- Email: [email protected]
A robust NMR method to measure porosity of low porosity rocks
Microporous and Mesoporous Materials xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
A robust NMR method to measure porosity of low porosity rocks Weichao Yan∗, Jianmeng Sun, Yang Sun, Naser Golsanami School of Geosciences, China University of Petroleum (East China), Qingdao, 266580, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Magic-sandwich echo Porosity Homonuclear dipolar coupling TOC content
Rocks in low porosity reservoirs contain a considerable number of micro-pores which make the accurate measurement of total porosity more and more difficult. Nuclear magnetic resonance (NMR) is wildly used in petrophysical analyses; however, when using traditional NMR porosity measurement method, it is hard to detect signals of micro-pores due to the limitation of echo spacing. In the current research study, we used a robust NMR method, i.e. magic-sandwich echo (MSE) pulse sequence, to measure the porosities of various types of hydrocarbon-bearing rocks, and compared the results with those of the conventional measurement methods. It is worth mentioning that clay minerals have a great influence on NMR porosity measurement results. Therefore, it was needed to measure the magnetic resonance signal of dry rock to reduce the influences of clay minerals on the rock porosity measurement by NMR approach. After conducting experiments on different low porosity rocks, the results showed that the relative error of our method was 13.50%, which was more accurate than traditional NMR porosity measurement method. In addition, based on porosity deviations of shales and homonuclear dipolar coupling, we put forward a novel method to estimate shale total organic carbon (TOC) content which is a helpful tool for evaluation of unconventional shale reservoirs.
1. Introduction Low porosity reservoirs having a high production rate have been found in different parts of the world and could include such kinds of reservoirs as tight sandstone reservoirs, tight carbonate reservoirs, and organic shale reservoirs. Petrophysical properties of low porosity rocks have received much interest because of their different pore structures compared with those of the conventional rocks. Nuclear magnetic resonance (NMR) is wildly used in such petrophysical analyses as porosity evaluation, and measurement of irreducible water saturation and wettability. Accurate rock porosity measurements are critical for oil and gas reservoir evaluation, especially for complex reservoirs with low porosities. When rocks are saturated with water, the NMR signals reflect total volume of fluid in the pores. Typical laboratory NMR porosity measurements are divided into two parts: first, preparing the standard samples and establishing the standard equation, and second, testing water saturated rock samples [1]. The pulse sequence used in traditional measurements is Carr-Purcell-Meiboom-Gill (CPMG), and five factors mainly affect the accuracy of NMR total porosity measurement of a rock sample which include repetition time (RT), echo spacing (TE), hydrogen index (HI), rock temperature, and magnetic field strength. Subsequently, the echo signal is measured by choosing the appropriate measurement parameters and exciting a series of RF pulses. The initial amplitude of the recorded echo signal indicates the total amount of
∗
fluids in the sample, and the rock porosity could be calculated by using this amplitude and an established standard equation. Laboratory measurements on conventional rock samples have yielded some successes in acquiring porosity by using NMR. However, due to the insufficient and short echo spacing, NMR porosity measurements of low porosity rocks usually underestimate the real porosity of the rock. Although numerous researchers have always tried to measure porosities of tight rocks by using NMR technology, they have merely chosen the proper measurement parameters rather than changing the entire pulse sequence. Considering the above-mentioned facts, the present research aimed to investigate application of magic-sandwich echo (MSE) pulse sequence for measuring rock porosities by the NMR method. MSE is famous for its capability of providing a reasonably long final delay that covers the dead time [2]. This approach has not yet found its way into application for NMR porosity measurements, and we believe that it could be a robust method to detect fluids in micro pores of low porosity rocks. This ideas was satisfying verified by the conducted experiments in the current research work and provided an improved methodology for deeper understanding of the finely porous rocks. Total organic carbon (TOC) content is one of the most critical parameters in evaluating the abundance of organic matter in shale and its hydrocarbon generation potential. The traditional laboratory method used for measuring TOC content is the well-known geochemical
Corresponding author. E-mail address: [email protected] (W. Yan).
https://doi.org/10.1016/j.micromeso.2018.02.022 Received 14 January 2017; Received in revised form 21 December 2017; Accepted 19 February 2018 1387-1811/ © 2018 Elsevier Inc. All rights reserved.
