Introduction

In situ stress is a natural stress formed in a long geological age and not disturbed by engineering. It has a major impact on permeability (Chen et al. 2017, 2018; Cao et al. 2021; Wu et al. 2021), gas content (Li et al. 2014), coal body structure (Cheng & Pan, 2020; Zhang et al. 2021), borehole trajectory and wellbore stability (Zoback et al. 2003; Liu et al. 2004) of the coal reservoir. Therefore, accurate prediction of the properties of in situ stress is beneficial to the CBM development.

According to the measurement principle, in situ stress assessment approaches can be classified as direct and indirect methods. Direct measurement methods include the acoustic emission method, the borehole caving method and the hydraulic fracturing method, while indirect methods mainly include the over-coring stress relief method and the strain recovery method (Zhang et al. 2012). Notably, the various measurement methods have different limitations. Acoustic emission is related to elastic wave propagation, and high strength brittle rocks usually exhibit a significant Kaiser effect. While coal has the characteristics of fracture development and low strength (Zhao et al. 2019), the Kaiser effect is not distinct, resulting in a lower accuracy of stress test results. The borehole caving method has a relatively small range of application conditions, because it can only be applied to the well wall caving zone, and the specific value of in situ stress cannot be calculated. At the same time, the heterogeneity of coal can potentially disturb the caving direction, reduce the accuracy of measurement and lead to errors in determining the stress orientation (Ljunggren et al. 2003). There are many factors affecting the strain recovery method, including temperature, reservoir pressure, anisotropy, strain recovery time and core positioning accuracy. This leads to a low measurement accuracy of strain recovery method, which is unfavorable to in situ stress measurement (Zhang et al. 2012). The over-coring stress relief method and the hydraulic fracturing method, which are characterized by being easily operable, economical and practical, as well as having a high accuracy, are currently the main methods for measuring crustal stress, suggested by the IRMTPC (International Rock Mechanics Testing Professional Committee) (Haimson & Cornet, 2003; Sjöberg et al. 2003). In the field of CBM development, the hydraulic fracturing method is frequently applied to the evaluation for in situ stress in coal seams (Chen et al. 2017, 2018; Wang & Zhang, 2018).

Coal seam permeability consists of matrix and fracture permeabilities; the latter are the main source of coal reservoir permeability (Moore, 2012; Fu et al. 2020; Zhu et al. 2020; Men et al. 2021). Matrix permeability is appreciably lower than that of the fractures. The combined effect of tectonic stress field and coalification affects the distribution rules of natural fractures, which directly influences fracture permeability. The distribution rules of natural fractures are regulated by the paleo-tectonic stress field, whereas the modern in situ stress field controls mainly the opening degree of the natural fractures, thereby affecting the permeability (Liu et al. 2017; Fu et al. 2020). Simultaneously, a significant factor influencing the gas content is the in situ stress mechanism. Qin et al. (1999) found that the development orientation of dominant fractures in overlying rock of coal seam in the central and southern Qinshui Basin is perpendicular to the modern tectonic stress field's maximum compressive stress, which enhances the sealing ability of the caprock to the coal reservoir, and is favorable for the preservation of CBM. Furthermore, different in situ stress mechanisms lead to different degrees of damage to the coal body structure, which can result in wellbore collapse and coal fines, that affect the drilling and drainage processes of CBM. Liu et al. (2004) indicated that the vertical borehole is the most stable and less vulnerable to shear failure when the vertical stress (Sv) is the highest stress. The vertical borehole is the most brittle and vulnerable to shear failure when Sv is the intermediate stress. Besides, when Sv is the lowest stress, the borehole parallel to the maximum horizontal stress (SH) is the most stable, and the borehole parallel to the minimum horizontal stress (Sh) is most prone to shear failure.

There are currently over 200 CBM wells in western Guizhou, most of which produce less than 500 m3/d gas on average, only a few exceeding 3000 m3/d (Zhang et al. 2019a). High yield wells are distributed mainly in the Faer syncline, Tucheng syncline and Zhuzang syncline in western Guizhou. The major reasons for the low production and large differences in productivity of CBM wells are the large variation in the geological attributes of coal reservoirs and the insufficient adaptability of existing engineering and technical measures, but there are few in-depth discussions on this topic at present. In view of this, this paper collected injection/falloff test data and gas content data of 70 coal seams in 28 wells in western Guizhou province, analyzed the variation rules of crustal stress, discussed the distribution pattern of stress fields and their impact on coal permeability and gas content, and provided a geological basis for CBM exploration selection and well location deployment.

