1 Introduction

With the rapid growth of global economy, the consumption and demand for various energies are increasing, but the amounts of conventional energy sources are decreasing. It is urgent to look for and develop new energy sources to fill the vacancies. Coalbed methane, a high-quality unconventional natural gas resource, has attracted more and more attentions and its development and utilization are conducive to the adjustment of the global energy structure (Guo et al. 2011; Sasmito et al. 2015; Li et al. 2019). However, it has been found during the long-term practices that the coal seam gas permeability is low. Hydraulic fracturing can create cracks in coal seam and increase the seepage area and conductivity to increase the production of coalbed methane (Wang 2019; Thakur et al. 2022; Wang et al. 2022; Hu et al. 2023; Xu et al. 2023). It has been widely used for pressure relief and permeability enhancement of low-permeability coal seams, which can significantly increase the natural gas production (Economides and Martin 2013; Tang and Hu 2018; Yow and Wong 2019).

Fracturing fluid is an indispensable part of hydraulic fracturing, and thus has always been the research focus in the field of hydraulic fracturing. Due to the disadvantages of the oil-based fracturing fluids, such as unstable viscosity and high flammability, and guar gum fracturing fluids, such as difficult gel breaking process and damages to coal reservoirs, in the late 1990s, Schlumberger developed a clean fracturing fluid with the advantages of stable viscosity, easy gel breaking process, less residual gel after gel breaking, low friction resistance, and less damage to coal reservoirs (Tan et al. 2013). It has received extensive attentions worldwide in recent years (Osiptsov 2017).

First, the mass fraction of surfactant in clean fracturing fluid affects its viscosity, which in turn affects the fracture prolongation and morphology in coal seam (Mazzotti et al. 2008). The in-situ stress of the coal seam is another factor affecting the fracturing performance. Coal seams are usually characterized with low permeability, large pore specific surface area, strong gas adsorption capacity, and large gas storage capacity (Cong et al. 2007; Bo et al. 2017). Therefore, it is of great significance to clarify the influence of the rheological properties of clean fracturing fluid and horizontal in-situ stress difference on the fracture formation for the coal seam pressure relief, permeability enhancement and natural gas extraction.

The influences of fracturing fluid viscosity and in-situ stress on coal seam fracturing have been extensively studied. Luo et al. (2018) analyzed the advantages and disadvantages of different fracturing fluids and their applications, and explained that the high-pressure rheological property of fracturing fluid is one of the key factors affecting the fracturing performance (Qin et al. 2021). Through numerical simulation, Found that the fracturing fluids with viscosities slow the prolongation of natural fractures. Chen (2011) and Chong et al. (2014) respectively demonstrated by numerical simulation that the fracture length decreased, and the fracture width and height increased with the increase of fracturing fluid viscosity (Fan and Zhang 2014). Found through true triaxial experiments that the complexity of hydraulic fracturing is greatly reduced with the increase of injection rate or fracturing fluid viscosity. It can be summarized from these studies that the fracturing with low viscosity fracturing fluids results in more complex fracture morphologies (Liu and Zhang 2011). By establishing a mathematical model, Found that controllable repetitive fracturing by optimizing the design stress field can form a network of seams on the vertical wellbore plane (Kao et al. 2018). Through the true triaxial hydraulic fracturing test. Found that, in deep shale, fracture propagation tended to form complex fracture networks under low stress differences and high stress differences likely produced relatively simple long straight fractures. Shi and Lin (2021) concluded that in-situ stress was the most important factor affecting fracture propagation, and it decided the fracture propagation direction and fracture morphology. López-Comino et al. (2021) studied the effects of horizontal stress on fracturing by acoustic emission (AE) and concluded that fractures extended in the direction under the minimum horizontal stress Wu et al. (2017) studied the AE characteristics of the pulse hydraulic fracturing processes of coal rock with the fracturing fluids of different viscosities, and found that the fracturing with the fluids of higher viscosities released weaker AE energies and resulted in lower fracture development degrees. Tan et al. (2017, 2020, 2021) studied the factors affecting the initiation and propagation of hydraulic fracturing and the migration of proppants in multiple hydraulic fractures through physical experiments and numerical simulations. There results suggest that there are certain relationships between in-situ stress and acoustic emission energy and fracture propagation.

