Article Cite This: Energy Fuels 2019, 33, 6008−6019
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Nanoscale Pore Structure Characterization of Tight Oil Formation: A Case Study of the Bakken Formation Chunxiao Li,† Lingyun Kong,*,‡ Mehdi Ostadhassan,‡ and Thomas Gentzis¶ †
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Harold Hamm School of Geology & Geological Engineering, University of North Dakota, 81 Cornell Street, Grand Forks, North Dakota 58202, United States ‡ Department of Petroleum Engineering, University of North Dakota, 2844 Campus Road, Grand Forks, North Dakota 58202, United States ¶ Core Laboratories, Reservoir Geology Group, Houston, Texas, United States ABSTRACT: Pore structure of unconventional reservoir is fundamental for understanding hydrocarbon storage, fluid transport, and geomechanics. The pore structure of shale gas reservoirs has been studied extensively, while investigation regarding the pore structure of shale oil reservoirs remains limited. The Bakken formation is one of the largest contributors to the growth of unconventional oil in the U.S. In this study, 13 core samples collected from the Bakken formation were examined through a series of experiments to investigate the geochemical properties and mineralogy, especially pore structure. Mineralogy analysis through the X-ray diffraction (XRD) test showed that quartz and illite are the major components for the upper and lower shale members, while quartz, feldspar, and dolomite dominate the middle member of the Bakken formation. Rock-Eval source rock analysis illustrated that all of the shale samples contain a significant percentage of organic matter. Nitrogen and carbon dioxide adsorption results showed that isotherm curves obtained from nitrogen adsorption are reserved S-shaped (typical type II curve), indicating that pores are mainly micro- and mesopores. Linear regression analysis of pore structure parameters with respect to total organic carbon (TOC) and mineral composition reveals that the TOC content has a positive relationship with micropore volume, while meso- and macropores are controlled by clay content. Development of micropores in organic matter is thermal-maturity-related. Shale samples with vitrinite reflectance higher than 1.0% have a higher surface area, suggesting that more micropores were developed in the organic matter after maturities of shales reached oil window level. In addition, results of the fractal analysis showed that samples with higher fractal dimension values are featured by more micropore volume, smaller pore diameter, and larger specific surface area.
1. INTRODUCTION Over the past two decades, the unconventional shale revolution dramatically changed the global energy structure and became the exploration target in the U.S. The Bakken formation in the Williston Basin is one of the largest unconventional oil target formations in the U.S.1 The U.S. portion of the Bakken formation carried 3.65 billion barrels of oil and 1.85 trillion ft3 of gas based on the USGS assessment in 2008.2 As porous media, shale rocks are heterogeneous and complex in composition and pore structures. Pore structures are crucial for understanding the storage and transport of hydrocarbon during development and the geomechanical properties of shale reservoirs during production.3,4 Pore structures are controlled by various factors, for instance, the mineral composition, thermal maturities of rocks, etc.5 Researchers investigated the pore structure of unconventional rocks based on indirect and direct methods. In the direct methods, field emission scanning electron microscopy combined with ion-milled techniques, FIB-SEM, microcomputer tomography (micro-CT), and nano- computer tomography (nano-CT) could directly provide ways to observe pore morphology, distribution, and pore connectivity.6−8 In the indirect approaches, gas (nitrogen/carbon dioxide/ methane) adsorption and mercury intrusion porosimetry (MIP) could provide us the quantitative information of volume, surface area, and size of pores by analyzing the © 2019 American Chemical Society
adsorbed gas volume or intruded mercury volume with respect to the applied pressure.5,9,10 Helium pycnometer is another indirect method to measure the total effective porosity of the sample. Moreover, each characterization method can only depict a specific size range of pore structure due to the technical limitation of each technique.11 Comparison of these methods in regard to resolution limits is given in Figure 1. Gas adsorption is one of the most widely used techniques for pore structure characterization. Previous research has proved that gas adsorption can be successfully used in nanosize pore characterization of geomaterials.9,12,13 The most commonly used adsorption gas includes nitrogen, argon, and carbon dioxide, depending on the properties of the porous materials and information required.14 Nitrogen at a temperature of 77 K is used as a standard adsorptive for pore analysis, which is capable of detecting pores in the range of 2−200 nm, while carbon dioxide adsorption is used in the range of 0.5 and region A for P/P0 < 0.5 (Figure 10). Region A Table 4. Fractal Dimension Calculated from Nitrogen Adsorption sample ID
