Porosity of Drill-Cuttings Using Multinuclear 19F and 1H NMR

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Porosity of drill-cuttings using multinuclear F and H NMR measurements Kamilla Fellah, Shin Utsuzawa, Yi-Qiao Song, and Ravinath Kausik Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01350 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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Porosity of drill-cuttings using multinuclear 19F and 1H NMR measurements Kamilla Fellah, Shin Utsuzawa, Yi-Qiao Song and Ravinath Kausik* Schlumberger-Doll Research Center, One Hampshire Street, Cambridge, MA 02139 USA

ABSTRACT Petrophysical measurements of drill cuttings are vital for determining reservoir quality in organic mudstone plays, where the highly deviated (horizontal/lateral) production wells are rarely logged. We demonstrate a methodology to measure the porosity of irregularly shaped drillcutting samples using multinuclear 1H-19F NMR measurements at medium frequencies (12 MHz). The methodology is successfully demonstrated on both core and irregularly shaped simulated cuttings from conventional and unconventional shale plays.

1. INTRODUCTION

NMR relaxometry has been gaining ground as a reliable approach for core analysis due to its capability to measure porosity and to characterize fluids of rock samples. NMR relaxometry (T1 or T2) measurements yield fluid saturations, wettability, and fluid types non-destructively and in a relatively quick fashion. Data on these rock properties aid in assessing reservoir quality, reserve estimates, and core-log integration1,2. The NMR measured porosities and fluid saturations have been compared favorably to alternative porosity measurements (helium gas porosity or buoyancy porosity) and additionally, the difference between the log and the core NMR porosities correlate to potentially producible fluid fractions.

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The NMR laboratory measurements are typically done at frequencies similar to those of logging tools (~2 MHz), for easy comparison, and on 1.5” x 1.5” cores that are trimmed and surface ground to fit in the NMR probe. The 1H (proton) NMR measurements are mainly used to obtain the volumes of the fluids in the pores (pore volume) and, in combination with the bulk volumes measured using calipers on cylindrical cores, the porosity is determined. Therefore, routine NMR laboratory measurements are made on regular shaped samples which result in a high filling factor in the radio frequency (RF) probe to achieve optimum signal-to-noise ratios (SNR).

In recent years the advent of improved horizontal drilling methods has revolutionized hydrocarbon production from organic-rich mudstones (referred to as unconventional shale plays). Although the value of NMR measurements and other logs of unconventional shale rocks has been shown for the vertical pilot wells3 -5, the absence of routine logs and cores from the horizontal production wells make it challenging to obtain a complete understanding of the reservoir.

In the absence of core samples, drill-cuttings can be a valuable source of information on the geology and reservoir quality; they correlate with depth and can help with understanding formation stratigraphy and finding pay zones6. Obtaining petrophysical answers from drillcuttings, such as porosity and its partition into different fractions, namely kerogen, bitumen and liquid hydrocarbon, would be very useful given the lack of information in the horizontal wells. The biggest challenge for porosity measurements of drill cuttings is a reliable measurement of the bulk volume of these irregularly shaped samples. Traditional measurements such as helium or mercury pycnometry which can potentially provide the porosity require extensive and timeconsuming sample cleaning steps to free the pore space of the residual water, oil and bitumen


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making them unsuitable for the analysis of organic mudstones7. X-ray computed tomography is unsuitable for small cuttings samples especially with a predominant porosity hosted in nanometer sized pore scales8. Imaging methods such as Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) or SEM are not practical for quick porosity measurements and require extensive sample preparation and others such as confocal microscopy do not have the required resolution. Therefore, there is a real need for quick and accurate porosity methods which require minimal sample preparation and cleaning.

In this paper, we describe a workflow for the routine measurement of porosity and fluid typing of drill-cuttings using multi-nuclei NMR methods. The drill cuttings have irregular shapes and sizes making their bulk volume measurements challenging. Additionally, the small irregularly-shaped samples have a lower filling factor in the RF probes resulting in lower SNR. We test a methodology for measuring bulk volume of irregularly shaped samples, such as cuttings, using fluorine 19F NMR. In combination with 1H NMR for the pore volume measurement, it provides a quick measure of porosity for unconventional shale samples16. Additionally, we demonstrate the use of higher frequency NMR measurements in comparison to the traditional core analysis (carried out at 2 MHz), thereby addressing the SNR challenge for the irregularly shaped cuttings samples encountered in the field. The use of higher frequency NMR systems also allow for shorter echo spacing, which is beneficial for detecting the short T2 components found in unconventional samples. Additionally, the T1 dependence of the clay associated water and viscous hydrocarbons enable their better separation3. An accurate porosity estimate combined with the ability to identify different organic and water components makes NMR a valuable analysis tool for unconventional reservoirs. The capability of measuring irregularly-shaped


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samples with minimal instrumentation and supervision could also open a way for automated cutting analysis at the wellsite.

