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Nanoscale Pore Structure Characterization of Tight oil Formation: A Case Study of the Bakken Formation Chunxiao Li, and Lingyun Kong Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00514 • Publication Date (Web): 13 May 2019 Downloaded from http://pubs.acs.org on May 15, 2019
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Energy & Fuels
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Nanoscale Pore Structure Characterization of Tight oil Formation: A
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Case Study of the Bakken Formation
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Chunxiao Li1, Lingyun Kong2*
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1 Harold Hamm School of Geology & Geological Engineering, University of North Dakota, 81
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Cornell Street, Grand Forks, ND 58202
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2 Department of Petroleum Engineering, University of North Dakota, 2844 Campus Road,
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Grand Forks, ND 58202
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Abstract
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Pore structure of unconventional reservoir is fundamental for understanding hydrocarbon
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storage, fluid transport, and geomechanics. The pore structure of shale gas reservoirs has been
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studied extensively while that of shale oil reservoirs remain limited investigation. The Bakken
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formation is one of the largest contributors to the growth of unconventional oil in the U.S. In this
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study, 13 core samples collected from the Bakken Formation were examined through a series of
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experiments to investigate the geochemical properties, mineralogy, and especially pore structure.
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Mineralogy analysis through X-ray diffraction (XRD) test showed that quartz and illite are the
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major components for the upper and lower shale members, while quartz, feldspar and dolomite
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dominate the middle member of Bakken Formation. Rock-Eval source rock analysis illustrated
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that all the shale samples contain a significant percentage of organic matter. Nitrogen and carbon
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dioxide adsorption results showed that isotherm curves obtained from nitrogen adsorption are
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reserved S-shaped (typical type II curve), indicating that pores are mainly micro- and meso-pores.
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Linear regression analysis of pore structure parameters with respect to TOC and mineral 1 ACS Paragon Plus Environment
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composition reveals that TOC content has a positive relationship with micropore volume, while
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meso- and macro-pores are controlled by clay content. Development of micropores in organic
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matter is thermal maturity related. Shale samples with vitrinite reflectance higher than 1.0% have
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higher surface area, suggesting that more micropores were developed in the organic matter after
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maturities of shales reached oil window level. In addition, results of the fractal analysis showed
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that samples with higher fractal dimension values are featured by more micro-pore volume,
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smaller pore diameter, and larger specific surface area.
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Keywords: Pore structure; Tight oil formation; Gas adsorption; The Bakken Formation; Fractal
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dimension
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1. Introduction
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In the last two decades, the unconventional shale revolution dramatically changed the global
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energy structure and became the exploration target in the U.S.. The Bakken formation in Williston
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Basin is one of the largest unconventional oil target formations in the U.S1. The U.S. portion of
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the Bakken Formation carried 3.65 billion barrels of oil and 1.85 trillion ft3 of gas based on the
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USGS assessment in 2008 2. As porous media, shale rocks are heterogeneous and complex in
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composition and pore structures. Pore structures are crucial for understanding the storage and
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transport of hydrocarbon during the development, and the geomechanical properties of shale
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reservoirs during production 3,4. Pore structures are controlled by various factors, for instance,
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the mineral composition, thermal maturities of rocks, et al 5.
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Researchers investigated the pore structure of unconventional rocks based on indirect and direct
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methods. In the direct methods, FE-SEM combined with ion-milled techniques, FIB-SEM, micro2 ACS Paragon Plus Environment
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Computer Tomography (micro-CT), and nano- Computer Tomography (nano-CT), could directly
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provide ways to observe pore morphology, distribution and pore connectivity 6–8. In the indirect
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approaches, gas (nitrogen/carbon dioxide/methane) adsorption and mercury intrusion
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porosimetry (MIP) could provide us the quantitative information of volume, surface area, and
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size of pores by analyzing the adsorbed gas volume or intruded mercury volume with respect to
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the applied pressure 5,9,10. Helium pycnometer is another indirect method to measure the total
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effective porosity the sample. Moreover, each characterization method can only depict a specific
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size range of pore structure due to the technical limitation of each technique11. Comparison of
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these methods in regards to resolution limits is given in Figure 1. Gas adsorption is one of the
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most widely used techniques for pore structure characterization. Previous researches have
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proved that gas adsorption can be successfully used in nano-size pore characterization of geo-
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materials
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dioxide, depending the properties of the porous materials and information required14. Nitrogen
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at temperature of 77K is used as a standard adsorptive for pore analysis, which is capable of
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detecting pores in the range of 2-200nm, while carbon dioxide adsorption used in the range of
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0.5, and Region A for P/P0
