Fractal Characteristics of Pores in the Longtan Shales of Guizhou, Southwest China

Huang, Yuqi; Zhang, Peng; Zhang, Jinchuan; Tang, Xuan; Liu, Chengwei; Yang, Junwei

doi:https://doi.org/10.1155/2020/8834758

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Abstract Introduction Results Discussion Conclusions Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

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Characterisation and Assessment Methods for Shale Plays

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Research Article | Open Access

Volume 2020 | Article ID 8834758 | https://doi.org/10.1155/2020/8834758

Fractal Characteristics of Pores in the Longtan Shales of Guizhou, Southwest China

Yuqi Huang,¹Peng Zhang,¹Jinchuan Zhang,²Xuan Tang,²Chengwei Liu,¹and Junwei Yang¹

Academic Editor: Zhouheng Chen

Received24 Jul 2020

Revised09 Oct 2020

Accepted30 Oct 2020

Published23 Nov 2020

Abstract

The pore structure of marine-continental transitional shales from the Longtan Formation in Guizhou, China, was investigated using fractal dimensions calculated by the FHH (Frenkel-Halsey-Hill) model based on low-temperature N₂ adsorption data. Results show that the overall (fractal dimension under low relative pressure, ) and (fractal dimension under high relative pressure, ) values of Longtan shales were relatively large, with average values of 2.7426 and 2.7838, respectively, indicating a strong adsorption and storage capacity and complex pore structure. The correlation analysis of fractal dimensions with specific surface area, average pore size, and maximum gas absorption volume indicates that can comprehensively characterize the adsorption and storage capacity of shales, while can effectively characterize the pore structure complexity. Further correlation among pore fractal dimension, shale organic geochemical parameters, and mineral composition parameters shows that there is a significant positive correlation between fractal dimensions and organic matter abundance as well as a complex correlation between fractal dimension and organic matter maturity. Fractal dimensions increase with an increase in clay mineral content and pyrite content but decrease with an increase in quartz content. Considering the actual geological evaluation and shale gas exploitation characteristics, a lower limit for and upper limit for should be set as evaluation criteria for favorable reservoirs. Combined with the shale gas-bearing property test results of Longtan shales in Guizhou, the favorable reservoir evaluation criteria are set as and . When is less than 2.60, the storage capacity of the shales is insufficient. When is greater than 2.85, the shale pore structure is too complicated, resulting in poor permeability and difficult exploitation.

1. Introduction

Shale is a type of heterogeneous porous rock with a complex pore structure, including various pore shapes and sizes [1, 2]. Currently, the mercury intrusion method, adsorption-desorption method, and structural fractal method are commonly used to quantitatively analyze shale pore structure characteristics [3–5]. Pore fractal theory plays a significant role in studying the complexity of pore structures. The Permian Longtan Formation in Guizhou is a coal-bearing formation containing marine-continental transitional facies, which is generally characterized by high organic matter content and good gas indicators, indicating a great natural gas resource potential [6–9]. With the development of exploration and research, the evaluation of shale reservoirs has become an important research topic. Reservoir evaluation is one of the key steps to determining the natural productivity and economic value of shale reservoirs. Scientists have found that the Longtan Formation contains a set of shale reservoirs with low porosity (0.89%-2.29%) and permeability (), strong heterogeneity, small pore size, and complex pore structure [10]. Using qualitative methods such as polarized light microscopy and scanning electron microscopy, a qualitative understanding of the morphology and structure of the Longtan Formation has been gained [1, 10, 11]. However, there is limited quantitative research and evaluation of the Longtan Formation pore structure, and a geological parameter to characterize the pore structure well has not yet been identified and incorporated into the reservoir evaluation system. As a result, the pore structure is often neglected in the current reservoir evaluation system.