Please cite this article as: Yan, W., Microporous and Mesoporous Materials (2018), https://doi.org/10.1016/j.micromeso.2018.02.022
Microporous and Mesoporous Materials xxx (xxxx) xxx–xxx
W. Yan et al.
NMR signal measuring methods, 39 low porosity rock samples (porosity ranging from 1% to 12%) were collected from exploration wells in different oilfields in China. These samples included 10 sandstone samples from Upper Permian Shihezi Formation in the southeast of Jiyang depression, 5 tight carbonate rock samples from Ordovician Badou Formation in Chezhen Sag, Jiyang Depression, and 24 shale samples from in the Cangdong depression, Bohai Gulf basin. Each sample, prepared as a cylindrical core plug, was cut parallel to the bedding planes with 25.4 mm in diameter and 36–47 mm in length. Because remaining oil in pores could influence total porosities, these samples were cleaned by methanol and toluene for 50 days for washing out the remaining oil and this was achieved when the fluorescence level of solution reached to 3 after washing the samples. Then, the samples were subjected to a high temperature inside the oven and were dried at 90 °C for 3 days to remove free fluids. Finally, these samples were saturated with water in vacuum pressure saturation machine. Two NMR porosity measurement methods were applied on watersaturated rock samples to determine the porosities using a NIUMAG low-field NMR instrument (MesoMR23-060H-I) operating at a proton Lamor frequency of approximately 23 MHz. The experiment temperature was set to 32 °C. And in order to detect signals of micro-pores as fully as possible, we used the minimum echo spacing in traditional method which was 0.1 ms. For MSE pulse sequence, τΦ is typically chosen to be as short as possible. In our research, we found out that 3 μs was the most suitable selection for our instrument.
analysis [3], which is expensive and time-consuming due to its complicated procedure. Accordingly, many other methodologies have been used to quantify TOC content, but there are only a few reports published on TOC content estimation by using NMR technology [4–6]. Relaxation mechanisms in shale contain both heteronuclear dipolar coupling and homonuclear dipolar coupling, which differ from relaxation mechanisms in rocks without organic matter. We believe that this difference in the relaxation mechanisms could be used in estimating TOC content and provide a time- and cost-effective alternative for the costly and complex laboratory measurements. In our research, we first considered the differences between CPMG and MSE pulse sequence. Then, a series of porosity measurement experiments were performed to optimize the application workflow of the suggested robust NMR method. In this step, we used different low porosity rocks to prove the accuracy and reliability of our method. In the next step, we studied the effects of clay minerals on porosity measurements. Finally, a novel method for estimating TOC content in shale was put forward by calculating the difference between water porosity and NMR-measured porosity. 2. Experiments and methods 2.1. NMR porosity measurement methods NMR is a process in which the nuclei in a magnetic field absorb and re-emit electromagnetic radiations. During rock NMR porosity measurement, NMR machine mainly detects the NMR signal of the hydrogen nucleus (1H) from fluids within the rock pores. Thus, when rock is saturated with water, the detected signal is proportional to the pore volume. After measuring rock volumes and signals of both rock samples and standard porosity samples, porosities of rock samples can be easily calculated. Generally, for measuring rock porosity by using NMR, two main steps are performed as follows: first, the standard equation (the relationship between NMR porosity and the volume of water) is established by measuring the signal of the standard porosity samples. Then, signals of fully water-saturated rock samples are acquired. In traditional NMR porosity measurement method, CPMG pulse sequence is used for getting relaxation decay curves of both standard porosity samples and rock samples as shown in Fig. 1 (a). We used the initial signal amplitude of the measured echo signal to calculate porosity values of the studied rock samples. Almost all NMR porosity measurements nowadays are using CPMG pulse sequence. However, when using this pulse sequence, it is hard to detect signals of micro-pores due to the limitation of echo spacing. Therefore, NMR-measured porosities of low porosity rocks are theoretically smaller than actual total porosities. In this research, a robust NMR method was developed to accurately measure rock porosity, especially for low porosity rocks. The main experiment steps were similar to traditional method, except that the employed pulse sequence was MSE as shown in Fig. 1 (b). MSE pulse sequence has been used in a variety of researches [7–9] such as solid-state imaging, polymer analysis, and biological tissues imaging experiments. Although there is no literature discussing the application of MSE in rock analyses, it would gain more interest because it is an efficient and robust method for detecting short relaxation components, which could help researchers in better understanding micro-pores of unconventional rocks. In this pulse sequence, τ = 2τp90 + 4τΦ, and τ’ = τ − τp90/2. The suitable duration of 90° pulse sequence (τp90) can be acquired by conducting measurements on standard samples. In practice, considering the dead time problem and the minimum switching delay, τΦ needs to be at least 2 μs [10].