In the existing studies, most of the hydraulic fracturing experiments are conducted with clean water or other fracturing fluids, while few are conducted with clean fracturing fluid and the in-depth study of the effects of clean fracturing fluid viscosity on coal fracture propagation is still insufficient. Herein, coal samples were collected from the Houwenjialiang Coal Mine in Ordos, Inner Mongolia, and clean fracturing fluids with different viscosities were prepared with CTAB cationic surfactant. A series of hydraulic fracturing tests were conducted on the coal samples with the clean fracturing fluids using an in-house developed true triaxial hydraulic fracturing simulator. The AE and fracturing fluid distribution in coal sample during fracturing were characterized to understand the influences of the fracturing fluid viscosity and horizontal in-situ stress difference on coal fracture propagation and explore the action mechanisms of the clean fracturing fluids with different viscosities on coal fracture propagation. This study was aimed to provide a scientific reference and basis for the design and optimization of field hydraulic fracturing parameters.

2 Experimental design

Figure 1 shows the experimental flow chart. The clean fracturing fluid viscosity was varied by changing the mass fraction of CTAB. The horizontal in-situ stress difference level was tuned by adjusting the triaxial stress. Coal samples were prepared accordingly to fit the true triaxial hydraulic fracturing simulator. The hydraulic fracturing experiment was then carried out using the true triaxial hydraulic fracturing system. The acoustic emission (AE) events and energy during fracturing were monitored in real time with an AE instrument. After the experiment, the coal sample was cleaved and observed for the fracturing fluid distribution. The Crack initiation and propagation law was explored based on the AE event three-dimensional (3D) localization map, time-pressure curve and acoustic emission energy curve.

Fig. 1
figure 1

Experimental flow chart

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2.1 Preparation of clean fracturing fluids

The clean fracturing fluids were prepared with sodium salicylate, potassium chloride (KCl) and CTAB in distilled water. Specifically, an appropriate amount of anti-swell reagent (KCl) was added into distilled water under stirring at room temperature. The auxiliary agent (sodium salicylate) and cationic surfactant (CTAB) were then added and stirred with a JJ-1A digital display booster electric mixer until completely dissolved. The solution was allowed to stay still until bubbles gradually floated to the surface and disappeared and disappeared.

The fracturing fluid viscosity was measured with an NDJ-9 viscometer. According to the four parameters of speed, rotor, measurement average and repeatability in the NDJ-9 s viscometer verification certificate, combined with the standards in SY/T5107-2005 “The evaluation measurement for properties of water-based fracturing fluid”, the average value was taken through multiple measurements, and finally the No. 2 rotor was selected, and the speed was set to 60 r/min, which was approximately equal to the viscosity determined at the shear rate of 170 s−1, and the maximum measured viscosity value was 2000 mPa·s (Xuan 2022; Shadfar Davoodi et al. 2023; SY-T 5107-2005). Other instruments used in our work included electronic balance and PH-10 pH testing pen. The compositions of the prepared clean fracturing fluids are listed in Table 1. The viscosity was varied from 130 to 700 mPa·s as the mass fraction of CTAB increased from 0.5% to 1.25%.

Table 1 Compositions and viscosities of prepared clean fracturing fluids
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Figure 2 shows the pick-up forms of the clean fracturing fluids prepared with different mass fractions of CTAB. As can be seen, the fracturing fluids prepared with higher mass fractions of CTAB are picked up more easily, indicating that the viscosity gradually increases with the increase of CTAB mass fraction.

Fig. 2
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Photos of the clean fracturing fluids with different viscosities being picked up with a glass rod

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Fluorescein sodium was added into the clean fracturing fluids as a tracer. With the fluorescence tracer, the distribution of clean fracturing fluid in coal sample was visualized under UV light irradiation after the fracturing test. The viscosity measurements confirmed that the addition of the tracer had no significant effects on the viscosity of clean fracturing fluid (Wang et al. 2019).

2.2 Preparation of coal sample

The coal sample collected from the Houwenjialiang Coal Mine in Erdos. This coal is long-flame coal, the mining depth is 65–70 m, it is a kind of bituminous coal with the lowest degree of metamorphism, high volatile content and porosity of 3%. By performing the uniaxial compression test and Brazilian splitting test on the coal samples, it is found that the uniaxial compression strength of the coal samples was 10.29 MPa, the modulus of elasticity was 1.27 GPa, Poisson’s ratio was 0.25, and the tensile strength was 1.7 MPa. Coal sample was cut into cubes (100 mm × 100 mm × 100 mm) with non-parallelism and non-perpendicularity less than 0.02 mm using a large rock cutting machine. The cubic coal samples were subjected to the pre-treatments, such as hole drilling, pipe embedding, and measurement, for the fracturing experiment. After weighing, the quality of the coal samples used in the experiment remained basically consistent.

