Multiscale Fracturing in Medium- to Low-Rank Coals and Its Implications on Coalbed Methane Production in the Baode Area, Eastern Ordos Basin, China

Zhang, Wei; Zhao, Baoshan; Zhang, Qingfeng; Zhang, Jikun; Li, Ziling; Sun, Xiaoguang; Yan, Mingming

doi:https://doi.org/10.1155/2022/4549153

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Advances in Enhancing Coalbed Methane and Shale Gas Recovery 2021

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Volume 2022 | Article ID 4549153 | https://doi.org/10.1155/2022/4549153

Multiscale Fracturing in Medium- to Low-Rank Coals and Its Implications on Coalbed Methane Production in the Baode Area, Eastern Ordos Basin, China

Wei Zhang,¹Baoshan Zhao,¹Qingfeng Zhang,¹Jikun Zhang,¹Ziling Li,¹Xiaoguang Sun,¹and Mingming Yan¹

Academic Editor: Yong Li

Received01 Mar 2022

Revised24 Mar 2022

Accepted29 Mar 2022

Published20 Apr 2022

Abstract

Coal fractures are crucial in affecting the production of methane from coal. Multiscale fracturing and its implications on coalbed methane production have still not been fully understood. Herein, we present a case study, combining underground coal mine surveying and specimen, thin section, and scanning electron microscope observations for illustrating the ~m-, ~cm-, ~mm-, and ~μm-scale fractures present in the Baode area, eastern Ordos Basin, China. Then, the fracture connectivity is evaluated by helium permeability and mercury porosimetry measurement. The coals are mainly of semibright, semidull, and dull macrolithotypes. And main maceral composition is vitrinite, accounting for 73%~95%, with around 26% inertinite. The coals are ultralow-ash and low-ash content, belonging to high-volatile bituminous coal. The ~m scale fractures can penetrate the whole coal seams, dominant by S-N and following E-W direction, which were generated during the Yanshanian and Himalayan movements. The ~cm fractures are generally parallel to the lamina, influenced by the bright and dull coal band extension caused by the depositional differences. The ~mm fractures are mainly shown as endogenous fractures perpendicular to the lamina restricted within bright macrolithotypes. There are also ~mm fractures that are perpendicular to the lamina while penetrating dull components and fractures parallel to the lamina. The ~μm fractures are widely distributed and connect each other. Some of the fractures are filled with carbonate and clay minerals and are beneficial for methane migration, caused by hydraulic fracturing. The average mercury withdrawal efficiency of the coals was 75%. The helium permeability of the coals was between and μm², indicating good fracture connectivity. The study findings, which indicated the presence of fractures of different scales in the coals studied, can be used for fully understanding the coalbed methane performance of medium- and low-rank coals.

1. Introduction

The total coalbed methane (CBM) resources in China that are within 2000 m from the ground are estimated to be 36.81 trillion cubic meters of which medium- and low-rank coals accounted for 7.8 and 14.7 trillion cubic meters, respectively [1, 2]. The CBM production in China is currently dominated by medium (maximum vitrinite reflectance, )-to-high () rank coals; most of the production wells are located in the southern Qinshui Basin and eastern Ordos Basin [3, 4]. Low-rank CBM has not yet been widely developed in China despite its huge production potential. The coals used from the Powder River Basin in the United States and Surat Basin in Australia for commercial production of CBM are typically low-rank coals [5–7]. The Baode area is the most successful middle-to-low rank–CBM production area in China. The coal reservoir characteristics have not been widely explored thus far.