3. Results and discussion 3.1. Influences of clay minerals Fig. 2 shows the conventional NMR porosity model of rocks, which is also the common model in NMR well logging interpretations. This model assumes that no signals of hydroxyl groups can be detected in clay minerals, and NMR T2 spectrum contains fluid components without solid parts. This model is useful for old NMR instruments, but it is necessary to examine the application effectiveness of MSE pulse sequence because of its capability of capturing signals of short relaxations. In addition, following the technological progresses in designing and manufacturing NMR tools, the sensor of the NMR logging tool has become more advanced, and the minimum echo spacing can be smaller than it used to be in the past. If signals of hydroxyl groups or interlayer water in clay minerals are detected and recorded on the final decay curve, the calculated rock porosity will be larger than the actual value because these components are not pores in reality. We dried pure clay minerals, including illite, montmorillonite, chlorite and kaolin, to study their effects on NMR signals. Meanwhile, pure quartz was used for comparison. A parameter, called signal-mass ratio, was put forward to illustrate influences of different clay minerals. It is a ratio of detected NMR signal to mass of clay mineral. As is indicated in Fig. 3, montmorillonite had the biggest signal-mass ratio, which would cause more porosity deviation if the rock contained a lot of montmorillonite. In addition, other clay minerals also considerably influenced detected NMR signals, while quartz had no influence. The main reason for this phenomenon is that the hydrogen protons of hydroxyl groups (OHe) and interlayer water (nH2O) in clay minerals can be detected by MSE pulse sequence or CPMG pulse sequence in short TE value (0.1 ms). Therefore, in order to reduce the influences of clay minerals on the rock porosity, it was needed to measure the magnetic resonance signals of dry rocks. Consequently, in the process of porosity calculation, these signals were removed from water-saturated rocks' signals. 3.2. Porosity results
2.2. Samples and experiments In this step, 39 rock samples were used to study their porosities. Herein, three methods were implemented that included traditional
In order to compare porosity values resulted from two different 2
Microporous and Mesoporous Materials xxx (xxxx) xxx–xxx
W. Yan et al.
Fig. 1. (a) CPMG pulse sequence. (b) MSE pulse sequence.
porosity measurement results, we would make the conclusion that the MSE pulse sequence detects more fluid volume, which is a better approach to measure porosities of rock samples. 3.3. TOC content estimation After the success in measuring porosities of rock samples, we tried to measure shale porosity by using our method. Fig. 5 shows the resulted porosity values of 24 shale samples. Unfortunately, our method had not a perfect performance in measuring porosity of shale directly. When shale samples are saturated with water, the interactions between water molecule and organic matter cause homonuclear dipolar coupling, leading to incorrect porosity measurement results. Even though our method was not very effective in measuring shale porosity, it still has its particular advantages in other shale-related applications. In other word, throughout our measurements, we noticed that when using the MSE pulse sequence for measuring porosity values of shale, deviations of measured values were different, which might be caused by some specific relaxation mechanisms. It should be noted that with regard to the rock components, the main difference between shale and tight sandstone is the presence of organic matter. This means that for the shale, homonuclear dipolar coupling changes relaxation due to the existence of organic matter. Therefore, TOC content in shale can be estimated based on porosity deviations. The relative TOC content could be expressed by equation (1) as follows:
Fig. 2. Conventional NMR porosity model of rock.