2. METHOD

Unique to this approach is the use of 19F NMR to measure bulk volume and its integration with 1

H NMR to obtain porosity. The method requires consistent sensitivity to volume for 19F and 1H

measurements. Therefore, a dual-tuned probe is used for these measurements. Fluorine resonates at a Larmor frequency 5.9% less than that of 1H, which is within the tuning range of most probes. As we are observing subtle changes in volume, system stability and calibration have a significant impact on the outcome. Higher field NMR systems yield better SNR, which is beneficial to measuring samples with low filling factor. And such systems also generally enable measurements at shorter echo spacings, which are useful for shale rock fluid typing and samples of low porosity.

Calibrations for both nuclei involve measuring the signal amplitude of known fluid volumes; water for 1H and a fluorocarbon for 19F. An additional calibration is made at 19F frequency using a “full” tube of fluorocarbon, meaning just enough fluid to fill the entire measurable volume (or region) of the RF coil (Figure 1). Samples can then be added to the same full tube of fluorocarbon and the signal amplitude acquired again. The difference in signal amplitude between the fluorocarbon only and the fluorocarbon plus sample is the contribution from a volume occupied by the sample. In Figure 1, sample tubes are labeled A-C and D refers to the rock cuttings sample. Using the signal amplitude of the tubes, SD = SB - SC, the bulk volumes of


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the cuttings is given by VD = (VA * SD) / SA. SB and SC are the signal amplitudes of tubes B and C and the difference is SD, the signal of the sample space. VA is the volume of fluorocarbon in tube A and SA is the signal amplitude of tube A. From the bulk volume measured with

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F and the

pore volume measured with 1H, porosity (i.e. pore volume/bulk volume) of irregularly-shaped samples is obtained independent of grain density or mass. The sequence of measurements has the flexibility to be done in any order. When the probe is tuned to the correct frequency, fluorine signal will not interfere with proton signal and vice versa so the samples can sit in a tube of fluorocarbon and be measured at 1H frequency without difficulty.

Figure 1. The procedure of the bulk volume measurements at

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F frequency (left) and resulting

NMR signal (right). The sample tube is filled with a fluorocarbon (C10HF22N), indicated by the striped pattern. The red dashed line denotes the coil’s sensitive volume.

3. EXPERIMENTAL 3.1. Sample Preparation


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To demonstrate the porosity measurement using the proposed method, we prepared two types of samples: irregularly shaped simulated cutting samples (hence referred as cuttings) and cylindrical core samples. Four quarried conventional rock samples (two limestones, a dolomite, and a sandstone) and four unconventional shale samples from the Eagle Ford formation were chosen for the experiments. The conventional rocks were cut into cores 20 mm long by 7 mm in diameter and used to benchmark the NMR bulk volume with caliper measurements. Simulated cutting samples were made by breaking up core plugs into small irregular pieces and sieved to collect cuttings of sizes between 1 – 5 mm. These were used to validate the bulk volume measurement of irregularly-shaped samples. The conventional rock samples were dried to 110° Celsius in a vacuum oven and then saturated with water at 1000 psi to achieve complete saturation. The shale samples were not cleaned nor oven dried to keep residual hydrocarbons and clay associated water in place. Similar to the conventional rock samples, the shales were cut to regular core sizes (20 mm x 7mm) and the cuttings were made from twin plugs to obtain small broken samples and sieved to collect samples between 1 – 5 mm. At ambient temperature, the shale samples were saturated with dodecane at 2000 psi in a custom made Temco reactor and held under pressure for 48 hours to fill the empty porosity with moveable oil3. The cutting samples weighed between 0.6g – 1g and for each rock type about 5-6 cuttings were used. A fluorocarbon, fluorinert FC-40 (C10HF22N, contains 1 hydrogen atom) purchased from 3M was used for the bulk fluorine measurements and was reused between measurements.

3.2. NMR Experiments


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All NMR measurements were performed on a Niumag permanent-magnet based benchtop NMR system, operating at 12 MHz with a RF probe size of 10 mm. The T2 relaxation times of the rock and bulk fluorocarbon were measured using a Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence. For

1

H NMR measurements of the rock samples, an echo spacing of 200

microseconds, and the number of echoes of 1000 were used. Using 64 scans for signal averaging, the measurement and processing time took a total of about 10 minutes per sample. For the bulk fluorocarbon measurements at

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F frequency, an echo spacing of 400 microseconds and the

number of echoes of 1000 were used. There was no background signal from the probe housing made of Teflon due to the relatively long echo spacings.

