hydrocarbon reservoirs.

Claims

1. A method for detection of solid organic matter and fluids in a geological formation sample by means of low field NMR (Nuclear Magnetic Resonance) in a single measurement, the method comprising the steps of: i. providing a rock sample; ii. submitting the sample to a 2D Nuclear Magnetic Resonance (NMR) assay comprising the application of a 2D pulse sequence with a saturation-recovery, or inversion-recovery, in an indirect dimension and a FID-CPMG in a direct dimension; iii. obtaining a 2D T.sub.1-T.sub.2 map from the NMR assay; iv. determining relaxation times; v. assigning solid organic matter and fluid contributions based on the relaxation times, wherein solid organic matter is assigned transverse relaxation times shorter than approximately 200 microseconds; vi. quantifying the amount of .sup.1H nuclei in predefined regions in the 2D map by using a pre-calibration of an equipment response.

2. The method of claim 1, wherein the 2D T.sub.1-T.sub.2 map is obtained by applying a 2D numerical inversion algorithm.

3. The method of claim 1, wherein the rock sample is selected from tight gas sandstones, oil or tar sands, heavy oil, gas shales, coalbed methane, oil shales, gas hydrates, shale gas, shale oil, other low-permeability tight formations, solid bitumen and extracted organic matter.

4. The method of claim 1, wherein the rock sample is in a form selected from a cylindrical plug of various dimensions, a sidewall core, drill cuttings, or ground rock.

5. The method of claim 1, wherein in step ii) a dipolar refocusing pulse sequence such as dipolar echo or magic sandwich echo is applied prior to the FID-CPMG in the direct dimension.

6. The method of claim 1, wherein in step v) fluids are assigned transverse relaxation times longer than approximately 200 microseconds.

7. The method of claim 1, wherein the pre-calibration of step vi) is carried out using a calibration sample, such as a known volume of water or water doped with CuSO.sub.4, with a transverse relaxation time in the order of 100 ms.

8. The method of claim 1, wherein the method is carried out by using an NMR equipment with magnetic field strength corresponding to a .sup.1H resonance frequency of 1.5 MHz to 60 MHz.

9. The method of claim 1, wherein a receiver dead time is shorter than 50 μs and preferably with an active resonant circuit Q-factor modulation.

10. The method of claim 1, further comprising a variable data acquisition time, or dwell time, and using programmable digital filters or analog filters.

11. The method of claim 1, further comprising characterizing the total organic content of the sample.

12. The method of claim 1, further comprising the steps of: acquiring the decay after a Hahn Echo following the first FID decay; or acquiring the decay of the FID and CPMG in any other single experiment.

13. The method of claim 1, wherein the method is carried out using a NMR laboratory or a well-logging tool, either at low or high field.

14. The method of claim 9, wherein the method is carried out using a NMR laboratory or a well-logging tool, either at low or high field.

15. A device for detecting solid organic matter and fluids in a shale rock, the device comprising: a. an NMR equipment configured to: perform a 2D NMR assay comprising the use of a 2D pulse sequence with a saturation-recovery, or inversion-recovery, in an indirect dimension and an FID-CPMG in a direct dimension; obtain a 2D T.sub.1-T.sub.2 map from the NMR assay by using a two-dimensional numerical inversion algorithm; and b. processing means configured to analyze data from the 2D T.sub.1-T.sub.2 map in order to: assign solid and fluid contributions based on relaxation times, wherein solid organic matter is assigned transverse relaxation times shorter than approximately 200 microseconds; and quantify the amount of .sup.1H nuclei in predefined regions in the 2D map by using a pre-calibration of the equipment response.

16. The device of claim 15, wherein the NMR equipment is an instrument with magnetic field strength corresponding to a .sup.1H resonance frequency of 1.5 MHz to 60 MHz.

17. The device of claim 15, wherein the NMR equipment is a laboratory instrument or a well-logging tool, either at low or high field.

18. The device of claim 15, wherein the NMR equipment has a receiver dead time shorter than 50 μs with active resonant circuit Q-factor modulation.

19. The device of claim 15, wherein the pre-calibration of the response is done using a sample with a known volume of water or water doped with CuSO.sub.4 with a transverse relaxation time in the order of 100 ms.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1A shows a diagram of the standard SR-CPMG pulse sequence to acquire a T.sub.1-T.sub.2 map using saturation recovery (SR) for T.sub.1 encoding and acquiring a CPMG for T.sub.2. The dots represent the acquired data.