Both fracture initiation and propagation are energy release processes and thus can be characterized with Acoustic emission (AE). AE refers to the phenomenon that a material releases strain energy in the form of stress wave due to deformation or fracture under the concentrated local stress caused by an external or internal force. If the released strain energy is large enough, the sound can be heard by human ear. The weak released energies can be extracted with an acoustic emission instrument. Therefore, AE is an important and effective means to study energy release (He et al. 2009).

In order to improve the accuracy of acoustic emission monitoring, 16 acoustic emission sensors were arranged in the front, back, left and right directions of each cubic coal sample for real-time monitoring, collection and storage of transmitted acoustic emission signals (Fig. 3). In order to ensure the accuracy of the acoustic emission instrument, after the coal samples were in close contact with the acoustic emission sensor, different surfaces of the coal samples were tapped. By observing the 3D location map of the acoustic emission event number, it is found that yellow event points are produced in the corresponding positions.

Fig. 3
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Distribution and design of AE channels on coal sample

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The type of acoustic emission instrument is the DS5-16C, the sampling frequency of the acoustic emission signal is generally 100–400 kHz, and the center frequency is generally 150–200 kHz, which can accurately monitor and record the process of crack generation and expansion. The sensor model is an RS-2W waterproof sensor, which is connected to the signal line. Each sensor is embedded within a rigid metal plate through a spring, and is fixed inside. The sensor is slightly higher than the metal plate. When the indenter is close to the surface of the specimen, the acoustic emission probe compresses the spring to ensure close contact between the acoustic emission probe and the specimen. Before the experiment, the coupling agent (Vaseline) was applied to each sensor to reduce the energy loss. To improve the accuracy of acoustic emission energy monitoring, the minimum threshold was set to 100 mv, and the signal was amplified 40 dB by a preamplifier.

2.3 Hydraulic fracturing experiment

As shown in Fig. 4, In order to complete the indoor hydraulic fracturing physical simulation experiment, a set of true triaxial hydraulic fracturing test systems has been independently developed, which is mainly composed of frame rack, stress loading system, hydraulic servo system, data acquisition and control system, acoustic emission monitoring system, etc.

Fig. 4
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True triaxial hydraulic fracturing simulation system for coal rock

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To eliminate the boundary effect generated during the experiment of 100 mm × 100 mm × 100 mm sample size, first of all, our loading method is six-axis synchronous centripetal loading, which can avoid as much as possible the boundary friction effect caused by one-axis static and one-axis loading in the direction of a single stress. In addition, by injecting hydraulic oil into the true triaxial pressure chamber, the pressure in the whole true triaxial pressure chamber is equal to σ3, which realizes that every part of the specimen is subjected to stress loading, and the boundary effect is eliminated. This device improves the measurement of data and control method based on the true triaxial hydraulic fracturing system studied by previous researchers and adds an acoustic emission monitoring system, which can reflect the fracture expansion and fracturing effect of the coal sample in real-time during the experiment (Heng et al. 2014; Jiang et al. 2016).

The fracturing experiment was conducted following four steps. First, debug the experimental system, and the AE sensors were covered with Vaseline. The pretreated coal sample was delivered into the chamber via a guide rail. The displacement sensor, water injection pipe and AE cable were installed and the chamber was closed tightly. The clean fracturing fluid was injected into the coal sample at the flow rate of 10 mL/s with the triaxial stress loaded to the pre-set value in the constant stress mode. Meanwhile, the data acquisition system was turned on to record the system parameters including water injection pressure, AE 3D localization map and AE energy. The experiment was terminated when the pressure suddenly dropped and became stable. The triaxial stress was then removed and the pressure monitoring instrument, AE acquisition software and pulse pressure generation device were turned off after the pump pressure became stable. The coal sample was taken out from the chamber and observe and record the destruction of coal samples.