The probe dimensions and coil length were taken into consideration during sample preparation. It is important to note that, for these measurements, having samples within the center region of the coil is vital. Figure 2 shows where the samples were positioned in the coil. The sample tube stops about midway down the coil, from where it is about 30 mm to the top, where the sensitive volume ends. It is necessary to have samples well below the upper limit but large enough to utilize the available coil’s sensitive volume. Therefore, samples were kept to 20 mm in length.


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Figure 2. The photo of a 10-mm diameter sample tube on the outside of the probe. The arrow indicates where the sample tube stops when placed inside the Teflon housing. From the bottom of the tube to the top of the coil is roughly 30 mm.

4. RESULTS AND DISCUSSION

Figure 3A shows the caliper measured bulk volume of the cores versus the 19F NMR measured bulk volume, and they are in good agreement. The porosity measured using the bulk volume from the caliper measurement agrees well with the porosity from the NMR measured bulk volumes as shown in Figure 3B. Note Figure 3A and B are only the core samples as the bulk volume of cuttings cannot be measured by caliper. To measure the porosity of all samples universally, we used the NMR measured bulk and pore volumes and found that comparable porosities were obtained for both core and cutting samples (Figure 4).


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Figure 3. (A) The comparison between caliper measured bulk volume and NMR measured bulk volume. The horizontal axis represents bulk volume using a caliper to measure the diameter and the length of each sample. The vertical axis is the bulk volume measured using the 19F NMR measurement as described in the previous section. (B) The corresponding porosity values determined using the respective bulk volumes. Dashed lines representing 2 p.u. error bounds are shown as a guide for the eye.


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Figure 4. The correlation between the core porosity and the cuttings porosity from NMR measurements. Dashed lines representing 2 p.u. error bounds are shown as a guide for the eye.

The results indicate that

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F NMR is a viable method to obtain the bulk volume of irregularly

shaped rock samples and can be used to determine porosity of cutting samples without the knowledge of weight or bulk density. In combination with the fluid types determined using multidimensional 1H T1-T2 2D NMR relaxation measurements5, this technique could be a powerful contributor to petrophysical measurements in unconventional shale rocks.

Improvement in SNR is achieved by going to higher magnetic field as the SNR is proportional to B01 ~ 7/4, where B0 is the nominal field strength of the magnet, depending on the noise source10,11. However, in NMR analysis of conventional formations, pore-scale magnetic field distortions (socalled “internal gradients”) caused by the solid/fluid susceptibility contrast can bring about complications; molecular diffusion through these internal gradients introduces an enhanced signal decay, leading to uncertainty in T2 measurements9. Since the internal gradients increase


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with the field strength, low field strength of 0.05 T (corresponding to a resonance frequency of 2 MHz for 1H) is considered the industry standard to provide quantitative measurements as well as for well-log calibration. On the other hand, the nanometer-scale pores in shale samples ensure that the spins explore the pore multiple times during a measurement and hence the gradient effects across a pore average out12. Under these conditions, an increase in B0 from 0.05 T to 0.3 T results in only a slight increase in the rate of signal decay due to diffusion in the internal gradients, whilst attaining much better SNR13. The cutting samples used in this study were kept between 1 – 5 mm, with most samples skewed to the larger end of this range. In the case of drill cuttings from the oil wells, it is likely that larger samples should be used for better porosity estimates14, for example, by filtering out the smaller particles to avoid mud additives (typically smaller than 1 mm). For the measurements presented in this paper, apart from wiping away the outside fluid, there was no further preparation of the sample prior to the NMR measurement. For the field cutting samples, the cuttings will need to first be collected from the shale shaker, then sieved to remove the smaller particles, and to have the outside mud removed, before they are ready for NMR. This procedure avoids any further time-consuming sample preparation steps such as sample crushing, solvent/thermal-cleaning, drying, or weighing the sample that is required for other rock characterization measurements15. Such simplified sample preparation procedure could be critical for the practical viability of such measurement in the field.

5. CONCLUSION The analysis of drill-cuttings is a vital step forward in better understanding unconventional shale rock plays, and we demonstrate how the fundamental petrophysical property, porosity can be


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measured using NMR methods on irregularly shaped rock samples. The challenges for measuring porosity of drill-cuttings by conventional NMR methods have been addressed in this study by modifying the procedure to include a higher frequency multi-nuclear NMR approach. These NMR porosity measurements on drill-cuttings could be carried out at either the well-site or in the laboratories, saving time and cost. The lack of complex sample preparation makes these measurements quick. Further work will include application of this technique to drill-cuttings from different basins and its integration with other spectroscopy methods for superior formation evaluation.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS

The authors are grateful to Schlumberger for supporting the research and permitting the publishing of the results. The authors are extremely grateful to Dr. Jerome Ackerman and the Athinoula Martinos Center, MGH, for hosting the NMR system and for useful discussions, and Dr. Hans Wang from Niumag corporation for help with the setup.