Coal permeability is critical in influencing CBM migration and production, which are both affected by coal matrix pores and fractures [8–10]. Even though hydraulic fracturing is typically used during CBM production, the initial permeability of coal is still a dominant controlling factor of CBM well performance [11, 12]. The permeability of coal is mainly influenced by its fractures, including intrageneric cleats and exogenous joints. The fracture size and connectivity are the main factors affecting the permeability of coal reservoirs [13, 14]. Extensive studies using methods, such as gas adsorption and desorption experiments with N₂, CO₂, or Ar, as the medium; three-dimensional imaging, such as micro- and nano-X-ray computed-tomography, focused ion beam scanning electron microscopy, and helium ion microscopy; and fluid intrusion techniques, such as mercury intrusion porosimetry and nuclear magnetic resonance, have been conducted for exploring micropore occurrence in coal [15–20]. However, as for the traditional and direct observation of the fracture developed in the coals, it is still lacking clear recognition [21]. The calculation of the number of fractures is difficult because of the complexity and heterogeneity of the fractures. Mechanical parameters are used to estimate coal fracture, fracture occurrence, and fracture numbers [22–24]. Coal fractures have to be studied using coal core and coal mining surface observations and geophysical methods.

The Baode area, one of the most successful CBM production areas in China, was selected as the study area. A series of comprehensive studies were conducted for investigating fracture occurrence in coal, followed by a discussion on its influence on CBM production. The key purpose of the study was to identify the fracture extension in low-to-middle rank coals, which could help understand methane migration and production. The results would help understand low rank–CBM production in other basins where coals with similar properties are present.

2. Geological Setting

The Baode area in Shanxi is on the eastern margin of the Ordos Basin, and its tectonic location is along the northern segment of the Jinxi fold [25]. The area, which has a stable structure, is located in the transitional area between the basin margin and the inner basin (Figure 1). Large faults and folds are less developed, with most small fault strikes along the near north-south direction. The coal exhibits a monoclinic structure along the near north-south direction with a westward inclination. The dip angle of the coal seam is small. The coal measure mainly comprises fine gravel sandstone, siltstone, and carbonaceous mudstone. It has small amounts of medium-grained and coarse-grained sandstones. The coal-bearing strata comprise the Upper Carboniferous-to-Permian Benxi, Taiyuan, and Shanxi Formations. Ordovician limestone is below the coal measure, whereas Triassic fluvial deposits are above it.

3. Materials and Methods

3.1. Coal Seams and Specimen Observation

Exogenous fractures in the coal seams of the Baode Coal Mine were observed via underground observations. The anatomical points were mainly concentrated on the coal wall of the excavation roadway. The 240 m wide 308 working face of the Baode Coal Mine was continuously tracked and observed while focusing on the development of small structures and microstructures, such as exogenous joints and small faults. The roadway excavation was straight, and small folds were not apparent. The occurrence of exogenous fractures was observed, and their density was estimated. The location and partial filling characteristics of the exogenous fractures associated with the coal seam were marked.

All of the samples were collected from the No. 8 coal seam of the study area. The hand specimen samples could be described using features, such as sample color, structure, composition, macroscopic coal type, luster, fracture, hardness, firmness coefficient, endogenous fracture development characteristics, exogenous fracture development characteristics, and filling characteristics. Coal macrolithotypes can be classified as dull, semidull, semibright, and bright coal. Furthermore, the ~μm scale fracture was detected by a scanning electronic microscope (SEM) [4].

3.2. Proximate, Maceral Composition, and Vitrinite Reflectance Analyses

Proximate analysis, including moisture, ash yield, volatile matter, and fixed carbon content analysis, was performed on 12 samples based on GB/T 212-2001 [26]. Maceral composition analysis was conducted according to ISO 7404–3 : 2009, and the total maceral composition was measured by counting all the macerals present using volume percentages [27, 28]. The values were analyzed using a Leitz MPV3-SP microscope in accordance with the international standard ISO17246: 2010 and China Petroleum Industry Standard SY/T 5124-2012. At least 50 readings from each sample were recorded [29].