The fracture morphology and prolongation of hydraulic fracturing in coal seams are greatly affected by in-situ stress, and the influences can be measured with the coefficient of variation of horizontal in-situ stress. The coefficient of variation of horizontal in-situ stress is defined as:

$$k_{\text{H}} = (\sigma_{\text{H}} - \sigma_{\text{h}} )/\sigma_{\text{h}}$$
(1)

where \(\sigma_{\text{H}}\) is the maximum horizontal principal stress and \(\sigma_{\text{h}}\) is the minimum horizontal principal stress. The larger \(k_{\text{H}}\) is, the greater the difference between the stresses in these two directions is.

Hydraulic fracturing experiment was conducted with four clean fracturing fluids with different viscosities at four different levels of in-situ stress difference to understand their effects on coal fracture propagation (Fan and Zhang 2014; Chen et al. 2017; Huang 2020; Cheng et al. 2021). The specific experimental parameters are shown in Table 2.

Table 2 Fracturing experiment parameters
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3 Results

3.1 Effects of clean fracturing fluid viscosity on hydraulic fracturing

Figure 5 shows the time-pressure curves during the hydraulic fracturing with the clean fracturing fluids of different viscosities. The pressure change in the coal sample can be divided into 5 stages.

Fig. 5
figure 5

Time-pressure curves in coal sample during the fracturing with the clean fracturing fluids of different viscosities

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At the initial stage (①), the X, Y and Z triaxial stresses are loaded to \(\sigma_{\text{h}}\) = 3.7 MPa, \(\sigma_{\text{v}}\) = 4.5 MPa, and \(\sigma_{\text{H}}\) = 5.1 MPa, respectively, and the hydraulic pressure is maintained at ~ 0 MPa. At the stage ②, the clean fracturing fluid is injected into the coal sample, and the fracturing fluid quickly fills the entire borehole. The pressure rapidly increases due to the pressure buildup in the borehole. At the stage ③, the clean fracturing fluid is gradually filtrated into the primary pores and fractures of the coal sample. Hydraulic fractures are formed as the injection rate becomes greater than the filtration rate and the pressure drops dramatically. At the stage ④, the hydraulic seepage channels are formed, and the fracturing fluid flows in the hydraulic fractures. The pressure gradually becomes stable. At the stage ⑤, the pumping of clean fracturing fluid is stopped, and the pressure rapidly drops to 0 MPa.

Figure 6 shows changes in penetration pressure and pressure drop with the change of fracturing fluid viscosity. The pressure corresponding to the maximum value of the time-pressure curve is defined as the through pressure, and the average value of pressure in the fourth stage is the pressure drop. These results along with those present in Fig. 5 suggest that the pressure for penetration fracture generation increases and the increase gradually becomes slower with the increase of clean fracturing fluid viscosity. In addition, after the fracture penetrates the coal sample, the pressure drop gradually increases with the increase clean fracturing fluid viscosity. A1 curve was obtained by injecting low viscosity clean frac fluid (130 mPa·s). Due to low viscosity and stronger fluidity, the injection process is easy to flow and filter in the primary cracks and micro-cracks of the coal samples, the forefront pressure is easier to reach the value of the initial fracture pressure. The clean fracturing fluid penetrates while fracturing, resulting in an insignificant breakdown pressure. In the fourth stage, through cracks from the water injection hole to the outer wall of the coal samples were produced, the permeability increased significantly, and the pressure decreased significantly at this time. However, since the injection did not stop, it was still maintained at a low-pressure value. The viscosity of the clean fracturing fluid injected in the B1 curve increased (320 mPa·s), its fluidity weakened, and the fluid loss decreased. In the process of fracturing, micro-cracks gradually sprouted and expanded, and the fluidity of fracturing fluid showed zigzag fluctuation with the expansion of micro-cracks during injection. Therefore, the pressure is prone to zigzag fluctuations until macroscopic cracks are formed, which is also a significant feature of complex fractures formed in the hydraulic fracturing process. The C1 and D1 curves were infused with cleaner fracturing fluids with higher viscosity, which resulted in poorer flow and further reduced filtration loss. After injection, it tended to accumulate at the bottom of the borehole, causing the initiation pressure to rise rapidly and reach a high value. Until the penetrating crack was produced, the pressure gradually decreased. As the fracture volume, fluid loss rate and the volume of pumped fracturing fluid are consistent, the pressure becomes stable, and the system enters a steady state.