For conventional sandstone reservoirs, quantitative methods such as the mercury injection capillary pressure (MICP) experiment can be used to study pore structure characteristics [2, 12, 13]. Tight reservoirs, such as shale reservoirs, have high pore displacement pressure, requiring a higher experiment pressure to measure the pore size. However, high-pressure mercury injection can damage the pore structure of shale [14]. To address this problem, fractal theory, which is an effective approach for studying the irregular surface and heterogeneity of porous materials, was introduced into the study of shale pores [14–18]. Fractal theory considers the complicated pore fracture as a whole and quantitatively evaluates the heterogeneity and irregular surface of pores in shale reservoirs [5, 9].

Therefore, this paper systematically studied the pore structures and fractal characteristics of marine-continental transitional facies shales in Guizhou as well as the relationship with other geological parameters (reservoir parameters, organic geochemical parameters, and mineral composition parameters) based on low-temperature N₂ adsorption experiments, field emission scanning electron microscopy (FE-SEM) observations, X-ray diffraction (XRD) analysis of rock mineral composition, organic geochemical experiments, and gas content tests. In this study, the characterization function and geological significance of fractal dimension on pore structure are clarified. Also, the fractal dimension division standard is determined for the dominant shale reservoirs of the Longtan Formation in Guizhou Province, which provides the basis for further exploration, research, and evaluation.

2. Geological Conditions for Shale Development

The main portion of the study area is located in the Qianzhong Uplift and the depression in the northern Dianqian area, belonging to the Upper Yangtze plate (Figure 1). The regional structure is characterized by NNE-trending and NE-trending folds and faults. The study area is intersected by the Hezhang—Zunyi NE-trending fault to the north and surrounded by the Nayong fault to the southeast [19, 20]. Since the formation of the continent in the Middle Permian, the crust has been exposed and eroded. The Longtan Formation, a coal-bearing series of marine-continental sedimentary strata, has been widely deposited in northwest Guizhou. The alternating sedimentary facies combinations, including lagoon facies, tidal-control delta facies, tidal-flat facies, and peat swamp facies, were developed during the Longtan Period [21]. The shale has a small single layer thickness but large cumulative thickness, containing multiple interlayers and coal seams [20, 22]. The lithology is dominated by black carbon shale, silty mudstone, sandstone, siltstone, marlite, and coal (Figure 1).

3. Samples and Experimental Methods

Samples in this study are all unweathered Longtan core samples from Well-XY1 (X-1 to X-6 in Table 1), Well-FY1 (F-1to F-3 in Table 1), and Well-JS1 (the locations of the three wells are shown in Figure 1). All samples were taken from gas-bearing layers and contain the primary Longtan shale reservoir lithology. The depth and lithology of the samples are listed in Tables 1 and 2.

A complete experimental project was conducted, including measurements of organic carbon (TOC) content, kerogen microcomponents, vitrinite reflectance (), XRD analysis of rock mineral composition, argon ion polishing—field emission scanning electron microscopy, low-temperature N₂ adsorption, isothermal adsorption, and gas content analyses (direct method).

The TOC tests were conducted using a LECO-CS230 carbon and sulfur analyzer. The powder samples (100 mg) were treated with 5% HCl (hydrochloric acid) at 80°C to remove inorganic carbon and then washed with pure water to remove residual HCl. The treated samples were placed into the apparatus to measure TOC content. The experiments conformed to the national standard GB/T19145-2003 (No. GB/T19145-2003 of the National Standards of the People’s Republic of China). An Axio Scope A1 microphotometer was used to measure with SY/T5124-2012 and SY/T5125-2014 as the test bases (No. SY/T5124-2012 and SY/T5125-2014 of the Petroleum Industry Standard of the People’s Republic of China).

An X’Pert Powder X-ray diffractometer (PANalytical Company, Netherlands) was used to measure the mineralogical compositions of the Longtan Shale samples. Samples were crushed into powders with grain size ranging from 200 to 300 mesh before the XRD analysis. The experiments conformed to the industry standard SY/T5163-2010.