(2) FIG. 1B shows a diagram of the proposed SR-FID-CPMG sequence which consists of a T.sub.1-T.sub.2 pulse sequence where the FID decay is also acquired to obtain the contribution of the fast-decaying components. The dots represent the acquired data.

(3) FIG. 1C shows a diagram of the proposed SR-FID-ECHO-CPMG sequence which consists of a T.sub.1-T.sub.2 pulse sequence where the Hahn Echo is also acquired after the FID decay to obtain the contribution of fast-decaying components. The dots represent the acquired data.

(4) FIG. 2A shows an NMR (2.2 MHz) 2D T.sub.1-T.sub.2 map of the as-received core plug sample Shale #1 using an SR-CPMG.

(5) FIG. 2B shows an NMR (2.2 MHz) 2D T.sub.1-T.sub.2 map of the as-received core plug sample Shale #1 using an SR-FID-CPMG.

(6) FIG. 2C shows an NMR (2.2 MHz) 2D T.sub.1-T.sub.2 map of the as-received core plug sample Shale #1 using an SR-FID-ECHO-CPMG.

(7) FIG. 3A shows an NMR (2.2 MHz) 2D T.sub.1-T.sub.2 map of the as-received core plug sample Shale #2 using an SR-CPMG.

(8) FIG. 3B shows an NMR (2.2 MHz) 2D T.sub.1-T.sub.2 map of the as-received core plug sample Shale #2 using an SR-FID-CPMG.

(9) FIG. 3C shows an NMR (2.2 MHz) 2D T.sub.1-T.sub.2 map of the as-received core plug sample Shale #2 using an SR-FID-ECHO-CPMG.

(10) FIG. 3D shows an NMR (19.9 MHz) 2D T.sub.1-T.sub.2 map of the as-received core plug sample Shale #2 using an SR-CPMG.

(11) FIG. 3E shows an NMR (19.9 MHz) 2D T.sub.1-T.sub.2 map of the as-received core plug sample Shale #2 using an SR-FID-CPMG.

(12) FIG. 4A shows an NMR (2.2 MHz) 2D T.sub.1-T.sub.2 map of the as-received outcrop sample Shale #3 using an SR-FID-CPMG.

(13) FIG. 4B shows an NMR (2.2 MHz) 2D T.sub.1-T.sub.2 map of the as-received outcrop sample Shale #3 using an SR-FID-ECHO-CPMG.

(14) FIG. 4C shows an NMR (19.9 MHz) 2D T.sub.1-T.sub.2 map of the as-received outcrop sample Shale #3 using an SR-FID-CPMG.

(15) FIG. 4D shows an NMR (19.9 MHz) 2D T.sub.1-T.sub.2 map of the as-received outcrop sample Shale #3 using an SR-FID.

(16) FIG. 5A shows the total signal per unit sample mass in the region labeled as “OM region” in the NMR (2.2 MHz) 2D T.sub.1-T.sub.2 map in FIG. 4A, as a function of TOC, for Shale #3 and twenty-eight other as-received samples from the same outcrop. An FID-CPMG was used for the T.sub.2 acquisition and a saturation recovery was used for T.sub.1 encoding. R.sup.2 stands for the coefficient of determination of the least squares linear regression represented by the dashed line.

(17) FIG. 5B shows the total signal per unit sample mass in the region labeled as “OM region” in the NMR (2.2 MHz) 2D T.sub.1-T.sub.2 map in FIG. 4B, as a function of TOC, for Shale #3 and twenty-eight other as-received samples from the same outcrop. An FID-ECHO-CPMG was used for the T.sub.2 acquisition and a saturation recovery was used for T.sub.1 encoding. R.sup.2 stands for the coefficient of determination of the least squares linear regression represented by the dashed line.

DETAILED DESCRIPTION OF THE INVENTION

(18) Applying the 2D pulse sequences of the present invention, the organic matter contained in shale rock can be detected and quantified in the laboratory with low-field NMR in cores, plugs, sidewall cores, drill cuttings, and ground rock. The same sequence could be programmed in the NMR well-logging tool in order to detect the signal of organic matter in a well profile.