NMR porosity measurement, our novel method, and water porosity measurement. We regarded water porosity measurement results as standard values. This is because, theoretically, all water injected in pores should be detected by NMR methods. After measuring the signal of water-saturated samples, these samples were dried and the NMR signals were measured once again in order to eliminate signals of clay minerals. Due to the existence of organic matter in shale, we separated the rock samples into two categories to prove the validity of our method. These categories involved (a) rocks without organic matter (tight sandstones and tight carbonate rocks) and (b) shale samples. Fig. 4 shows the porosity measurement results of tight sandstones and tight carbonate rocks. All NMR porosity results measured by MSE and CPMG pulse sequence were smaller than porosities of water-saturated samples, and this phenomenon indicated that some part of water signal was missing. While the relative error of MSE porosity was 13.50%, the relative error of CPMG porosity was 24.54%. Comparing MSE and CPMG
TOC ∗ =
100(ϕw − ϕMSE ) Vt × ϕw
(1)
Where ϕw and ϕMSE are measured water porosity and MSE porosity respectively (%); and Vt is total rock volume (cm3). TOC∗ is called 3
Microporous and Mesoporous Materials xxx (xxxx) xxx–xxx
W. Yan et al.
Fig. 3. Acquired signal-mass ratios of different samples.
Fig. 6. Relationship between TOC∗ and TOC content of 24 shale samples. Fig. 4. Porosity measurement results for different tight sandstones and tight carbonate rocks.
samples to establish the conversion equation. Fig. 6 shows correlation between TOC∗ and TOC content of 24 shale samples. Through calculation using the established equation, TOC content would be easily estimated. Even if the actual TOC content data is insufficient, TOC∗ is still helpful for fast unconventional reservoir evaluation. While conventional TOC content experiment is quite time consuming, by adopting our introduced method of acquiring TOC∗, the favorable shale layers which are rich in organic matter would be selected effectively and quickly. 4. Conclusions This research study developed a robust NMR method to measure porosity of low porosity rocks. According to the accomplished results, the following conclusions could be drawn: (1) Compared with the traditional NMR porosity measurement method, MSE pulse sequence is of superior performance in investigating the micro pores of complex porous rocks. While MSE is effectively applicable in tight sandstone and tight carbonate rocks, it's not very useful in shale medium due to the occurrence of specific relaxation mechanisms. (2) When measuring NMR rock porosity, signal of dry rock is needed to be measured because clay minerals have significant effects on total NMR signals.
Fig. 5. Porosity measurement results for shale samples.
relative TOC content because it is not the real TOC content. To put it simply, TOC∗ means porosity deviations per pore volume. Due to the linear relationship between TOC∗ and TOC content, TOC content can be calculated if there exists the experimental data of several 4
Microporous and Mesoporous Materials xxx (xxxx) xxx–xxx
W. Yan et al.
References
(3) Based on porosity deviations of shale, shale TOC content estimation method was put forward. Even if there is no actual TOC content data, this method can still be used for quick selection of shale layers being rich in organic matter. However, it is not suitable for accurate TOC content measurement. (4) Considering the wide application of NMR measurements for investigating varying types of porous media in different scientific disciplines, we believe that the MSE pulse sequence would even come in handy in better comprehension of the media other than hydrocarbon-bearing rocks. This is the area that requires further investigation by the relevant researchers.
[1] A. Timur, J. Petrol. Technol. 21 (1969) 775–786. [2] A. Maus, C. Hertein, K. Saalwächter, Macromol. Chem. Phys. 207 (2006) 1150–1158. [3] E. Utuedor, Y. Yikarebogha, F.C. Okpala, Int. J. Sci. Invent. Today 4 (2015) 430–438. [4] X. Ge, Y. Fan, Y. Cao, J. Li, J. Cai, J. Liu, S. Wei, Energy Fuels 30 (2016) 4699–4709. [5] Y. Mehmani, A.K. Burnham, M.D.V. Berg, F. Gelin, H. Tchelepi, Fuel 177 (2016) 63–75. [6] P. Wang, Z. Chen, X. Pang, K. Hu, M. Sun, X. Chen, Mar. Petrol. Geol. 70 (2016) 304–319. [7] S. Hafner, D.E. Demco, R. Kimmich, Solid State Nucl. Magn. Reson. 6 (1996) 275–293. [8] M. Pieruccini, S. Sturniolo, M. Corti, A. Rigamonti, Eur. Phys. J. B 88 (2015) 1–7. [9] R.R. Regatte, M.E. Schweitzer, A. Jerschow, R. Reddy, Magn. Reson. Imaging 25 (2007) 433–438. [10] A. Maus, Ph.D Thesis, University of Freiburg (2005).