Samples were polished by Argon ion using an Ilion II697 argon-ion polisher (Gatan Company). Then, the polished samples were scanned using a Merlin Compact field emission scanning electron microscope (ZEISS Company) to obtain the micromorphology images with an acceleration voltage of 10.0 kV and working distance ranging from 2 to 15 mm. The experiments conformed to the industry standard GB/T 16594-2008 and SY/T 5162-1997.

Low-temperature N₂ adsorption experiments were performed using a Quadrasorb SI specific surface analyzer. The crushed samples (60-80 mesh) were degassed for 4 h at 150°C. Then, N₂ adsorption experiments were conducted at -196°C under a relative pressure ranging from 0.040 to 0.997. The specific surface area and average pore size were calculated using the Brunauer-Emmett-Teller (BET) method (Brunauer et al., 1938). The experiments conformed to the industry standard GB/T19587-2004.

Isothermal adsorption tests were conducted using a GAI-100 high-pressure gas isothermal adsorption apparatus with a pressure range of 0.007-17.300 MPa at 30°C with SY/T 6132-1995 as the test basis. The desorbed gas content and residual gas content data were directly measured using a tubeless field desorption instrument and a fully sealed residual gas analyzer independently developed by the China University of Geosciences (Beijing). The lost gas content was obtained by mathematical calculation. ISO18871-2015 was used as the reference standard.

4. Results

4.1. Reservoir Characteristics

According to the geochemical test results from Longtan shale samples, the TOC of shales ranges from 1.01% to 8.62%, with an average value of 3.68%, indicating that the overall organic matter content is relatively high. values primarily range from 2.00% to 3.00% with an average value of 2.61% (Table 1), indicating that Longtan shale is in the early overmature stage with good gas generation conditions. Longtan shales are primarily composed of quartz, feldspar, carbonate minerals, and clay minerals. The quartz content ranges from 10.46% to 38.19% with an average value of 28.99%, and clay mineral content ranges from 25.94% to 47.37% with an average value of 40.37%, which is relatively large. In addition, clay minerals are primarily composed of illite/montmorillonite and illite, while chlorite, kaolinite, and montmorillonite account for a small proportion (Table 1).

Through FE-SEM observations, Longtan shale pores can be divided into four types: inorganic intergranular pores, inorganic intragranular pores, organic pores, and microfractures based on their relationships with grain and pore morphology. Inorganic intergranular pores are mostly intercrystalline pores and shrink fractures along the grain (Figures 2(a) and 2(f)), with large pore sizes ranging from mesopores (2-50 nm) to macropores (≥50 nm). With simple structure and good connectivity, inorganic pores are primarily intercrystalline pores between clay minerals and dissolution pores (Figures 2(b) and 2(c)). The intercrystalline pores of clay minerals are mostly slit-shaped with good connectivity, and dissolution pores are mostly round or irregular with relatively poor connectivity, including both micropores (<2 nm) and macropores. Both the organic abundance and maturity can control the development of organic pores. In the Guizhou area, Longtan shales have high organic abundance, and the maturity is primarily in the early overmature stage. Organic pores are mostly round and elliptical in shape with relatively poor connectivity (Figures 2(g), 2(h), and 2(i)), including both micropores and macropores. Microfractures are most likely related to tectonic activity and commonly appear within brittle mineral crystals (Figures 2(a), 2(g), and 2(h)). They play an important role in increasing pore connectivity and improving the reservoir properties of the Longtan Formation.

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(d)

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Figure 2

FE-SEM images of pores in Longtan shale samples ((a) sample from Well-XY1, 430.0 m, black carbonaceous shale; (b) sample from Well- XY1, 443.5 m, black carbonaceous shale; (c) sample from Well- XY1, 430.0 m, black carbonaceous shale; (d) sample from Well-JS1, 751.0 m, dark grey silty mudstone; (e) sample from Well-XY1, 439.0 m, black carbonaceous shale; (f) sample from Well-JS1, 781.4 m, black carbonaceous shale; (g) sample from Well-JS1, 781.4 m, black carbonaceous shale; (h) sample from Well-JS1, 779.3 m, black carbonaceous shale; (i) sample from Well-JS1,781.4 m, black carbonaceous shale).

