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Characterization of Nanoporous Systems in Gas Shales by Low Field NMR Cryoporometry Bing Zhou, Qian Han, and Peiqiang Yang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01780 • Publication Date (Web): 04 Oct 2016 Downloaded from http://pubs.acs.org on October 8, 2016
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Characterization of Nanoporous Systems in Gas Shales by Low Field NMR Cryoporometry Bing Zhou*§, Qian Han ǂ, and Peiqiang Yang ǂ
§
School of Materials Science and Engineering, Tongji University, China, 210000 ǂ
Suzhou Niumag Analytical Instrument Corporation, China, 215100
KEYWORDS: organic shale, nano-sized pore, PSD, NMRC, NMRR, low magnetic field
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ABSTRACT
Shale gas and oil is an increasingly important source of unconventional energy. Shale gas and oil reservoirs differ from their conventional counterparts mainly in the nanoporous structures of the former, which play a critical role not only for the resource estimation but also for the shale gas/oil extraction and development. However, the traditional methods for characterizing rock porosities, such as gas sorption (the Brunauer-Emmett-Teller technique, BET), mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), cannot satisfactorily and adequately measure and characterize the nanoporous structures. Nuclear magnetic resonance (NMR) spectroscopy is known for its sensitivity to local environments at the atomic level and, therefore, can provide an alternative method for the investigation of nanoporous structures in gas shales. This study has refined the low field NMR cryoporometry (NMRC) method and applied it along with other methods such as NMR relaxometry (NMRR) to measure and characterize the nanoporous structures (i.e., the pore size distribution, PSD) of selected shale samples from the Sinian-Cambrian-Ordovician strata at the Low Yangzi Plateau, China. Our NMRC measurements of a controlled porous glass (CPG) and shale samples show that the organic compound octamethylcyclotetrasiloxane (OMCTS) is a superior NMR probe liquid in terms of improved spectral resolution and signal/noise (S/N) ratio. Comparisons of the NMRC shale PSD results with those from NMRR and gas sorption show that NMRC is an independent and effective method for determining the distribution of nano-sized pores in gas shales. Moreover, important parameters such as porosity can also be estimated from the low field NMR cryoporometry.
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INTRODUCTION The shale gas revolution in the USA over the past decade has made shale gas an increasingly important source of unconventional energy and has inspired a broad range of research into evaluating and developing shale gas resources in many countries worldwide. 1-4 The most salient feature of gas shales is their nanoporous structures, which not only facilitate the adsorption and preservation of gas but also pose significant difficulties in gas extraction due to the very low permeability and high capillary pressure caused by such pores. As such, knowledge about the nanoporous structures in shales is crucially important for resource estimation as well as the development and extraction processes. Therefore measurement and characterization of nanoporous structures, especially pore size distribution (PSD), in shales are extremely important in the evaluation and development of shale gas reservoirs. 4-6 The nanoporous nature of such shales presents significant challenges for the traditional methods of porosity measurements such as gas sorption (the Brunauer-Emmett-Teller technique, BET), mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). MIP, for example, only measures the porous throats, but the porous structures in shales may be destroyed during the forceful intrusion procedure. Likewise, the time-consuming and semi-quantitative nature of gas sorption measurement at low pressure for nanopores make this method unfeasible and unattractive as well. The imaging-based methods, such as SEM and TEM, are usually restricted to local pores on the micrometer scale and therefore may not provide the true porosity in shales. Nuclear magnetic resonance (NMR) spectroscopy, on the other hand, is very sensitive to local structural environments and dynamic processes at the atomic/molecular level. As a result, NMR has been widely applied to investigate geochemical processes at the atomic level as well as local
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structural features, such as bonding in minerals. 7-9 NMR, therefore, is an ideal approach for characterizing the nanoporous structures in shales. Indeed, NMR relaxometry (NMRR) has already been utilized to characterize PSD in porous media such as cements, sandstones and shales.
5-6,10-25
Magnetic Resonance Imaging (MRI) of freezing water as a function of
temperature has also been successfully employed to investigate the spatially resolved PSD in cements.