Geological Background

Western Guizhou, located in southwest China, is an important enrichment area of CBM in China (Fig. 1a, b) (Tao et al. 2019; Zhang et al. 2019a, b, 2020). The study area includes mainly Tucheng syncline, Panguan syncline, Bide syncline and Zhuzang syncline (Fig. 1c). The major coal-bearing formations in study area are in the Late Permian, including mainly the Longtan and Changxing Formations, deposited in the coastal plain-delta sedimentary system. The eastern margin of the Kangdian ancient land in the west transited to the Xuanwei Formation dominated by continental facies (alluvial plain), and to the southeast became the Wujiaping Formation of shallow sea carbonate platform facies (Qin et al. 2020). According to lithological characteristics, coal-bearing properties and facies transformation factors, the Longtan Formation is further classified into upper and lower coal-bearing sub-sections (Fig. 1d).

Figure 1
figure 1

Geological location and coal-bearing strata columnar diagram of western Guizhou: a Map of China; b Map of Guizhou province; c Geological location map; d Coal-bearing strata columnar diagram

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The structure of the coal accumulation period significantly affects the development features of coal-bearing formation and coal seams of Late Permian, while the structure after the coal accumulation period determines the preservation degree of coal-bearing formation. In fact, the study area experienced the Indo-China, Yanshan and Himalayan movements successively. Western Guizhou was influenced mainly by the Yanshan period, which caused the pre-Late Cretaceous strata to be generally folded, establishing the basic pattern of modern coal-bearing structures (Qin & Gao, 2012). The methane contents of Zhina coalfield and Liupanshui coalfield in western Guizhou are 0.24–29.21 m3/t (avg. 13.81) and 3.87–29.16 m3/t (avg. 12.79), respectively. The permeability ranges are 0.1074–0.5002 mDFootnote 1* (avg. 0.2797) and 0.0004–0.4800 mD (avg. 0.0741), respectively. The well test pressure of coal reservoir is 2.97–12.89 MPa (avg. 6.47), and the pressure gradient is 0.64–1.29 MPa/hm (avg. 1.02) (Qin & Gao, 2012). It is worth noting that the injection/falloff testing data of 28 wells were collected in this paper, of which 11 wells in western Guizhou were from the work of Xu et al. (2016). The exact locations of these wells were not provided in their work; therefore, there were only 17 wells marked in Figure 1c.

Generally, Western Guizhou is characterized by abundant CBM resources, a large permeability variation, high reservoir pressure, a complex structure and high gas content (Gao et al. 2009). These characteristics indicate that the development potential of CBM in western Guizhou is very big, but also show that the development conditions of CBM are complex, which means more advanced technology for the engineering implementation of CBM field is required.

Test Methods and Data Processing

CBM well testing is an important way to obtain coal reservoir parameters. Through the well test of the target coal seam, the key parameters (permeability, reservoir pressure (P0), skin factor, reservoir temperature, fracture pressure (Pf) and closure pressure (Pc)) are obtained, which provide reliable parameter basis for the evaluation of CBM production potential.

Test Methods

In full compliance with the China National Standard GB/T 24504-2009, the injection/falloff well test was performed. After installing the test equipment, the pressure testing of the test string was conducted. When the test pressure exceeded the designed maximum surface injection pressure, the strength and tightness of the pipeline system were verified. The packer was then set, and the setting effect of the packer was checked. Afterward, the well was shut-in for 2–4 h to balance the pressure. According to the designed pressure, displacement and time, clean water was selected for the injection, and the time interval for recording the test data should not exceed 10 min. During the injection process, the fluctuation value of injection displacement should not exceed 10%, and the wellhead pressure should be controlled below the designed maximum injection pressure value on the ground. In addition, the injection time must be greater than the end time of the wellbore storage, otherwise the injection time should be increased.