Low-temperature N₂ adsorption-desorption experiments show that the specific surface area of Longtan shales ranges from 8.32-31.18 m²/g (average 15.79 m²/g). The proportion of micropores is high, primarily distributed from 3 nm to 5 nm (Figure 3), but there are also some mesopores and macropores larger than 25 nm. Adsorption-desorption curves show that the adsorption volume of the Longtan shales in the low-pressure region is small, and the slopes of the curves vary without any significant inflection points. The higher the relative pressure is, the larger the adsorption volume is. Capillary condensation of the adsorbate occurs, and the isotherm rises rapidly. Desorption lag occurs due to the capillary force. According to the IUPAC classification [23–28], the increasing hysteresis is H2 type and H3 type (Figure 4), indicating that quite a few flaky grains, primarily clay minerals, are present within the shale. Pores are primarily slit, wedge, and ink bottle-shaped [4, 11, 26].

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4.2. Shale Pore Fractal Dimension

Pore fractal dimensions were calculated based on the fractal FHH (Frenkel-Halsey-Hill) model using low-temperature N₂ adsorption data for the Longtan shale (Table 2) [26, 28–30]. The calculation formula is where —the volume of adsorbed gas at the equilibrium pressure P, cm³/g; —saturated vapor pressure, MPa; —equilibrium pressure, MPa; —fractal dimension.

According to the fractal FHH model, (fractal dimension) can be determined by the slope of the fitting line in the plot of . Generally, ranges from 2 to 3 [31, 32]. When is closer to 2, the pore structure is simple with a smooth and regular surface. On the other hand, when is closer to 3, the pore structure is complex with an irregular surface, strong heterogeneity, and strong fluid flow resistance. A high fractal dimension usually means a complex pore structure, which is more favorable for gas adsorption and storage than for gas transport [25, 26].

The piecewise least-square fitting line obtained from the experimental data reflects two different fractal characteristics in the shale pores (Table 2) (Figure 5). The fractal dimension value calculated using a curve with relatively low pressure () was recorded as , and the fractal dimension calculated using a curve with relatively high pressure was recorded as (). The plots of versus are shown in Figures 5 and 6. ranges from 2.6655 to 2.8648 with an average value of 2.7426, and ranges from 2.6841 to 2.9256 with an average value of 2.7838. and are large and close to 3. Also, is greater than (Figure 6).

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5. Discussion

5.1. Characterization Function of Fractal Dimensions

The correlation between the calculated fractal dimensions and the specific surface area (Figure 7(a)) as well as the average pore size (Figure 7(b)) is examined to determine the fractal dimension characterization function of the pore structures and shale storage properties. and increase with the increase in specific surface area and decrease in pore size. The correlation between and specific surface area is slightly better than that of , while the correlation between and average pore size is slightly better than that of . is more effective for comprehensively characterizing the shale adsorbed gas storage capacity, while can effectively characterize pore structure complexity and reservoir heterogeneity. Shale isothermal adsorption tests showed that the maximum adsorption gas volume has a significant positive correlation with (Figure 7(c)), which further verifies the ability of to characterize the gas adsorption and storage capacity of shales. According to the and values for the marine-continental transitional facies Longtan shales in the Guizhou area, the overall adsorbed gas adsorption and storage capacity is good, indicating that the Longtan shales are a good reservoir for adsorbed natural gas. However, the pore size is small, and the pore structure is complex with relatively poor connectivity.