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The PSD data obtained with NMRR, however, sometimes appear to be independent
of the samples, rendering it unusable for the purpose of fingerprinting. This may be the result of improper assumptions for surface relaxation, as well as the non-stable nature of the Inversion Laplace Transformation (ILT) employed by NMRR.
17,25
The most significant problem for
NMRR lies in the fact that the diffusional displacement of molecules over a period comparable to the transverse relaxation time T2 stretches over many pores, and therefore one measures only the average value of T2. In the present study, the NMR cryoporometry (NMRC) method has been investigated and refined to reliably characterize the nanoporous structures in shales. In particular we hypothesize that the diffusion of NMR probe liquids in nanoporous shales is limited by the confinement in very small pore spaces, as well as occlusion by the frozen phase (i.e., pore blocking on freezing, premolten layers). At low magnetic fields, the effect of internal field gradients produced in nanoporous shales is expected to be very small too, especially for NMR measurements using the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequences. 25 Similarly, the effects of paramagnetic impurities in shales will be also reduced significantly at low magnetic fields. Furthermore, NMRC has a significant advantage over NMRR and other methods in that it can be used even when liquids and volatile components already exist in the pores. 16 NMRC can thus directly probe the pores accessible to the imbibing liquid. 27
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In comparison with the complexity of relaxation mechanisms encountered with NMRR,
25
NMRC, a thermodynamics-based method, is more straightforward. Nanometer scale structure will change the liquid Gibbs-Free Enthalpy, as surface tension is directly equivalent to the volumetric energy 16. Based on the differences in the Helmholtz Free Energy for fluids at the surface and center of a pore, the melting point depression (MPD) can be calculated via: 28 ∆T = −
୴ౢ ஓ౩ౢ బ ୗ౩ ∆ୌ ౩
= −்ீܭ
ୗ౩ ౩
(1)
where T0 as the bulk solid-liquid transition temperature, ∆H is the bulk enthalpy of fusion, ɣsl is liquid-solid surface energy, vl is the molar volume, while Vs and Ss donate the volume and surface of the solid core, respectively. Further details of the equation can be found in reference 28. If we assume spherical pores with radius D, the liquid-solid equilibrium from Equation (1) is shifted to liquid, 29 as shown by the Gibbs-Thomas Equation (2), with KGT of 58.2 K·nm for H2O: 16,30
∆T=-KGT/D
(2)
Thus, as indicated by Equation (2), the corresponding MPD is directly related to the pore size. Traditionally phase transitions are detected with differential scanning calorimetry (DSC). However, freezing/melting phase transitions can also be detected by measuring the NMR signal from the probe liquid, since the frozen NMR probe liquid and the structural OH are nonobservable or can be filtered out of NMR measurements because of their fast relaxation times (T2 ~6 µs for frozen water). 19 In other words, the signal intensity quantitatively gives the amount of the NMR probe liquid in the pores at each temperature, thus, providing the PSD of the porous media. The NMRC method had been applied to the PSD analysis of cements and other model porous materials such as controlled porous glass (CPG).
11,28,31,32-33
A few NMRC studies have
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attempted to analyze the PSD in shales using water as the probe liquid and were able to measure the pores up to 1 µm in diameter. 16,30 From Equation (2), the MPD becomes negligible for pore diameters over 100 nm, although sizes of ~10 µm had been reported by Webber et al.. 16 Even if the conditions for MPD still hold for such large pores, the temperature resolution must be better than 0.006°C for a pore size of 1 µm if H2O is used as the probe liquid. Such a temperature resolution is technically difficult for current commercial NMR spectrometers, although Webber (2014)
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reported to have developed such a system with exceptional thermal stability and
resolution within 10 mK. In order to extend the upper limit of detectable pore sizes (up to ~1 µm), NMR probe liquids with larger KGT may be employed for NMRC. 11,33 For the lower limit, considering surface relaxivity (ρ) of 1 to 10 µm/s, T2 for H2O in nanopores of ~1 nm is ~0.1 ms, which can be detected by most commercial NMR instruments. 20 Therefore, in principal, pore sizes as low as ~1 nm may be measured by NMRC, although in practice nanopores