slope
D1
slope
D2
1 2 3 4 5 6 7 8 9 10 11 12 13
−1.2076 −1.5066 −0.7101 −0.8699 −1.682 −1.1028 −1.5676 −1.705 −0.2609 −0.3335 −0.1794 −0.3866 −0.2827
1.7924 1.4934 2.2899 2.1301 1.318 1.8972 1.4324 1.295 2.7391 2.6665 2.8206 2.6134 2.7173
−0.209 −0.2355 −0.2406 −0.3809 −0.1515 −0.2631 −0.0896 −0.1139 −0.5386 −0.3916 −0.5413 −0.5023 −0.4459
2.791 2.7645 2.7594 2.6191 2.8485 2.7369 2.9104 2.8861 2.4614 2.6084 2.4587 2.4977 2.5541
5. DISCUSSION 5.1. Effect of TOC and Mineralogy on Pore Structure. Organic matter plays a critical role in pore development for shale rocks. Previous studies of pore structures of shale gas 6014
DOI: 10.1021/acs.energyfuels.9b00514 Energy Fuels 2019, 33, 6008−6019
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Figure 11. Correlation between shale samples (from UBM and LBM) comprising contents and pore volumes. Cross-plots of (a) TOC, (c) clay, (e) quartz, and feldspar versus meso- and macropore (left column), and cross-plots of (b) TOC, (d) clay, (f) quartz, and feldspar versus micropore parameters (right column). Blue dots correspond to the left Y-axis and yellow dots are associated with the right Y-axis. Dashed lines represent linear regression between two variables.
increasing TOC content, an obvious increasing trend of micropore volume was observed (Figure 11b), which suggests that the development of micropores is associated with organic matter. The cross-plots between major constituent (clays, quartz, and feldspars) with pore volumes are given in Figure 11c−f. For meso- and macropore volumes determined by nitrogen adsorption, it was suggested that clay content has a weak positive relationship with it, while no obvious relationship between quartz and feldspar was observed. Moreover, the
reservoirs reported that TOC shows a significant association with the total pore volume.33 Numerous studies of the pore structure of shale gas reservoirs reported a positive relationship between TOC and micro- or mesopore volume.34−36 In this paper, unlike most of the results reported for shale gas reservoirs, a decreasing trend in meso- and macropore volumes (determined by nitrogen adsorption) with increasing TOC suggests that the TOC content takes a negative role in mesoand macropore volumes and the pores are not associated with organic matter in this case (Figure 11a). However, with 6015
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thermal maturity on pore structures. Curtis et al. found secondary pores developed within Woodford shale samples of Ro (vitrinite reflection) values greater than 1.23%, but no secondary porosity was found in shales with Ro smaller than 0.90%.37 Loucks et al. reported that in Barnett shale samples, organic pores developed in organic matter with Ro higher than 0.8%, and no organic pores were found in shales below Ro of 0.7%.38 Mastalerz et al. studied pore evolution in Mississippian New Albany shales and revealed that as thermal maturity gets to the gas window level, the secondary, organic-related pores are contributed by the thermal cracking of bitumen and oil to natural gas.13 In the U.S. part of the Williston Basin, the thermal evolution level of organic matter in the Bakken shales is relatively low compared to that of shale gas reservoirs, with most part of the Bakken formation in the thermal maturity level of immature to oil window.23 Table 1 shows that the samples used in this study have Ro in the range of 0.55−1.01%. From the linear regression analysis of the thermal evolution degree with respect to pore structure parameters (Figure 13), it is found that, in
relationship between a specific area with minerals was also given in the secondary axis, in which the very low R2 values show that the specific surface area is not strongly associated with mineral contents in samples. For micropores determined by carbon dioxide adsorption, samples have less clay content, and greater quartz and feldspar contents tend to have more micropores, and higher R2 values were found, compared to the values of the correlations for meso- and macropores, suggesting that micropores are more related to mineral content. For MBS, the increase in pore volumes with an increase in both clay contents and carbonate contents shows that most of the pore volumes are related to these minerals (Figure 12a,b), while silicious minerals have a negative role in pore development (Figure 12c).