Fig. 6
figure 6

Viscosity-through pressure and viscosity-pressure drop curves in the coal sample during hydraulic fracturing

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In addition, it can be seen from Fig. 5 that the time to start pumping fracturing fluid (stage ②) decreased gradually as the viscosity of clean fracturing fluid increased. The analysis concluded that: due to the viscous and elastic nature of the clean fracturing fluid, when the viscosity is small, clean fracturing fluid tends to be compressed into the fine cracks of the coal, resulting in a longer time to reach the initiation pressure. As the viscosity of clean fracturing fluid increases, the network structure becomes denser and more difficult to be compressed into the fine fracture, so the initiation pressure can be reached in a relatively short time. This is also the difference between clean fracturing fluid and the use of water or other fracturing fluid before.

The AE energy curves in Fig. 7 show the energy release process during the fracturing under different conditions. The predecessors only have analyzed the variation law of the acoustic emission energy curve. In fact, there is a certain correspondence between the acoustic emission energy curve and the time-pressure curve. Since the curves were recorded from the time when the clean fracturing fluid was pumped, the stages I, II, and III correspond to the stages ②, ③, and ④ in Fig. 5, respectively.

Fig. 7
figure 7

AE energy curves of the fracturing tests with the fracturing fluids of different viscosities

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In the stage I, the coal sample is elastically deformed under the combined action of hydraulic pressure and external loading as the clean fracturing fluid is injected. The input energy is stored as elastic energy. The coal sample remains intact at this point, and thus no AE energy is detected, even the fluid pressure is increased. In the stage II, the coal sample cannot absorb more energies as the external energy further input. The external energy input becomes the driving force for the fracture initiation. The coal sample is destructed, which emits actively high acoustic energies. At this time, the pressure begins to decrease from the highest point. The fracture propagation tends to be stable in stage III, and the AE energy drops and becomes stable. At this time, the pressure also drops to a stable value.

The fracture propagation in the coal sample during fracturing can be deduced from the AE energy curve. As can be seen from Fig. 7 for the AE energy curves. the higher the viscosity of the clean fracturing fluid, the less the AE energy released by the fracture prolongation. The analysis shows that when the viscosity is small, the clean fracturing fluid is easier to communicate the original cracks and micro cracks, forming more complex cracks, and the damage variable is large, resulting in a higher acoustic emission energy value. The increase of viscosity leads to the increase of flow resistance of clean fracturing fluid, the limitation of fracture propagation, only a single fracture, and the small damage variable. The released energy can be detected by the AE meter when the energy is accumulated to a relatively high level. The more fractures are generated, the higher the AE energy is.

The hexahedral coal sample after fracturing was photographed by the high-definition camera to form a hexahedral expansion diagram. A simple sketch drawing was drawn according to the unfolded map, and the coal sample was opened along the hydraulic fractures, so as to analyze the fracture propagation law more comprehensively and accurately. Figure 8 shows the coal samples fractured with different fracturing fluids. In the acoustic emission 3D localization map, the blue points, red points and yellow points represent the acoustic emission sensors, detected fractures, and new fractures, respectively. Many tiny fractures are observed in the X+, Y+, and Z+ directions in the coal sample A1 that is fractured with the fracturing fluid of 130 mPa·s. The overall width of the fractures is narrow and gradually decreases from the borehole to the coal wall. The sectioning along the X and Z directions reveals a large amount of fracturing fluid remained in the entire section. In coal sample B1 (fracturing fluid viscosity, 320 mPa·s), a small penetrating crack perpendicular to the X direction is produced. The width of the fractures is increased compared to A1. The clean fracturing fluid residues are found in the section along the Z+ direction, but not in the Z− direction. A long straight penetration fracture is formed in the X and Y+ direction of the coal sample C1 (fracturing fluid viscosity, 510 mPa·s). Clean fracturing fluid residues are found near the borehole and in the X+ direction. As shown in the AE 3D localization map, intensive event points are distributed along the direction of the sample midline. The fracturing of coal sample D1 (fracturing fluid viscosity, 700 mPa·s) produces an obvious penetration fracture in the Z direction. The width of penetrating fractures is obviously larger than that of A1, B1 and C1. The examination of the section in Z direction suggests that clean fracturing fluid residues are remained only near the borehole, and do not expand in the hydraulic fracture. The AE 3D localization map shows that most of the events are concentrated near the borehole, which mutually confirms the hydraulic fracture location.