3.3. Helium Permeability

Coal permeability was tested using coal columns of 25 mm diameter. It was determined using Darcy’s formula, which says that the flow rate of a fluid passing through a rock sample will be directly proportional to the cross-sectional area of the core and pressure difference between core inlet and outlet and inversely proportional to the length of the rock sample, and viscosity of the fluid as given in where is the gas permeability, 10⁻³ μm²; is the cross-sectional area of the core, cm²; is the length of the tested core, cm; is fluid viscosity, mPa·s; is the flow rate at the core outlet under atmospheric pressure, cm³/s; is the atmospheric pressure, 0.1 MPa; is the pressure at the core inlet, MPa; and is the pressure at the core outlet, MPa. The helium viscosity was found to be 0.017 mPa·s when tested.

3.4. Mercury Intrusion Porosimetry

Mercury intrusion porosimetry analysis was performed only on six selected samples because other samples had either quality- or quantity-related issues. The experiment was conducted using a Micromeritics 9310 porosimeter. It was based on the Chinese Oil and Gas Industry Standard SY/T 5346-1994. Before performing the analysis, all samples were dried at 75°C for 48 h. The average pore radius, mercury saturation, and mercury withdrawal efficiency of the coal in the samples were all recorded [8, 30].

4. Results

4.1. Maceral Composition

Coal has a high vitrinite content of approximately 73%–95% (Table 1). Its inertinite content is low (<26%). The gelovitrinite content in vitrinite is high, mainly comprising homogeneous vitrinite and collodetrinite. Furthermore, a few samples contained telovitrinite. Vitrodetrinite was present in the samples, and small amounts of collodetrinite and telinite were observed (Figure 2(a)). The inertinite group was dominated by semifusinite and inertodetrinite. Well-preserved fusinite was rarely detected. The interlayers of collodetrinite and homogeneous vitrinite strips and corpogelinite could be observed, as shown in Figure 2(b). Collotelinite is commonly developed with internal pores, and pore deformation occurred during the coalification process (Figures 2(c) and 2(d)).

(a)

(b)

(c)

(d)

4.2. Proximate Analysis

The results of the proximate analysis of the samples indicated that they had stable moisture content of 2%–4% (Table 2). Thus, the coal could be classified as low-moisture coal. The ash yield of the coal varied depending on the constituent minerals. The highest and lowest ash yields were 29.52% and 5.1%, respectively. Thus, the coals found in the study area could be generally classified as ultralow-ash and low-ash coals. The volatile-matter content of the coal was between 28% and 41%. Most of the samples had volatile-matter content between 36% and 41%, indicating that the samples comprised high-volatile bituminous coal. Two samples comprised a volatile-matter content of 28.04% and 30.59%, respectively, belonging to medium volatile bituminous coal. The values of most samples were between 0.7% and 0.9% with an average of 0.78%. Two samples had a of 1.21% and 1.40%, which indicated an abnormally high thermal maturity. Most samples contained gas coal and fat coal. A few samples comprised long flame coal.

4.3. Occurrence of m-Scale Fractures

The orientation of most of the exogenous fractures in the coal seam of the Baode Coal Mine was along the south-north direction, while the orientation of the other fractures was along the east-west direction. Most of the natural exogenous fractures that had large cross-beds were steeply inclined and tended to dip westward (Figure 3). The linear density of the fractures was 0.7–1.3 strips/10 m, and the distribution of single strips was dominant. Exogenous joints and endogenous fractures were present. Exogenous joints can pass through adjacent macroscopic coal rock-type layers. It is difficult for them to pass through thick gangue layers. The mechanically strong gangue lithology mostly comprises carbonaceous and sandy mudstones. Gangue can develop network joints. Horizontal natural fractures develop in coal reservoirs, mainly in the layers between the coal seam and roof and between the layers within the coal seam.