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 41374124, No. 41574122) and Fundamental Research Funds for the Central Universities (No.16CX06049A).
5
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
A robust NMR method to measure porosity of low porosity rocks Weichao Yan∗, Jianmeng Sun, Yang Sun, Naser Golsanami School of Geosciences, China University of Petroleum (East China), Qingdao, 266580, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Magic-sandwich echo Porosity Homonuclear dipolar coupling TOC content
Rocks in low porosity reservoirs contain a considerable number of micro-pores which make the accurate measurement of total porosity more and more difficult. Nuclear magnetic resonance (NMR) is wildly used in petrophysical analyses; however, when using traditional NMR porosity measurement method, it is hard to detect signals of micro-pores due to the limitation of echo spacing. In the current research study, we used a robust NMR method, i.e. magic-sandwich echo (MSE) pulse sequence, to measure the porosities of various types of hydrocarbon-bearing rocks, and compared the results with those of the conventional measurement methods. It is worth mentioning that clay minerals have a great influence on NMR porosity measurement results. Therefore, it was needed to measure the magnetic resonance signal of dry rock to reduce the influences of clay minerals on the rock porosity measurement by NMR approach. After conducting experiments on different low porosity rocks, the results showed that the relative error of our method was 13.50%, which was more accurate than traditional NMR porosity measurement method. In addition, based on porosity deviations of shales and homonuclear dipolar coupling, we put forward a novel method to estimate shale total organic carbon (TOC) content which is a helpful tool for evaluation of unconventional shale reservoirs.
1. Introduction Low porosity reservoirs having a high production rate have been found in different parts of the world and could include such kinds of reservoirs as tight sandstone reservoirs, tight carbonate reservoirs, and organic shale reservoirs. Petrophysical properties of low porosity rocks have received much interest because of their different pore structures compared with those of the conventional rocks. Nuclear magnetic resonance (NMR) is wildly used in such petrophysical analyses as porosity evaluation, and measurement of irreducible water saturation and wettability. Accurate rock porosity measurements are critical for oil and gas reservoir evaluation, especially for complex reservoirs with low porosities. When rocks are saturated with water, the NMR signals reflect total volume of fluid in the pores. Typical laboratory NMR porosity measurements are divided into two parts: first, preparing the standard samples and establishing the standard equation, and second, testing water saturated rock samples [1]. The pulse sequence used in traditional measurements is Carr-Purcell-Meiboom-Gill (CPMG), and five factors mainly affect the accuracy of NMR total porosity measurement of a rock sample which include repetition time (RT), echo spacing (TE), hydrogen index (HI), rock temperature, and magnetic field strength. Subsequently, the echo signal is measured by choosing the appropriate measurement parameters and exciting a series of RF pulses. The initial amplitude of the recorded echo signal indicates the total amount of
∗
fluids in the sample, and the rock porosity could be calculated by using this amplitude and an established standard equation. Laboratory measurements on conventional rock samples have yielded some successes in acquiring porosity by using NMR. However, due to the insufficient and short echo spacing, NMR porosity measurements of low porosity rocks usually underestimate the real porosity of the rock. Although numerous researchers have always tried to measure porosities of tight rocks by using NMR technology, they have merely chosen the proper measurement parameters rather than changing the entire pulse sequence. Considering the above-mentioned facts, the present research aimed to investigate application of magic-sandwich echo (MSE) pulse sequence for measuring rock porosities by the NMR method. MSE is famous for its capability of providing a reasonably long final delay that covers the dead time [2]. This approach has not yet found its way into application for NMR porosity measurements, and we believe that it could be a robust method to detect fluids in micro pores of low porosity rocks. This ideas was satisfying verified by the conducted experiments in the current research work and provided an improved methodology for deeper understanding of the finely porous rocks. Total organic carbon (TOC) content is one of the most critical parameters in evaluating the abundance of organic matter in shale and its hydrocarbon generation potential. The traditional laboratory method used for measuring TOC content is the well-known geochemical
Corresponding author. E-mail address: [email protected] (W. Yan).