After the injection, the downhole shut-in was used, the wellhead valve was closed at the same time, and the pressure variation at the wellhead was observed to verify the sealing condition of the downhole shut-in work. Meanwhile, the shut-in time should be not less than double the injection time (Fig. 2). Pressure gauge was used to record the time-varying process of the bottom-hole pressure with time. By analyzing the data, the parameters of the coal seam were obtained. Because it was difficult to control the extremely unstable displacement during the injection stage, which causes the fluctuation of the bottom-hole pressure, the most representative data analysis usually comes from the pressure drop phase (Fig. 2). Therefore, the initial reservoir pressure and permeability were analyzed using the semi-logarithmic method. The double-logarithmic method was used for the fitting analysis and the verification. The pressure history matching curve was used for the reliability testing to ensure that the obtained permeability data were true and reliable.

Figure 2
figure 2

Injection / falloff well test curve of typical CBM well

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The in situ stress analysis was carried out according to the hydraulic fracturing method in the China Seismic Standard DB/T 14-2000 after the injection/falloff test was completed. The test process contained no less than three re-opening cycles, and each cycle included three steps: fluid injection, pump shut-off, and pressure falloff (Fig. 3). Each cycle was at least two minutes long, and the time interval between two cycles was at least one minute. After the installation and inspection of the whole measuring system, a constant flow rate of no less than 4 l/min should be maintained during the pressure injection process. For hydraulic fracturing, the pump can be shut down and the closing pressure can be observed after the hole wall rupture or rupture retention according to the pressure–time curve recorded on site. After each fracturing cycle was completed, the pressure should be completely relieved so that the pressure pipeline was the same as the atmosphere pressure. To interpret in situ stress, the pressure–time curve reported in the field was used as the basic data. From the four cycles of stress measurements, 2–3 cycles with relatively good fracturing effect were selected to analyze the falloff data in shut-in stages. Closure pressure was calculated using the time square root method and verified by the double logarithm method (Zhao et al. 2019).

Figure 3
figure 3

Stress measurement curve of typical CBM well

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Data Analysis

In the hydraulic fracturing process, with the continuous growth of hydraulic pressure, the maximum tangential stress of the rock fracture at the borehole wall gradually transforms into tensile stress. Finally, the strain reaches the rock's ultimate tensile bearing limit, which induces the rock's tensile failure and the propagation of the fracture. According to the theory of elasticity, SH can be calculated by as (Hubbert & Willis, 1957; Bredehoeft et al., 1976):

$$S_{H} = 3P_{c} - P_{f} - P_{0} + S_{t}$$
(1)

where SH, Pc, Pf, P0 and St are tensile strengths (in MPa). During stress measurement, at least three re-opening cycles are included. Because the coal seam has been fractured in the first re-opening cycle, the tensile strength St is zero during the subsequent re-opening cycles. Therefore, SH can be calculated as:

$$S_{H} (S_{t} = 0) = 3P_{c} - P_{f} - P_{0}$$
(2)

According to the development rules of vertical fractures in vertical wells, the propagation direction of vertical fractures is perpendicular to SH, and Pc is equal to Sh (Haimson & Cornet, 2003; Chen et al., 2021). Therefore, Sh (in MPa) is calculated as:

$$S_{h} = P_{c}$$
(3)

Sv (in MPa) is primarily subjected to the gravity of overlying rock, and Sv of coal seam can be predicted from the overlying rock. Brown and Hoek (1980) established the relationship between Sv and depth by sorting out the global in situ stress measurement results (Eq. (4)), which has been widely used in the prediction of Sv in diverse regions (Xu et al. 2016; Ju et al., 2018a; Zhao et al. 2019; Ju et al., 2020). This relation is also adopted in the present work:

$$S_{v} = 0.027H$$
(4)

where H is burial depth (in m).