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5.2. Relationship between Fractal Dimensions and Organic Geochemical Characteristics

Shale is typically rich in organic matter. The nanomicroorganic pores present in the organic matter are important storage spaces for gas [22, 33, 34]. Therefore, a significant relationship exists between the organic geochemical characteristics and fractal dimensions. By analyzing the correlation between fractal dimensions and TOC (Figure 8(a)) as well as (Figure 8(b)), it is found that a good positive correlation exists between fractal dimensions and TOC. The fractal dimensions increase with increasing organic matter abundance because the organic pores and clay mineral-related inorganic pores are the most important type of shale reservoirs. In marine-continental transitional facies shales, organic matter is commonly associated with clay minerals, indicating that both organic pores and inorganic pores are well developed in organic-rich shales. All these pores have small diameters and complex structures resulting in a significant increase in fractal dimensions. The correlation between fractal dimensions and is not obvious. The fitting lines show that when ranges from 2.0-2.5%, fractal dimensions increase with increasing . and both reach the peak when is around 2.5%, then fractal dimensions show a negative correlation with . However, the two fitting line confidences are low, because the type-III kerogen is still in thermal pyrolysis in the early overmature stage, and organic pores can be developed along with gas generation. The development of the organic pores determines the increase in fractal dimensions. However, along with this process, the graphitization of organic matter also occurs [35], which reduces the adsorption capacity of shales and even collapses the graphitized organic pores. Under the simultaneous influence of these two effects, the fractal dimension and the organic maturity show a complicated relationship. The specific change trend of the fractal dimensions is related to the relative strength of these two effects. When the thermal pyrolysis gas generation dominates, fractal dimensions increase. Conversely, when graphitization dominates, fractal dimensions decrease.

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5.3. Relationship between Fractal Dimensions and Mineral Components

As different minerals develop pores with different structures, the mineral composition of shales is also an important factor influencing fractal dimensions. Quartz and clay minerals are the primary minerals within the Longtan shales. Fractal dimensions are negatively correlated with quartz content (Figure 9(a)) because the Longtan Formation is a set of marine-continental transitional shales, and the quartz contained therein is primarily terrestrial quartz. Therefore, organic abundance is negatively correlated with quartz content (Figure 10). Furthermore, the grain size of quartz minerals is relatively large. Hence, pores related to quartz are intergranular pores and microfractures (macropores to mesopores) with simple structures. The increase in the quartz content results in an increase in simple macropores in shales, causing a decrease in fractal dimensions. On the contrary, fractal dimensions have a good positive correlation with clay mineral content (Figure 9(b)). As previously mentioned (Section 4.2), the organic abundance in marine-continental transitional shales is positively correlated with clay mineral content (Figure 10), indicating that shales rich in clay minerals are also organic-rich. Thus, inorganic as well as organic micropores with complex structures can be developed in these shales. Shales rich in clay minerals have strong gas adsorption and storage capacity, with complex pore structures and strong heterogeneity with large fractal dimensions. In addition to quartz and clay content, pyrite content also has a significant effect on the fractal dimension (Figure 9(c)). Numerous studies have shown that pyrite primarily develops in an oxygen-deficient environment, which is conducive to the preservation of organic matter [6, 36–38]. Organic abundance shows a good positive correlation with pyrite content (Figure 10). Framboid pyrites, a type of pyrite which predominantly influences pore structures [39, 40], are the primary type found in Longtan shales. Intercrystalline pores are developed inside framboid pyrites and are commonly filled with organic matter (Figures 2(d) and 2(e)). Pyrite crystals can serve as supporting frameworks to protect organic matter and organic pores within them [22, 36, 41, 42]. Therefore, the shale samples with large pyrite content in this study contain abundant organic matter and well-developed organic pores, thus having a large fractal dimension.