https://doi.org/10.1016/j.micromeso.2018.02.022 Received 14 January 2017; Received in revised form 21 December 2017; Accepted 19 February 2018 1387-1811/ © 2018 Elsevier Inc. All rights reserved.
Please cite this article as: Yan, W., Microporous and Mesoporous Materials (2018), https://doi.org/10.1016/j.micromeso.2018.02.022
Microporous and Mesoporous Materials xxx (xxxx) xxx–xxx
W. Yan et al.
NMR signal measuring methods, 39 low porosity rock samples (porosity ranging from 1% to 12%) were collected from exploration wells in different oilfields in China. These samples included 10 sandstone samples from Upper Permian Shihezi Formation in the southeast of Jiyang depression, 5 tight carbonate rock samples from Ordovician Badou Formation in Chezhen Sag, Jiyang Depression, and 24 shale samples from in the Cangdong depression, Bohai Gulf basin. Each sample, prepared as a cylindrical core plug, was cut parallel to the bedding planes with 25.4 mm in diameter and 36–47 mm in length. Because remaining oil in pores could influence total porosities, these samples were cleaned by methanol and toluene for 50 days for washing out the remaining oil and this was achieved when the fluorescence level of solution reached to 3 after washing the samples. Then, the samples were subjected to a high temperature inside the oven and were dried at 90 °C for 3 days to remove free fluids. Finally, these samples were saturated with water in vacuum pressure saturation machine. Two NMR porosity measurement methods were applied on watersaturated rock samples to determine the porosities using a NIUMAG low-field NMR instrument (MesoMR23-060H-I) operating at a proton Lamor frequency of approximately 23 MHz. The experiment temperature was set to 32 °C. And in order to detect signals of micro-pores as fully as possible, we used the minimum echo spacing in traditional method which was 0.1 ms. For MSE pulse sequence, τΦ is typically chosen to be as short as possible. In our research, we found out that 3 μs was the most suitable selection for our instrument.
analysis [3], which is expensive and time-consuming due to its complicated procedure. Accordingly, many other methodologies have been used to quantify TOC content, but there are only a few reports published on TOC content estimation by using NMR technology [4–6]. Relaxation mechanisms in shale contain both heteronuclear dipolar coupling and homonuclear dipolar coupling, which differ from relaxation mechanisms in rocks without organic matter. We believe that this difference in the relaxation mechanisms could be used in estimating TOC content and provide a time- and cost-effective alternative for the costly and complex laboratory measurements. In our research, we first considered the differences between CPMG and MSE pulse sequence. Then, a series of porosity measurement experiments were performed to optimize the application workflow of the suggested robust NMR method. In this step, we used different low porosity rocks to prove the accuracy and reliability of our method. In the next step, we studied the effects of clay minerals on porosity measurements. Finally, a novel method for estimating TOC content in shale was put forward by calculating the difference between water porosity and NMR-measured porosity. 2. Experiments and methods 2.1. NMR porosity measurement methods NMR is a process in which the nuclei in a magnetic field absorb and re-emit electromagnetic radiations. During rock NMR porosity measurement, NMR machine mainly detects the NMR signal of the hydrogen nucleus (1H) from fluids within the rock pores. Thus, when rock is saturated with water, the detected signal is proportional to the pore volume. After measuring rock volumes and signals of both rock samples and standard porosity samples, porosities of rock samples can be easily calculated. Generally, for measuring rock porosity by using NMR, two main steps are performed as follows: first, the standard equation (the relationship between NMR porosity and the volume of water) is established by measuring the signal of the standard porosity samples. Then, signals of fully water-saturated rock samples