Results and Discussion

Well Test Results and Analysis

According to the well test data of 28 wells (70 coal seam) in western Guizhou, the depth of coal seam is 215.25–1243.6 m (avg. 584.8), P0 is 1.04–12.89 MPa (avg. 5.9), and pressure gradient (G0) is 0.47–1.71 MPa/hm (avg. 1.01), therefore, being abnormal high pressure. Pf is between 5.41 and 32.33 MPa (avg. 13.57), and pressure gradient (Gf) is between 1.37 and 4.62 MPa/hm (avg. 2.41) (Fig. 4). The coal seam permeability is between 0.0002 and 0.573 mD (avg. 0.1041), much lower than the coal seam permeability of other CBM development blocks in China (Zhao et al. 2016; Chen et al. 2017; Fu et al. 2020). It is a typical reservoir with low permeability.

Figure 4
figure 4

Reservoir parameters obtained by well testing in western Guizhou

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According to Eqs. (2)–(4), SH, Sh and Sv in western Guizhou were calculated. In western Guizhou, SH was 4.35–40.39 MPa (avg. 16.15), the stress gradient (GH) was 1.38–6.08 MPa (avg. 2.87), Sh was 4.25–27.36 MPa (avg. 11.87), and the stress gradient (Gh) was 1.18–3.61 MPa (avg. 2.1) (Fig. 4). According to stress intensity, Kang et al. (2009) divided the stress area into four types, namely the low stress area (< 10 MPa), the medium stress area (10–18 MPa), the high stress area (18–30 MPa) and the ultra-high stress area (> 30 MPa). Based on this classification standard, western Guizhou belongs to medium stress area, whereas some coal seams belong to ultra-high stress area. Compared with the typical CBM enrichment basins in foreign countries (McKee et al. 1988; Enever & Hennig, 1997), the stress value in the study area is relatively high, which may be the fundamental factor for why the permeability of the study area is 1–2 orders of magnitude lower than that in foreign countries (Scott et al. 1994).

Variation Rules of Vertical Stress Field

A significant basis for judging the type of stress field is the relative magnitude relationship of the three principal stresses. According to the relationship of the three principal stresses, Anderson (1951) divided the stress field into the reverse fault stress field (SH > Sh > Sv), the strike slip fault stress field (SH > Sv > Sh) and the normal fault stress field (Sv > SH > Sh). In this paper, the vertical stress field in western Guizhou is divided according to this standard.

Datasets with 70 stress measurements are collected in western Guizhou, including 33 with Sv > SH > Sh, 27 with SH > Sv > Sh, and 10 with SH > Sh > Sv. In the vertical direction, SH > Sv > Sh and SH > Sh > Sv occur mainly in coal seams with burial depth of less than 500 m and more than 1000 m, respectively, while Sv > SH > Sh appears mainly at burial depth of less than 1000 m. Based on the above results, the vertical stress field can be grouped into three sections (Fig. 5).

Figure 5
figure 5

Distribution characteristics of stress field in western Guizhou

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(I) There were 27 data points above 500 m. Among them, 14 points represent SH > Sv > Sh, accounting for 51.85%; 8 points exhibit SH > Sh > Sv, accounting for 29.63%; the other 5 points correspond to Sv > SH > Sh, accounting for 18.52%. If burial depth was less than 500 m, with the horizontal stress playing a dominant role, the stress field showed strike slip fault type.

(II) When burial depth was 500–1000 m, there were 39 data points. Among them, 28 points represent Sv > SH > Sh, accounting for 71.79%; 9 points correspond to SH > Sv > Sh, accounting for 23.08%; the other 2 points exhibit SH > Sh > Sv, accounting for 5.13%. If burial depth was 500–1000, with vertical stress playing a dominant role, the stress field transformed into a normal fault type.

(III) There were 4 data points under 1000 m. They all show SH > Sv > Sh, showing that the stress field was again dominated by horizontal stress, and the stress field was again transformed into a strike slip fault type.

Generally, the vertical stress fields in western Guizhou are mainly normal fault and strike slip stress fields. In the normal fault stress field, the vertical borehole was the most stable type. In comparison, the horizontal borehole drilled along the direction of SH was the most unstable and prone to shear failure and tensile leakage (Liu et al. 2004). Therefore, vertical wells should be deployed in the normal fault stress field. In the strike slip fault stress field, the horizontal borehole drilled along the direction of SH was the most stable type, while the vertical borehole was the most unstable, which is prone to shear failure and tensile leakage (Liu et al. 2004). Therefore, horizontal wells along the direction of SH should be deployed in the strike slip fault stress field.