Fig. 8
figure 8

Experimental results of hydraulic fracturing with the clean fracturing fluids of different viscosities

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From the above analysis, it can be concluded that the low-viscosity clean fracturing fluid pumped is easy to be compressed into fine fractures due to its loose network structure, resulting in small and complex fractures in the coal samples. However, when high-viscosity clean fracturing fluid is pumped, the network structure is relatively dense and it is not easy to be compressed, resulting in a single fracture shape with less branching and only a single long straight-through fracture in the coal sample. This is where clean fracturing fluid differs in terms of fracture formation mechanisms compared to other fracturing fluids. In Sect. 4, the mechanism of viscosity of clean fracturing fluid on crack growth in coal rock will be further discussed. In addition, the clean fracturing fluid distribution gradually concentrates around the borehole from the entire section as the clean fracturing fluid viscosity increase.

3.2 Effects of in situ stress on hydraulic fracturing

The experiment was conducted with varied triaxial stresses, but the same clean fracturing fluid (viscosity, 320 mPa·s), and the fracture initiation and propagation were then analyzed. Figure 9 shows the time-pressure curves under different levels of in-situ stress difference. The stage ① in the time-pressure curve is where the triaxial stress is loaded to the pre-set value, and the subsequent four stages are the same as those discussed above for the fracturing with the clean fracturing fluids of different viscosities (Fig. 5).

Fig. 9
figure 9

Time-pressure curves during the fracturing under different horizontal in situ stress differences

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Figure 10 shows the variations of fracturing pressure and pressure drop with the horizontal in-situ stress difference. As can be seen, the fracturing pressure of the coal sample gradually decreases, and the pressure drop after the penetration fracture formed becomes smaller with the increase of horizontal in-situ stress difference. In the case of no horizontal in situ stress difference (B2 curve), the constraint ability of the cracks extension path is strong and the hydraulic energy is relatively concentrated. This is mainly because the stress of the front and back directions and the left and right directions of the coal sample are the same and the direction of the drilling hole is parallel to the direction of the coal sample bedding. When the clean fracturing fluid is injected into the coal sample, the generation and expansion of cracks are inhibited. The pressure in the hole can only produce cracks by overcoming the external equilibrium load. So, the initiation pressure is large and obvious. As the clean fracturing fluid continues to pump, some of the primary fractures near the drilling hole are connected. These cracks are prone to steering and migration, resulting in a decrease in pressure, but remain relatively stable. With the increase of horizontal in-situ stress difference (B1, B3 and B4 curves), the stress imbalance in front and back directions and left and right directions is enhanced, which changes from inhibiting to promoting the formation and propagation of cracks. After the clean fracturing fluid is injected into the coal samples, the horizontal in situ stress has a low constraint on the cracks’ extension path. Therefore, when the pressure rises to a certain value, the coal sample cracking pressure will be reached. Although the value of the initiation pressure is not obvious, it is also greater than the minimum value of the triaxial stress, which is easy to produce more obvious through cracks.

Fig. 10
figure 10

Variations of fracturing pressure and pressure drop with the coefficient of variation of horizontal in-situ stress difference

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Figure 11 shows the AE energy curves during the fracturing under different levels horizontal in-situ stress difference. The three stages are the same as those for the fracturing with the clean fracturing fluids of different viscosities (Fig. 7).

Fig. 11
figure 11

AE energy curve during fracturing under different horizontal in situ stress differences

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As can be seen, the AE energy detected during the hydraulic fracturing with zero horizontal in-situ stress difference is low. With the zero horizontal in-situ stress difference, the external stress load is greater than the internal pressure after the clean fracturing fluid injected. The fracturing fluid can only be filtrated into the primary fractures of the coal sample. No hydraulic fractures are formed, and the coal sample remains intact at this point. The damage variable is small and consequently, the AE energy is low. With the increase of the horizontal in-situ stress difference, the imbalance of horizontal stress is gradually enhanced, and the external loading eventually becomes lower than the pumping pressure. Therefore, the coal sample is more prone to fracture, and the integrity of the coal is damaged, which releases high AE energies.

The fracturing experiment results of the coal samples with different horizontal in situ stress differences are present in Fig. 12. No obvious hydraulic fractures are formed in sample B2. It can also be seen from the sketch that the width of the fractures is narrow and scattered. The sectioning along the bedding direction reveals a large amount of clean fracturing fluid in the entire section. The AE 3D localization suggests that AE event points are scattered in the whole coal sample. A relative obvious hydraulic penetration fracture is generated in sample B3 in the Z direction, and micro-fractures are generated near the penetration fracture. The width of the fractures is increased compared with B2. The clean fracturing fluid residues are found around the borehole where more AE event points are also localized. The fracturing of coal sample B4 with the coefficient of variation of 1.55 results in an obvious hydraulic fracture in the X direction. The width of the fractures is larger than that of B1, B2 and B3. The fracturing fluid residues are only found around the borehole in the section of the X direction. In addition, most of the AE events are distributed around the borehole, as shown in the AE 3D localization map.