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6. Geological Significance of Fractal Dimensions

As mentioned above, the fractal dimension can be used as an effective quantitative characterization parameter for pore structure, gas adsorption, storage capacity, and pore complexity of shales in shale reservoir evaluations. can reflect the gas adsorption and storage capacity of shales. Shales with larger have a stronger gas adsorption capacity and are better natural gas reservoirs, especially for adsorbed gas. can characterize pore structure complexity and heterogeneity. Shales with larger have a higher proportion of micropores, more complex pore structures, and strong heterogeneity. All these will reduce the permeability of shales and make gas desorption and diffusion more difficult, thus impeding gas flow in the shale reservoir [14, 43–47]. Therefore, for evaluating shale gas potential, the larger the value is, the better the reservoir properties of the shale, while should be in the appropriate range to ensure that the reservoir is not too complex for exploitation. In the reservoir evaluation process, should have a lower limit, and should have an upper limit [22], such that a favorable reservoir characterized by fractal dimension would also have good gas adsorption, good storage capacity, and an acceptable pore structure complexity [48–51]. The gas content ratio of natural desorption (the sum of lost gas content and desorbed gas content) to the total gas content (the sum of lost gas content, desorbed gas content, and residual gas content) can be regarded as the approximate proportion of gas that can be produced naturally during the exploitation process (with no reservoir fracture reconstruction). Since the lost gas content is obtained mathematically, there is a certain error compared to the actual value. However, the ratio has a certain significance for the current analysis. A larger ratio means more gas produced naturally and lower exploitation difficulty [49, 52, 53]. Correlation between the ratio and (Figure 11) indicates that when exceeds 2.85, the proportion of desorbed gas tends to decrease.

Combined with isothermal adsorption test data, favorable reservoir evaluation criteria of Longtan shales in Guizhou can be formulated. The lower limit of is set as 2.60. When is less than 2.60, the gas adsorption and storage capacity of shale are insufficient. The upper limit of is set as 2.85. When is larger than 2.85, the proportion of desorbed gas in the shale decreases, and the pores become too complex. Consequently, gas productivity can be affected. When the clay mineral content of the shale is greater than 40.0% (Figure 9(a)) and the quartz content is less than 30.0% (Figure 9(b)), the shale brittleness is poor, which can hinder hydraulic fracturing [44]. Exploitation of this kind of shale () requires a more detailed recoverability evaluation and complex fracture scheme.

7. Conclusions

(1)The characteristic fractal dimensions of marine-continental transitional Longtan shales in Guizhou are determined by piecewise fitting based on the FHH model. Fractal dimensions under low relative pressure () are recorded as and range from 2.6655 to 2.8648 with an average value of 2.7426. Fractal dimensions under high relative pressure () are recorded as and range from 2.6841 to 2.9256 with an average value of 2.7838. High and values (closer to 3) indicate that the Longtan shales in Guizhou have good gas storage capacity and complex pore structures(2)Both and show positive correlations with specific surface areas and negative correlations with pore size. also shows a good positive correlation with the maximum gas absorption capacity of the shale. and increase with increasing organic matter and have complex correlations with organic maturity. There are positive correlations between fractal dimensions and clay mineral content and pyrite content, as well as a negative correlation between fractal dimensions and quartz content(3)For the set of marine-continental transitional shales of the Longtan Formation in Guizhou, the lower limit of is set as 2.60, and the upper limit of is set as 2.85. When is less than 2.60, the gas adsorption and storage capacity of the shale are insufficient. When is larger than 2.85, the shale pores are so complex that the permeability is poor and natural productivity is low, indicating that the recoverability of the shale reservoir requires further evaluation

Data Availability

The data used to support the findings of this study are included within the article and are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This research was jointly supported by the Technical Fund Project of the Guizhou Science and Technology Department (QKHJC No.[2019]1293), Fund Project of the Education Department of Guizhou Province (QJH No.KY [2018]029), Fund Program of the Science and Technology Department of Liupanshui City (52020-2018-03-03), and Fund Program of the Science and Technology Department of Liupanshui City (52020-2019-05-03).

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Copyright © 2020 Yuqi Huang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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