4.4. Occurrence of cm-Scale Fractures

The samples collected were mainly made of dull coal, semidull coal, and semibright coal, with some bright coal present. The coal in most of the samples had a unified structure or ribbon structure, influenced by peat deposition intervals. The endogenous fractures in the samples were present mainly in bright coal. Yellow-to-white minerals were observed mostly in the endogenous fractures. Four continuous bright coal bands with uneven thicknesses can be observed in Figure 4(a). Their average thicknesses were 4, 3, 1.5, and 0.5 mm. Most of the bright bands had discontinuous fracture distributions. Dull coal bands, with a thickness of approximately 2–6 mm, were present between continuous bright coal bands and discontinuous bright coal bands. Microfractures were well developed in the bright coal bands and were perpendicular to the lamina (Figure 4(b)). They were parallel to each other. At the center of the sample, the fracture density was approximately 5 over a length of 7 cm. Some fractures were connected to one another. The fractures intersected the bright coal bands at a high angle. However, these fractures generally cannot penetrate the dull bands.

(a)

(b)

Microcracks were prominent in the continuous bright coal bands. A few microfractures are present in the discontinuous bright coal strips. Microcracks are developed perpendicular to the layers and are parallel to one another. Yellow-white fillings can be seen in the bright coal strip in the middle. The mineral-filled fissures have a density of 5-7 cm with a spacing of 1.5–11 mm and penetrated the bright coal strip and extended into the dark coal layer. The density of the microcracks that are filled with minerals is 1/cm, and their spacing is between 2 and 5 mm. The dark coal layers have not been penetrated, and there are parallel fractures between the two parallel fractures to connect them. Mineral-filled fractures are less developed in the rest of the belts, and the fractures not filled with minerals are similarly developed with a density of approximately 1/cm.

4.5. Occurrence of mm-Scale Fractures

In the thin section, microfractures are widely distributed, penetrating bright and dull coal bands. The fractures are generally unstable and are root-shaped, geese-shaped, cone-shaped, and side column-shaped with coarse fracture surfaces. These fractures are not fully filled, and some of the broken coal matrix particles could be seen. The fractures are connected even though most of the fractures are confined to the bright coal band (Figure 5).

The mm-scale fractures can be divided into three categories. The first type of fractures is perpendicular to the lamina and confined to the bright coal bands. A few have small openings and are filled with minerals. They are not connected to other fractures. The second type of fractures, which are either root-shaped or brush-shaped, is perpendicular to the lamina. They extend into bright and dull coal bands. Most of these fractures have large openings filled with minerals. The minerals and coal components are quite apart from each other, not completely filled. The fractures generated during the geological evolution of coal are connected together, which benefits methane migration and production. The third type of fractures is parallel to the lamina, and they are mainly at the junctions of the bright coal bands and silk charcoal or within silk charcoal itself.

4.6. Occurrence of μm-Scale Fractures

The SEM images indicated that endogenous fractures were widely distributed in the samples tested. Some fractures were parallel to each other, with some being closed while some are open, as shown in Figures 6(a) and 6(b). The fractures indicated that the inner connectivity of the coal in the study area was satisfactory, which is beneficial for methane migration. Similar to the fractures, the pores, namely, gas pores and residual plant tissue pores, were well developed, as shown in Figures 6(c) and 6(d). The coals had been generating methane as indicated by their thermal maturity (Table 2). The coals, however, were in their early coalification stage with residual pores apparent, which is favorable for methane migration and production.

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(b)

(c)

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(e)

(f)

5. Discussion

5.1. Fracture Connectivity and Its Influences

The formation of endogenous fractures in coal depends on the type of coal. Endogenous fractures often develop in bright coal bands (Figure 7). They are irregularly distributed, and the directions of some fractures were visible. One group of fractures was perpendicular to the lamina, whereas the fractures in the other two groups were parallel to the lamina, and some of the fractures in the first group were connected to the fractures in the other group at a certain angle, as shown in Figures 7(a) and 7(c). The fracture surfaces were flat, and some fractures were filled with minerals. The endogenous fissures on the hand specimens formed an irregular network and were filled with minerals. SEM energy spectrum analysis revealed that the endogenous fractures were filled mostly with carbonate minerals, followed by clay minerals (Figure 6(f)). The closure of the fractures can decrease coal permeability, and the existence of closed fractures can be beneficial for methane production. The fractures that are not filled with minerals can get easily extended during hydraulic fracturing. Thus, the development of both forms of fractures is advantageous for CBM production [31].

