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Nuclear magnetic resonance cryoporometry is a newly developed technique that can characterize the pore size distribution of nano-scale porous materials. To date, this technique has scarcely been used for the testing of unconventional oil and gas reservoirs; thus, their micro- and nano-scale pore structures must still be investigated. The selection...
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... The characterization methods of the pore structure of shale mainly include image analysis, fluid injection, and non-material injection (Maex et al., 2003). The non-material injection methods refer to nuclear magnetic resonance (NMR) (Su and Zou, 2016;Zhu et al., 2019), smallangle scattering, computerized X-ray tomography (CT), and thermal porosimetry method (Felix et al., 2012;Jannot et al., 2018). At present, scanning electron microscopy (Chen et al., 2011;Wang et al., 2012;Wei et al., 2013), mercury intrusion porosimetry, low-pressure nitrogen adsorption (Yang et al., 2013), NMR, etc., have been widely used in shale pore characterization (Chen S. et al., 2015;Hu et al., 2015;Yang et al., 2016b;Hu et al., 2016;Chen et al., 2017;Gao and Hu, 2018;Yuan et al., 2018). ...
... The comprehensive reaction is expressed as: Clay mineral + feldspar + H 2 O + CO 2 ↔ Quartz + calcite + dolomite + kaolinite + siderite. Kaolinite reacts with carbonate minerals to form quartz and dolomite: [Fe/Mg]5Al 2 Si 3 O 10 (OH) 8 + 5CaCO 3 + 5CO 2 ↔ 5Ca [Fe/Mg](CO 3 ) 2 + Al 2 Si 2 O 5 (OH) 4 + SiO 2 + 2H 2 O. Carbonate rocks such as calcite and dolomite will undergo the following chemical reactions in acidic solutionsZhu et al., ...
The microstructure, mineral composition, total organic carbon content, etc., of gas shale are crucial parameters for shale reservoirs, which can directly/indirectly affect shale brittleness, fracturing effect, adsorption ability and production efficiency. The study proposed a workflow to characterize the physical and mechanical parameters of Lower Silurian Longmaxi shale outcrop samples extracted from the favorable block in Changning, Sichuan, southwest China. This study elaborated on the influence of these physical and mechanical characteristics and proposed a corresponding brittleness index on shale extraction. In addition, it put forward corresponding suggestions for development and risk control. For a better understanding the mechanisms of shale gas storage and production, XRD, XRF, SEM, low temperature Nitrogen adsorption method, nuclear magnetic resonance and other measurements were employed to analyze and study the mineral composition, microstructure, and adsorption performance of shale. The results demonstrated that the pores of shale are mainly slit pores; there are diverse pore types in shale, mainly including intergranular pores, mineral particle dissolution pores, and internal pores of organic matter; The samples with relatively low porosity also noticeably exhibit ultra-low permeability, and the nanopore structure is remarkably significant, with distribution primarily in range of 5–237 nm. Finally, a brittleness index considering the influence of water content and the mechanical properties was proposed, and the coupling interaction of various minerals components and mechanical properties on the brittleness index can more objectively reflect the brittleness characteristics of deep shale formation.
... According to the literature, the K GT range of water is between 45 and 58.5 nm K, 13,14,54 and using different K GT values leads to a translation of the PSD curve of the transformation. 16,55 The K GT value of 50 nm K that has been used in most studies was used in this study. 42,56 The experimental temperature range (−35 to −0.1°C) can be converted to the pore diameter range (1.43−500 nm). ...
Characterizing the pore and fluid distribution is critical for evaluating the reservoir potential of new areas. Nuclear magnetic resonance (NMR) is considered as an experimental method capable of full-scale characterization of pore characteristics. However, the T2 spectrum of a saturated sample is affected by a combination of sample and experimental parameters, and it is important to confirm whether the T2 spectrum fully reflects the sample pore information. Eight tight sandstone samples from the Julu area were selected for thin section identification, mercury intrusion porosimetry (MIP), NMR, NMR cryoporometry (NMRC), and centrifugation experiments to critically analyze the applicability of the NMR results. Two methods, the similarity method and Kozeny’s equation method, were used to calculate surface relaxivity, a critical parameter for converting NMR T2 signals into pore information. The discussion focuses on the applicability of the calculated surface relaxivity and the phenomenon of T2 signal changes in a short relaxation range after centrifugation. The main results are as follows: The surface relaxivity values calculated using the different methods differed significantly. The surface relaxivity calculated using the same method reflected the relative magnitude of the true surface relaxivity of the samples. For the samples with large surface relaxivity, there may be partial misses of the short relaxation signal, the NMR porosity was smaller than the gas-measured porosity, there was a variation in the T2 spectrum in the short relaxation range after centrifugation, and the calculated surface relaxivity was small. The surface relaxivity calculated using Kozeny’s equation was nearly accurate, but perhaps smaller than the true value. The T2 spectra mainly reflected macropore information. This study suggests that PSDs converted from T2 spectra of saturated samples should be judged with relative caution rather than solely based on the peak or range correspondence between the two curves, and the minimum centrifugal radius can be used as a constraint.