Figure 13. Plot of Ro% versus pore volume: (a) volume of macro- and mesopores and (b) volume of micropores.
general, the correlation relationship between the thermal evolution and the total pore volume is poor. Instead, a negative relationship between TOC and meso- and macropores was found in this study. One potential mechanism is that the control factor for meso- and macropores is an inorganic component, and also as illustrated in Figure 11c, the meso- and macropore volumes have a positive trend with clay content. Moreover, the specific surface area increases with the increase of thermal evolution degree. However, for samples with Ro lower than 0.7%, the relationship between Ro and specific surface area was poor, and higher-specific-surface-area values were then seen for samples with Ro values higher than 1.0%.
Figure 12. Correlation between MBS constituents and meso- and macropore volumes. Dashed lines represent linear regression between two variables.
5.2. Relationship between Thermal Maturity and Pore Structure. Previous studies revealed that the pore development of shales is strongly related to the evolution of thermal maturity and hydrocarbon generation, which could be utilized to help us identify the target reservoir and locate the favorable spots for oil and gas exploration. However, regarding the Bakken formation, limited studies discussed the impact of 6016
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Figure 14. Correlation relationship of fractal dimension versus pore structure parameters.
This observation is consistent with the previous SEM study of pore development for Bakken formation by Xu and Sonnerberg, which suggested that the organic porosity is maturity-dependent, and the porosity has a remarkable decreasing trend with increasing Ro in immature stages and then an increasing trend by entering mature stages.39 In our case, in the process of thermal evolution, organic-matterrelated micropores are not developed until the shale is mature enough (with Ro greater than 1.0%). The pores developed in the organic matter were relatively small (nanoscale); the increasing organic matter pores did not reflect on the obvious increase of pore volume but on the specific surface area since considering the same pore volume, smaller pores have a greater specific area than larger pores. 5.3. Relationship between Pore Structures and Pore Complexity. Plots of fractal dimensions versus various pore structure parameters are given in Figure 14. A good correlativity between fractal dimension with macropore volume (Figure 13c), specific surface area (Figure 13e), and average pore diameter was found (Figure 13f). Previous studies reported a similar trend among these parameters.7,33,40 The greater the fractal dimension is, the more complex the pore structure is, and the samples tend to have more micropores and fewer macropores. Existence of more micropores results in
larger specific surface area and smaller pore width. Moreover, a weak or no correlation relationships were observed for fractal dimension versus mesopore volume and total pore volumes (Figure 13c,d). Therefore, the values of fractal dimension are strongly correlated to those of the micropores, and the value of it can effectively indicate the complexity of pore structure.
6. CONCLUSIONS Comprehensive shale rock characterization, including XRD and Rock-Eval analysis, along with gas adsorption experiments was conducted to examine the pore structure of unconventional oil play using Bakken shales. The following conclusions were drawn in this study: • X-ray diffraction analysis shows that quartz and illite are the major components in the matrix of Bakken shales, while quartz, feldspar, dolomite, and clays dominate the MBS. • Rock-Eval source rock results illustrate that Bakken shales are significant organic carbon-rich, with various maturity levels, and the thermal maturity level is from immature to mature level. • Nitrogen isotherms for shale samples were mainly type II, while those for MBS were type III. Based on the hysteresis loop shape, the shale samples mainly contain 6017
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silt- and narrow-silt-like pores, while the pore shapes in sandstone and siltstone samples are similar to inkbottlelike. MBS have a larger specific surface area and pore volume than LBS and UBS. Pores in LBS and UBS are smaller than the ones in MBS. Shale samples from the lower and upper Bakken members have average pore diameters from 3 to 9 nm, while samples from MBM are in the range of 9−25 nm. Samples having more TOC component tend to have fewer meso- and macropores and more micropores. The content of clay minerals controls the meso- and macropore volumes in shale samples. For sandstone/ siltstone samples, clay and dolomite control the pore volume. Development of micropores in organic matter is thermal-maturity-related. Shale samples with vitrinite reflectance higher than 1.0% tend to have higher BET surface area, suggesting the development of organicmatter-related pores. Fractal dimension has positive relationships between micropore volume and BET surface area, and negative relationships between macropore volume and average pore diameters. It represents the complexity of pore structures.
AUTHOR INFORMATION
ORCID
Chunxiao Li: 0000-0003-2220-9369 Lingyun Kong: 0000-0001-9703-8299 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank the North Dakota Core Library for providing the samples. The authors would also like to thank the anonymous reviewers for constructive comments, which helped improve the manuscript. The authors would like to especially thank the Allen & Eleanor Martini Named Grant from the 2018 American Association of Petroleum Geologists Foundation Grants-in-Aid Program.
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