Influence of Stress Field on Permeability

Permeability is a measure for how easy it is for fluids to pass through reservoirs; it is also a pivotal evaluation parameter in CBM exploration and development stage. Many studies have found that permeability and effective stress show negative exponential relationship (Qin et al. 1999; Chen et al. 2017, 2018). In the formation, the permeability is not only affected by the effective principal stress in a single direction, but also by the joint action of three effective principal stresses. To accurately characterize the stress characteristics of coal reservoirs, this paper introduces the effective in situ stress (ESI) proposed by Chen et al. (2017):

$$ESI = \frac{{S_{H} + S_{h} + S_{v} }}{3} - P_{0}$$
(5)

where ESI is in MPa.

The relationship between permeability and ESI exhibits exponential decay (Fig. 6a), but the correlation is low (R2 = 0.0385). In addition, we applied the effective stress calculation method of Zhao et al. (2019) to calculate the relationship between the EIS and permeability (Fig. 6b, c and d), but the correlation is still very low. Our findings indicate that the factors affecting the permeability were complicated, and the effective stress was not the main controlling factor.

Figure 6
figure 6

Scatter plot of permeability and effective stress ESI (a), SH-P0 (b), Sh-P0 (c), SV-P0 (d) in western Guizhou

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Qin et al. (1999) discovered that when the direction of SH is consistent with the propagation direction of the dominant rock fractures, the fractures are under tension. The greater the principal stress difference is, the higher the tension effect is, which is more favorable for the increase of permeability. When the direction of SH is perpendicular to the propagation direction of dominant rock fractures, the fractures are compressed. The greater the principal stress difference is, the higher the compression effect is, leading to the reduction of the fracture wall space. In other words, the primary controlling factor affecting the permeability is the action type (tension and compression) of tectonic stress exerts on the natural fractures of coal reservoirs, and the secondary factor is the relative magnitude of tectonic stress.

The permeability of coal seams shows increase followed by decrease with increasing the depth: When the depth is less than 500 m, the permeability is between 0.0002 and 0.5002 mD (avg. 0.0565). When the depth range is 500–1000 m, the permeability is between 0.0002 and 0.573 mD (avg. 0.1472). When the depth is greater than 1000 m, the permeability is between 0.001 and 0.0096 mD (avg. 0.0047) (Fig. 7).

Figure 7
figure 7

Variation characteristics of permeability in western Guizhou

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During the measurement of dip angles of natural fractures in underground coal seams in the study area, it is found that the dip angles of natural fractures are vertical or sub-vertical. The dip angle of most fractures is more than 80°, and the strike direction is NW–SE and NE–SW (Jiang et al. 2017). The types of stress field include mainly normal fault and strike slip stress fields. In the strike slip fault stress field, horizontal stress becomes the dominant stress, and natural fractures are compressed. The greater the principal stress difference is, the higher the compression effect is, which leads to the fracture closure and the rapid decay of permeability. In the normal fault stress field, the vertical stress transforms into the dominant stress, and thus the natural fracture is tensioned. The greater the principal stress difference is, the higher the tension effect is and the higher the permeability is. The variation characteristics of vertical permeability in western Guizhou are essentially consistent with the stress field’s transformation position (Figs. 5 and 7), showing that the development rule of natural fractures and stress field jointly control the permeability of coal reservoirs in western Guizhou.

Influence of Stress Field on Gas Content

To study the vertical variation rules of coal seams gas content in western Guizhou, this paper has collected a large volume of coalfield borehole and coal seam desorption data on site. In western Guizhou, the gas content with depth of less than 500 m was 5.16–31.87 m3/t (avg. 13.77). When the coal seam was between 500 and 1000 m, the gas content was 4.07–41.15 m3/t (avg. 16.67). The gas content with depth of more than 1000 m was 8.83–40.62 m3/t (avg. 26.74). The results showed that the gas content increased steadily as burial depth increased (Fig. 8).