... As an emerging pore characterization technology, nuclear magnetic resonance cryoporometry (NMRC) does not only cover the measurement range of nanopores and realize the continuous characterization of a given sample but also allows the direct and efficient acquisition of information, such as PSD and porosity [13,20,27,28]. Unlike NMR, the theoretical basis of NMRC is the Gibbs-Thomson equation, which offers the possibility to directly obtain pore size data based on changes in the melting point of a solid [29]. ...
... Unlike NMR, the theoretical basis of NMRC is the Gibbs-Thomson equation, which offers the possibility to directly obtain pore size data based on changes in the melting point of a solid [29]. In an attempt to use this method to measure rock pore sizes, some scholars have used octamethylcyclotetrasiloxane (OMCTS) as the probe liquid, which does not only improve the accuracy of mesopore and macropore measurements but also meet the testing requirements for tight sandstone reservoir (between 4 and 1400 nm) characterization [13,28]. However, it greatly narrowed down the measurement range and cannot truly and comprehensively display the pore size distribution of the sample. ...
... NMRC is a new method for measuring the nanometer pore of coal and shale and based on the Gibbs-Thomson equation [28,38,39]. In recent years, this experiment has also been used for tight sandstone pore size analysis [13,28]. ...
Exploring appropriate methods to understanding the pore structure and overall pore size distribution (PSD) of tight sandstone reservoir is important for evaluating the quality of the reservoir. An integration analysis comprising X-ray diffraction (XRD), three beams of argon ion (TIB) polishing approach, scanning electron microscopy (SEM), high-pressure mercury intrusion (HPMI), nuclear magnetic resonance (NMR), and nuclear magnetic resonance cryoporometry (NMRC) was applied to determine the pore structure of the tight sandstone reservoirs in the Upper Paleozoic of Dongpu Depression as well as their relationship with the physical properties of the reservoirs. NMRC offered the possibility to obtain the nanoscale (4-1400 nm) pore structure of the reservoirs directly and accurately. Therefore, an attempt by combining NMRC and NMR can reveal the PSD of tight sandstone reservoirs with different pore structures. The results showed that the tight sandstone reservoirs consisted of intergranular, intragranular, and intercrystalline (clay mineral) pores. The full PSD intelligibly showed pore structure characteristics of four different types of reservoirs, with pore sizes ranging from 2 nm to dozens of microns. Specifically, the overall PSD of type I reservoirs showed a broad unimodal distribution pattern with the peaks in the range 0.1–2 μm, indicating an association with dissolution intergranular pores, and for type II reservoirs, the overall PSD showed a bimodal distribution pattern, with their left and right peaks, in the ranges 0.004–0.01 μm and 0.15–0.4 μm, respectively, showing similar amplitudes, implying the predominance of both intergranular (mesopores) and intergranular (macropores) pores. The full PSDs of type III and IV reservoirs showed much lower amplitudes than type I and II reservoirs, indicating a lower pore number and a complex pore structure. Furthermore, NMRC also demonstrated that different diagenesis resulted in a correlation between pore structure and reservoir physical properties.
... The sandstone samples were analyzed using a NMRC12-010V freeze-melt instrument. OMCTS with a low chemical activity was used as the probe fluid; it has an upper experimental limit of 1500 nm, and the lower limit is 4 nm, which is suitable for the analysis of tight sandstone (Liu et al., 2017a;Zhu et al., 2018Zhu et al., , 2019. Before the test, the sample needs to be pretreated. ...