are acquired. In traditional NMR porosity measurement method, CPMG pulse sequence is used for getting relaxation decay curves of both standard porosity samples and rock samples as shown in Fig. 1 (a). We used the initial signal amplitude of the measured echo signal to calculate porosity values of the studied rock samples. Almost all NMR porosity measurements nowadays are using CPMG pulse sequence. However, when using this pulse sequence, it is hard to detect signals of micro-pores due to the limitation of echo spacing. Therefore, NMR-measured porosities of low porosity rocks are theoretically smaller than actual total porosities. In this research, a robust NMR method was developed to accurately measure rock porosity, especially for low porosity rocks. The main experiment steps were similar to traditional method, except that the employed pulse sequence was MSE as shown in Fig. 1 (b). MSE pulse sequence has been used in a variety of researches [7–9] such as solid-state imaging, polymer analysis, and biological tissues imaging experiments. Although there is no literature discussing the application of MSE in rock analyses, it would gain more interest because it is an efficient and robust method for detecting short relaxation components, which could help researchers in better understanding micro-pores of unconventional rocks. In this pulse sequence, τ = 2τp90 + 4τΦ, and τ’ = τ − τp90/2. The suitable duration of 90° pulse sequence (τp90) can be acquired by conducting measurements on standard samples. In practice, considering the dead time problem and the minimum switching delay, τΦ needs to be at least 2 μs [10].
3. Results and discussion 3.1. Influences of clay minerals Fig. 2 shows the conventional NMR porosity model of rocks, which is also the common model in NMR well logging interpretations. This model assumes that no signals of hydroxyl groups can be detected in clay minerals, and NMR T2 spectrum contains fluid components without solid parts. This model is useful for old NMR instruments, but it is necessary to examine the application effectiveness of MSE pulse sequence because of its capability of capturing signals of short relaxations. In addition, following the technological progresses in designing and manufacturing NMR tools, the sensor of the NMR logging tool has become more advanced, and the minimum echo spacing can be smaller than it used to be in the past. If signals of hydroxyl groups or interlayer water in clay minerals are detected and recorded on the final decay curve, the calculated rock porosity will be larger than the actual value because these components are not pores in reality. We dried pure clay minerals, including illite, montmorillonite, chlorite and kaolin, to study their effects on NMR signals. Meanwhile, pure quartz was used for comparison. A parameter, called signal-mass ratio, was put forward to illustrate influences of different clay minerals. It is a ratio of detected NMR signal to mass of clay mineral. As is indicated in Fig. 3, montmorillonite had the biggest signal-mass ratio, which would cause more porosity deviation if the rock contained a lot of montmorillonite. In addition, other clay minerals also considerably influenced detected NMR signals, while quartz had no influence. The main reason for this phenomenon is that the hydrogen protons of hydroxyl groups (OHe) and interlayer water (nH2O) in clay minerals can be detected by MSE pulse sequence or CPMG pulse sequence in short TE value (0.1 ms). Therefore, in order to reduce the influences of clay minerals on the rock porosity, it was needed to measure the magnetic resonance signals of dry rocks. Consequently, in the process of porosity calculation, these signals were removed from water-saturated rocks' signals. 3.2. Porosity results
2.2. Samples and experiments In this step, 39 rock samples were used to study their porosities. Herein, three methods were implemented that included traditional
In order to compare porosity values resulted from two different 2
Microporous and Mesoporous Materials xxx (xxxx) xxx–xxx
W. Yan et al.
Fig. 1. (a) CPMG pulse sequence. (b) MSE pulse sequence.