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REFERENCES

1. Kausik, R.; Craddock, P. R.; Reeder, S. L.; Kleinberg, R. L.; Pomerantz, A. E.; Shray, F.; Lewis, R. E.; Rylander, E., Novel Reservoir Quality Indices for Tight Oil. In Unconventional Resources Technology Conference, Society of Petroleum Engineers: San Antonio, Texas, USA, 2015. 2. Jiang, T.; Rylander, E.; Singer, P.M.; Lewis. R.E.; Sinclair, S.M., Integrated Petrophysical Interpretation of Eagle Ford Shale with 1-D and 2-D Nuclear Magnetic Resonance (NMR). Society of Petrophysicists and Well-Log Analysts SPWLA 54th Annual Logging Symposium, June 22-26, 2013. 3. Kausik, R.; Fellah, K.; Rylander, E.; Singer, P. M.; Lewis, R. E.; Sinclair, S., NMR Petrophysics for Tight Oil Shale Enabled by Core Resaturation. International Symposium of the Society of Core Analysts, Avignon, France. 2014, 73. 4. Kausik, R.; Fellah, K.; Rylander, E.; Singer, P. M.; Lewis, R. E.; Sinclair, S. M., NMR Relaxometry in Shale and Implications for Logging. Petrophysics. 2016, 57, 339 – 350. 5. Kausik, R..; Fellah, K.,; Feng, L.,; Simpson, G.,. High- and Low-Field NMR Relaxometry and Diffusometry of the Bakken Petroleum System. Petrophysics. 2017, 58, 341-351 6. Santarelli, F.J.; Marsala, A.F.; Brignoli, M.; Rossi, E.; Bona, N., Formation Evaluation from Logging on Cuttings. Society of Petroleum Engineers Reservoir Evaluation & Engineering. 1998, 1, 238 – 244.


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7. Dang, S.T.; Rolke, M.M.; Sondergeld, C.H.; and Rai, C.S., Study of Drill Cuttings Porosity for Formation Evaluation. International Symposium of the Society of Core Analysts, Vienna, Austria. 2017, 75. 8. Siddiqui, S., Grader, A. S., Touati, M., Loermans, A. M., & Funk, J. J., Techniques for Extracting Reliable Density and Porosity Data From Cuttings. Society of Petroleum Engineers, 2005 9. J. Mitchell, T.C. Chandrasekera, M.L. Johns, L.F. Gladden, E.J. Fordham (2010) Nuclear magnetic resonance relaxation and diffusion in the presence of internal gradients: the effect of field strength Phys. Rev. E 81, 026101 10. Mitchell, J., Gladden, L.F., Chandrasekera, M.L., Fordham, E.J., (2014) Low-field permanent magnets for industrial process and quality control Prog. Nucl. Magn. Reson. Spectros. 76, 1-60. 11. Hoult, D.I., Richards, R.E., (1976) The signal-to-noise ratio of the nuclear magnetic resonance experiment J. Magn. Reson. 24, 71-85. 12. M. D. Hürlimann, K. G. Helmer, T. M. de Swiet, P. N. Sen, and C. H. Sotak (1995) Spin echoes in a constant gradient and in the presence of simple restriction J. Magn. Reson. Ser. A 113, 260 13. J. Mitchell, E.J. Fordham (2014) Contributed Review: Nuclear magnetic resonance core analysis at 0.3 T Rev. Sci. Instrum. 85, 111502. 14. Yu, Y.; and Menouar, H., An Experimental Method to Measure the Porosity from Cuttings: Evaluation and Error Analysis. Society of Petroleum Engineers Productions and Operations Symposium, Oklahoma City, Oklahoma, USA, 1-5 March, 2015.


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15. Pomerantz, Drew, Valenza, John, Borot de Battisti, Maxence, “Method and apparatus to prepare drill cuttings for petrophysical analysis by infrared spectroscopy and gas sorption, United States patent application No. 13/446985, filed April 13, 2012, 16. J. Mitchell, E.J. Fordham., “Determining properties of porous material by NMR” U.K. Patent 2542406 2018.


Porosity of Drill-Cuttings Using Multinuclear 19F and 1H NMR

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