Pore structure and connectivity are key factors that determine the quality of tight sandstone reservoirs, which are a frontier field for exploration worldwide in the 21st century but are quite challenging to characterize. Here we explored novel methodologies based on a case study of the well-known Chang-7 reservoir in the Upper Triassic Yanchang Formation, Ordos Basin, China. Pore size distribution (PSD) can be quantitatively characterized by nuclear magnetic resonance cryoporometry (NMRC), and the connectivity of the reservoir can be described by using a combination of Wood’s metal (WM) impregnation, environmental scanning electron microscopy (ESEM), and nano-computed tomography (CT). The PSD of tight sandstones with different oil-bearing levels exhibit obvious differences. The complexity of pore boundaries and orientations are key factors that affect reservoir physical properties and oiliness. The pore fracturing in tight sandstones can be divided into three types: poor, medium, and good connectivity. The ball-and-stick model is suitable for the study of reservoir connectivity, and the effect of tight sandstone heterogeneity on connectivity analysis can be reduced significantly. Our data suggest that multi-scale characterization is reliable for describing the pore structure and connectivity of tight reservoirs. It is found for the first time that the quality and oiliness of tight sandstone reservoirs are not determined solely by physical properties, but are also controlled by PSD and pore morphology and connectivity. This may lead to underestimation of the effective reservoir and potential resources as previously thought.
... The residual signal integral can be attributed to the remained frozen cyclohexane which is not relaxed during the applied echo time. [23,45] The broad pore size distribution has a maximum at diameter (d) ca. 110 nm ( Fig. 7.b) meaning that cyclohexane was detected in large, cylindrical channels. ...
Carbon aerogels prepared from resorcinol formaldehyde organic aerogels have a wide range of use due to their considerably large specific surface area. Since the applications mostly happen in wet form, e.g. in aqueous medium, NMR cryoporometry was employed to follow the porous behavior of an organic aerogel and its carbon derivative, as well as the textural changes after the pyrolysis. Water and cyclohexane were used as hydrophilic and hydrophobic probe molecules, respectively. In the polymer aerogel, saturated with water, by NMR we found spherical mesopores confined by the aerogel beads and wide channels in the macropore size-range separating the aggregated beads. After carbonization cylindrical pores were observed between the beads and the aggregates got closer to each other. On the other hand, the hydrophobic cyclohexane probed exclusively the macropores, which might be the result of local swelling. The micropore region both in the polymer and the carbonized form was explored only by the low temperature gas adsorption measurements. The comparison of the two methods confirmed that these techniques excellently complement each other in characterizing the micro-, meso- and macropores of solid porous materials: vapor adsorption is superior in characterizing the micro- and mesoporosity, while NMR cryoporometry provides information about the pore geometry and size distribution in the meso- and macropores.
The complex pore structure of unconventional oil and gas reservoirs is one of the reasons for the difficulties in resource evaluation and development. Therefore, it is crucial to comprehensively characterize the pore structure, understand reservoir heterogeneity from multiple perspectives, and gain an in-depth understanding of fluid migration and accumulation mechanisms. This review outlines the methods and basic principles for characterizing microporous systems in unconventional reservoirs, summarizes the fractal analysis corresponding to the different methods, sorts out the relationship between the fractals and reservoir macroscopic physical properties (porosity, permeability, etc.) with the reservoir microscopic pore structures (pore structure parameters, pore connectivity, etc.). The research focuses on cutting-edge applications of characterization techniques, such as improved characterization accuracy, calibration of PSD ranges, and identification of different hydrogen compositions in pore systems for dynamic assessment of unconventional reservoirs. Fractal dimension analysis can effectively identify the quality level of the reservoir; complex pore-throat structures reduce permeability and destroy free fluid storage space, and the saturation of removable fluids is negatively correlated with Df. As for the mineral composition, the fractal dimension is positively correlated with quartz, negatively correlated with feldspar, and weakly correlated with clay mineral content. In future qualitative characterization studies, the application and combination of contrast agents, molecular dynamics simulations, artificial intelligence techniques, and 4D imaging techniques can effectively improve the spatial resolution of the images and explore the adsorption/desorption of gases within the pores, and also help to reduce the computational cost of these processes; these could also attempt to link reservoir characterization to research on supercritical carbon dioxide-enhanced integrated shale gas recovery, carbon geological sequestration, and advanced underground hydrogen storage.