(a)

(b)

(c)

(d)

Mercury porosimetry can indicate the presence of fractures and help determine pore connectivity of coal. Table 3 shows that the mercury intrusion porosity of the sample ranges from 2.8% to 6.37% (). The pore throat diameter of the coal ranges from 6.53 to 16.18 nm, with an average of 7.9 nm. The approximate average mercury saturation is 50.94%, and the approximate average mercury withdrawal efficiency is 74.58%. The samples exhibit high mercury withdrawal efficiency, implying satisfactory pore and fracture connectivity. The amount of mercury injected gradually increases at first; the mercury injection saturation is within 10%. When the pressure reaches 20–50 MPa, the amount of mercury injected increases rapidly until mercury injection saturation is achieved, indicating that the micropores are rich in the sample. The mercury withdrawal curve is roughly similar to the mercury injection curve. The high mercury withdrawal efficiency indicates that the pores are mostly open and that they have good connectivity—favorable for CBM production [32, 33].

5.2. Influence of Fractures on Coal Permeability

Two series of coal permeability tests were conducted under different confining stresses to determine the influence of the fractures on the permeability of the coal (Table 4). The original permeabilities of the two series of coals with fractures were 40.78 and μm² while the permeabilities of the coals that did not have fractures were only and μm². Coal permeability was tested under a confining pressure of 7 MPa and at a coal burial depth of 700–800 m. Coal permeability was influenced by the confining pressure and decreased exponentially as the pressure was increased from 7 to 17 MPa (Figure 8). As the burial depth increased, the fractures started to influence coal permeability. Coals with a high initial permeability can improve the channel flow in the coal mine. The permeabilities of the two coals with fractures were 23.71 and μm² at 17 MPa while the permeabilities of the two coals without fractures were only 2.59 and μm² at 17 MPa. Compared with the permeability of the coals at other CBM production blocks, such as the Shizhuang Block, the permeability of the coals in the study area was high, which benefits CBM production [31].

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(b)

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6. Conclusions

Coal macrolithotypes contain mostly semibright, semidull, and dull coals. The main maceral group found in coal is vitrinite (73%–95%). The inertinite content in coal is approximately 26%. Collodetrinite and corpogelinite are the dominant subgroups of vitrinite, whereas semifusinite and inertodetrinite are the dominant subgroups of inertinite. High-volatile bituminous coal has ultralow-ash or low-ash content.

The m-scale fractures in coal seams are mostly in the south-north direction. Some fractures are in the east-west direction. The fractures can penetrate entire coal seams while was blocked the coal gauge. These fractures have been generated during the Yanshanian and Himalayan movements.

The cm-scale fractures are parallel to the lamina and are influenced by the bright and dull coal bands. The mm-scale fractures are of three types. Most of them are endogenous fractures perpendicular to the lamina and are confined to the bright coal bands. The other two types are the fractures perpendicular to the lamina, penetrating the dull coal bands and the fractures parallel to the lamina. The μm-scale fractures are widely distributed and connect coal matrix pores.

Some fractures are filled with carbonate and clay minerals and are arranged as connected networks, which are beneficial for methane production after fracturing. Mercury porosimetry measurements and helium tests show that the coal in the study area has high permeability in the wide range of μm². Coal permeability is exponentially related to the confining stress and is influenced by the fracture closure under high confining pressures.

Data Availability

The data has been included in the ms.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This research was funded by the National Major Science and Technology Projects of China (2016ZX05042). We would like to thank Yuan Zhang for helping in data curation and Prof. Shengwei Wang in China University of Geosciences (Wuhan) for conducting the original research.

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