Figure 8
figure 8

Vertical distribution characteristics of gas content in western Guizhou

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The gas content is affected mainly by the stress mechanism, reservoir temperature and reservoir pressure in the vertical direction (Qin & Shen, 2016; Li et al. 2020). Fu et al. (2020) pointed out that the coal seam is in a tension state in the normal fault stress field, which is not helpful to the preservation of CBM. In a strike slip fault stress field, the coal seam is in a compression state, which is helpful to the enrichment of CBM, resulting in an increase of gas content. In addition, when Qin and Shen (2016) studied the geological problems of deep CBM, they proposed that reservoir pressure and reservoir temperature have different effects on coal seam adsorption. Reservoir pressure shows a positive effect, which leads to increased adsorption and gas content in coal. Reservoir temperature shows a negative effect, which leads to decreased adsorption capacity and gas content in coal.

In the vertical direction, the western Guizhou stress field has successively experienced strike slip, normal fault and strike slip fault stress fields (Fig. 5). The transformation of the stress mechanism did not lead to an obvious variation of gas content. Moreover, when the coal seam depth increased, the reservoir temperature showed a gradually increasing trend (Fig. 9). The reservoir temperature did not lead to an obvious attenuation of gas content in the vertical direction. Preliminary analysis suggested that this might be related to the shallow depth and the smaller temperature. In the vertical direction, the variation rule of coal seam gas content in western Guizhou is consistent with reservoir pressure (Fig. 10), which turned out that the vertical variation characteristics of gas content in western Guizhou are controlled mainly by reservoir pressure.

Figure 9
figure 9

Scatter plot of depth and temperature in western Guizhou

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Figure 10
figure 10

The relationship between burial depth and P0 in western Guizhou

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Classification of CBM Development Potential

Gas content and permeability are not only essential parameters of CBM exploration and production, but they are also the main factors determining the productivity of CBM. The higher coal permeability and the gas content are, the greater the development potential of CBM is (Yang et al. 2018, 2019). Moreover, burial depth is also one of the important indices affecting the development potential of CBM wells. The fracture pressure showed a linear increasing trend with increase in burial depth. As illustrated in Figure 11, the fracture pressure was more than 25 MPa if the depth was greater than 1000 m. High fracture pressure demands strict requirements for fracturing technology and equipment of coal seams. According to the Chinese Geological and Mineral Industry Standard (DZ/T 0216-2010), when coal seam depth exceeded 1000 m, it belonged to deep CBM. Based on the development characteristics of gas content, permeability and burial depth of coal seams (Figs. 7, 8), the vertical CBM in western Guizhou was divided into three development potential regions (Fig. 12). In western Guizhou, when depth was less than 500 m (Region 1), the gas content and permeability were low, and the development potential of CBM was small. When depth was between 500 and 1000 m (Region 2), the gas content and permeability were high, and the development potential of CBM was great. When burial depth exceeded 1000 m (Region 3), it belonged to deep CBM. The deep coal seam had characteristics of high stress and high temperature, which meant strict requirements for engineering technology and equipment, and the development difficulty was high, which damaged the development of CBM. Overall, the most favorable vertical development section in western Guizhou was at 500–1000 m.

Figure 11
figure 11

Relationship between burial depth and Pf in western Guizhou

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Figure 12
figure 12

Schematic illustration of vertical CBM development potential in western Guizhou (development potential increased followed by decrease as depth increased)

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Conclusions

The study of the distribution pattern of in situ stress field and vertical development unit division of CBM in western Guizhou allowed drawing the following conclusions.

  1. 1.

    The development characteristics of vertical in situ stress field in western Guizhou are revealed. From the top to bottom, strike slip fault (< 500 m), normal fault (500–1000 m) and strike slip fault stress field (> 1000 m) were successively developed in western Guizhou.

  2. 2.

    The coal permeability in western Guizhou showed a trend of increase followed by decrease with increasing burial depth. The development rule of natural fractures and the vertical distribution characteristics of stress field jointly regulate the permeability of coal reservoirs.

  3. 3.

    On the strength of the characteristics of burial depth, permeability and gas content, the vertical CBM in western Guizhou is divided into three development potential regions. The most favorable vertical development region is 500–1000 m, which has the characteristics of high gas content, high permeability and moderate burial depth, which are beneficial to the development of CBM.