(19) The present invention will be described in detail on the basis of the following examples and in relation to the appended figures, which illustrate preferred embodiments of the invention.

(20) Two rock samples (plugs extracted from a core) corresponding to the oil window in the Vaca Muerta Formation in Argentina were designated as Shale #1 and Shale #2. Shale #3 is an outcrop sample from the Vaca Muerta Formation. The rocks were measured as-received.

(21) Shale #1 has a TOC of 4.34 wt % as calculated from a Rock-Eval 6 pyrolysis experiment.

(22) Shale #2 has a TOC of 4.59 wt % as calculated from a Rock-Eval 6 pyrolysis experiment.

(23) Shale #3 has a TOC of 10.4 wt % as calculated from a Rock-Eval 6 pyrolysis experiment.

(24) The NMR experiments at 2.2 MHz for .sup.1H were carried out in an Oxford Geospec2 instrument. The data for establishing a correlation of longitudinal and transverse relaxation times were acquired with saturation recovery (SR) to encode T.sub.1 varying the delay (τ.sub.1) from 21 μs to 390 ms in 50 logarithmically spaced steps. In the experimental setup, SR showed a better performance than the original Inversion-Recovery (IR) presented by Rondeau-Mouro et al. (ibid). The pulse durations are 9 μs and 18 μs for the 90° and 180° pulses, respectively. The T.sub.2 acquisition was done with different configurations:

(25) In FIG. 1A, a SR-CPMG pulse sequence is used with an echo time t.sub.E=200 μs and 500 echoes. Each datum was acquired at the top of the spin-echo, as schematized in the figure.

(26) In a second configuration, FIG. 1B, a SR-FID-CPMG pulse sequence was used. The CPMG is acquired with the parameters specified before, and the FID decay was acquired with a dwell time DW=8 μs. For the dwell time chosen, the dead time is 33 μs and 5 points can be acquired before the first 180° pulse, as schematized in the figure.

(27) In order to acquire more data points during the FID decay, a SR-FID-ECHO-CPMG pulse sequence was implemented, see FIG. 1C. For this configuration, 18 points were acquired in the first FID with a total decay time of τ=200 s and the CPMG was acquired with the same parameters as specified before.

(28) The numerical inversion was performed by using the adaptive truncation of matrix decomposition introduced by Teal and Eccles (P. D. Teal, C. Eccles, Adaptive truncation of matrix decompositions and efficient estimation of NMR relaxation distributions, Inverse Problems 31 (2015) 045010). However, there are other methods that can be used to invert the data as specified above.

(29) FIG. 2A shows the 2D T.sub.10-T.sub.2 map for Shale #1, as-received, using a CPMG pulse sequence for detection (FIG. 1A). Due to the echo time used (t.sub.E=200 μs), only the water and light oil (not heavy) present in both organic and inorganic pores contribute to the signal. However, the signal coming from solid components decays faster and those contributions do not show in the T.sub.1-T.sub.2 correlation map. In order to acquire these signals, the FID decay is also measured in the same experiment, as schematically shown in FIG. 1B. In FIG. 2B a component with T.sub.2=15 μs is present, which does not appear with the traditional approach that uses only the long echo time available in low field laboratory instruments and well-logging tools. The contribution which appears within T.sub.1/T.sub.2=4-100 agrees with what is disclosed by R. Kausik et al. (R. Kausik, K. Fellah, E. Rylander, P. Singer, R. Lewis, S. Sinclair, NMR relaxometry in shale and implications for logging, PETROPHYSICS 57 (2016) 339-350) as the signal from bitumen. Unexpectedly, an extra contribution was observed above T.sub.1/T.sub.2=100. According to R. Kausik et al., this signal with the shortest T.sub.2 and high T.sub.1, is correlated to kerogen. By acquiring the FID-CPMG decay, the instrumental limitation is therefore overcome. In FIG. 2C, an FID-ECHO-CPMG was used for T.sub.2 acquisition. The signal correlated to kerogen appears about a ratio T.sub.1/T.sub.2=450.

(30) The same experiments were carried out for a second sample (Shale #2) where a similar result was found, see FIGS. 3A to 3C. The signal correlated to kerogen appears above T.sub.1/T.sub.2=100 for Shale #2.