porosity measurement results, we would make the conclusion that the MSE pulse sequence detects more fluid volume, which is a better approach to measure porosities of rock samples. 3.3. TOC content estimation After the success in measuring porosities of rock samples, we tried to measure shale porosity by using our method. Fig. 5 shows the resulted porosity values of 24 shale samples. Unfortunately, our method had not a perfect performance in measuring porosity of shale directly. When shale samples are saturated with water, the interactions between water molecule and organic matter cause homonuclear dipolar coupling, leading to incorrect porosity measurement results. Even though our method was not very effective in measuring shale porosity, it still has its particular advantages in other shale-related applications. In other word, throughout our measurements, we noticed that when using the MSE pulse sequence for measuring porosity values of shale, deviations of measured values were different, which might be caused by some specific relaxation mechanisms. It should be noted that with regard to the rock components, the main difference between shale and tight sandstone is the presence of organic matter. This means that for the shale, homonuclear dipolar coupling changes relaxation due to the existence of organic matter. Therefore, TOC content in shale can be estimated based on porosity deviations. The relative TOC content could be expressed by equation (1) as follows:
Fig. 2. Conventional NMR porosity model of rock.
NMR porosity measurement, our novel method, and water porosity measurement. We regarded water porosity measurement results as standard values. This is because, theoretically, all water injected in pores should be detected by NMR methods. After measuring the signal of water-saturated samples, these samples were dried and the NMR signals were measured once again in order to eliminate signals of clay minerals. Due to the existence of organic matter in shale, we separated the rock samples into two categories to prove the validity of our method. These categories involved (a) rocks without organic matter (tight sandstones and tight carbonate rocks) and (b) shale samples. Fig. 4 shows the porosity measurement results of tight sandstones and tight carbonate rocks. All NMR porosity results measured by MSE and CPMG pulse sequence were smaller than porosities of water-saturated samples, and this phenomenon indicated that some part of water signal was missing. While the relative error of MSE porosity was 13.50%, the relative error of CPMG porosity was 24.54%. Comparing MSE and CPMG
TOC ∗ =
100(ϕw − ϕMSE ) Vt × ϕw
(1)
Where ϕw and ϕMSE are measured water porosity and MSE porosity respectively (%); and Vt is total rock volume (cm3). TOC∗ is called 3
Microporous and Mesoporous Materials xxx (xxxx) xxx–xxx
W. Yan et al.
Fig. 3. Acquired signal-mass ratios of different samples.
Fig. 6. Relationship between TOC∗ and TOC content of 24 shale samples. Fig. 4. Porosity measurement results for different tight sandstones and tight carbonate rocks.
samples to establish the conversion equation. Fig. 6 shows correlation between TOC∗ and TOC content of 24 shale samples. Through calculation using the established equation, TOC content would be easily estimated. Even if the actual TOC content data is insufficient, TOC∗ is still helpful for fast unconventional reservoir evaluation. While conventional TOC content experiment is quite time consuming, by adopting our introduced method of acquiring TOC∗, the favorable shale layers which are rich in organic matter would be selected effectively and quickly. 4. Conclusions This research study developed a robust NMR method to measure porosity of low porosity rocks. According to the accomplished results, the following conclusions could be drawn: (1) Compared with the traditional NMR porosity measurement method, MSE pulse sequence is of superior performance in investigating the micro pores of complex porous rocks. While MSE is effectively applicable in tight sandstone and tight carbonate rocks, it's not very useful in shale medium due to the occurrence of specific relaxation mechanisms. (2) When measuring NMR rock porosity, signal of dry rock is needed to be measured because clay minerals have significant effects on total NMR signals.
Fig. 5. Porosity measurement results for shale samples.
relative TOC content because it is not the real TOC content. To put it simply, TOC∗ means porosity deviations per pore volume. Due to the linear relationship between TOC∗ and TOC content, TOC content can be calculated if there exists the experimental data of several 4
Microporous and Mesoporous Materials xxx (xxxx) xxx–xxx
W. Yan et al.
References
(3) Based on porosity deviations of shale, shale TOC content estimation method was put forward. Even if there is no actual TOC content data, this method can still be used for quick selection of shale layers being rich in organic matter. However, it is not suitable for accurate TOC content measurement. (4) Considering the wide application of NMR measurements for investigating varying types of porous media in different scientific disciplines, we believe that the MSE pulse sequence would even come in handy in better comprehension of the media other than hydrocarbon-bearing rocks. This is the area that requires further investigation by the relevant researchers.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 41374124, No. 41574122) and Fundamental Research Funds for the Central Universities (No.16CX06049A).
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