(31) The same experiments using an SR-FID-CPMG and an SR-FID-ECHO-CPMG pulse sequence were carried out for a third sample (Shale #3) where still a similar result was found, see FIGS. 4A and 4B. The signal correlated to kerogen appears around the ratio T.sub.1/T.sub.2=750-770 for Shale #3. NMR experiments at 19.9 MHz were also carried out for Shale #2, see FIGS. 3D and 3E.

(32) The NMR experiments at 19.9 MHz were performed in a Bruker Minispec MQ20 Time-Domain Spectrometer, equipped with permanent magnets that provide an operating magnetic field of ca. 0.5 T. The correlation of longitudinal and transverse relaxation times was acquired with saturation recovery (SR) to encode T.sub.1 varying the delay (τ.sub.1) from 25 μs to 1500 ms in 50 logarithmically spaced steps. The pulse durations are 4 μs and 8 μs for the 90° and 180° pulses, respectively. The T.sub.2 acquisition was done with different configurations:

(33) In FIG. 3D, a SR-CPMG pulse sequence is used with an echo time t.sub.E=100 μs and 1000 echoes.

(34) In a second configuration, that corresponds to FIG. 3E, a SR-FID-CPMG pulse sequence was used. The CPMG is acquired with the parameters specified before, and the FID decay was acquired with 80 points and a dwell time DW=0.6 μs. The dead time is 14 μs.

(35) For shales collected from outcrop zones (Shale #3), the amount of fluid is negligible, and mainly organic matter and clay bound water are present. They have a rapid signal decay that can be acquired with a complete FID. The limitation is that the longest relaxation time must be shorter than the T.sub.2* of the equipment, which is of 1.4 ms for the used Bruker Minispec MQ20. For this kind of samples, it is sufficient to acquire an SR-FID sequence with the same list for τ.sub.1 and acquisition of 10000 points with DW=0.4 μs (M. Sadegh Zimiri, B. MacMillan, F. Marica, J. Guo, L. Romero-Zerón, B. J. Balcom, Petrophysical and geochemical evaluation of shales using magnetic resonance T.sub.1-T.sub.2* relaxation correlation, Fuel 284 (2021) 119014). This is shown in FIG. 4D. A comparison can be made with a SR-FID-CPMG for the same sample (FIG. 4C).

(36) The total organic carbon was obtained by Rock-Eval 6 pyrolysis for the as-received samples Shale #1, Shale #2, and Shale #3. The same procedure was performed for the other 28 as-received samples from the same outcrop as Shale #3.

(37) The TOC values for the 29 outcrop samples mentioned above strongly correlate with the total signal per unit mass in the region in the NMR (2.2 MHz) T.sub.1-T.sub.2 map defined by 3.Math.10.sup.−3 ms<T.sub.2<3.Math.10.sup.−2 ms and 1.5 ms<T.sub.1<10.sup.4 ms and labeled as “OM region” in FIGS. 4A and 4B. This positive correlation is found both when the NMR (2.2 MHz) T.sub.1-T.sub.2 map is acquired with either an SR-FID-CPMG or an SR-FID-ECHO-CPMG experiment, as shown in FIGS. 5A and 5B, respectively.

(38) The strategy of acquiring the already available magnetization after the first radiofrequency pulse is a great step towards the accurate characterization that will allow further developments, like the extension of the FID acquisition period by introducing a Hahn echo followed by a CPMG sequence. An alternative consists in the elimination of the blind window at short times due to the receiver dead time by using dipolar refocusing sequences such as those used to correlate short time signals with total organic carbon content.

(39) The present invention has demonstrated that the contribution of components with short relaxation times in unconventional reservoir rocks can be observed at low fields using the existing NMR instruments. By measuring T.sub.1-T.sub.2 maps with the acquisition of an SR-FID-CPMG or an SR-FID-ECHO-CPMG experiment, signals coming from environments with relaxation time shorter than the echo time can also be sensed. This methodology uses standard low field NMR instruments applying a new acquisition sequence, with no need for additional complements.

(40) It is to be understood that the above description is intended to be illustrative and not restrictive. Many embodiments will be apparent to those skilled in the art upon reviewing the above description. The scope of the invention should therefore, be determined not with reference to the above description, but should instead be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. Those skilled in the art will recognize, or will be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described. Such equivalents are intended to be encompassed by the following claims.