Fig. 12
figure 12

Fracturing results of the coal samples at different levels of horizontal in situ stress difference

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In all, the fracturing under small horizontal in-situ stress differences produces micro-fractures in the coal sample with no obvious main fracture, and the fracture morphology is complex. The fracturing fluid is subjected to serious filtration loss in the primary fractures. With the increase of the horizontal in-situ stress difference, the fracturing is less affected by the primary fractures, and more obvious hydraulic fractures are generated along the bedding direction. The fracture morphology also becomes simple. The distribution of the tracer suggests that the fracturing fluid is gradually concentrated around the borehole with the increase of the horizontal in-situ stress difference.

4 Discussion

The experimental results suggest that the fracturing with the low viscosity clean fracturing fluids forms more complex fracture morphologies, and the fracture morphology becomes simpler as the fracturing fluid viscosity gradually increased. The reason may be that clean fracturing fluid is both viscous and elastic. When the clean fracturing fluid is pumped into the coal, the network structure enhances its elasticity and is more easily to be compressed into the fine cracks of the coal, resulting in the formation of more complex branching fractures. Other fracturing fluids are similar in viscosity to clean fracturing fluid but less elastic. When the pump rate is the same, other fracturing fluids are not easily compressed and it is difficult to reach the fine fractures. As a result, clean fracturing fluid produces more complex fractures than other fracturing fluids (Gomaa et al. 2015; Hu et al. 2015; Sumanth and Kuru 2017; Afra 2020; Davoodi et al. 2023).

The clean fracturing fluid used in our study is prepared with constant mass fractions of sodium salicylate and KCl, but different mass fractions of CTAB to vary the viscoelasticity. As shown in Fig. 13, sodium salicylate and KCl interact with each other to form multiple individual rod-like micelles as a small amount of CTAB added. These micelles intertwin to form a reversible 3D viscoelastic network structure via van der Waals force and weak intermolecular chemical bonds. Due to low CTAB content, the network structure is relatively sparse, and the clean fracturing fluid exhibits strong elasticity and weak viscosity (Gao et al. 2019; Aleshina et al. 2020). Therefore, the low viscosity fracturing fluid shows good fluidity, but is easily compressed into small primary fractures as continuously injected into the coal body at a constant flow rate. The hydraulic pressure in the borehole builds up rapidly as the fracturing fluid reaches the undeformed fracture in the stress concentration zone. The fracturing fluid flow overcomes the anti-fracturing resistances of the weak surfaces of primary fractures in the coal body to form more fractures. The fractures expand, extend, and prolong, resulting in complex fracture patterns. Increasing the mass fraction of CTAB results in denser reversible 3D viscoelastic network structures. The fracturing fluids show weaker elasticities, higher viscosities (Hu et al. 2015), poor fluidities, and not easily compressed. The fracturing fluid of high viscosity accumulates at the bottom of the borehole or enters some relatively large primary fractures as continuously injected into the coal sample at the constant rate. The fracturing then causes a relatively obvious single hydraulic fracture based on the primary fractures connected in the early stage.

Fig. 13
figure 13

Action mechanism of clean fracturing fluid for coal seam fracturing

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In addition, it is speculated that without the horizontal in-situ stress difference, the clean fracturing fluid is pumped into the coal sample. Due to the strong horizontal stress balance, the generation and expansion of the fractures in the coal sample are inhibited. When the clean fracturing fluid is injected into the coal body at a constant speed, it is easy to be compressed into the small fractures and reach the tip of fracture, resulting in a large initiation pressure and more complex fractures. With the increase of horizontal in-situ stress difference, this balance gradually weakens, and the generation and expansion of fractures change from inhibition to promotion. The compression of clean fracturing fluid becomes small or even does not need to be compressed to cause fractures in coal body. Therefore, the fractures generated are also relatively single, and the fracture initiation pressure is also small.

5 Conclusions

In this study, the influence of different viscosity of clean fracturing fluid and different horizontal in-situ stress difference on coal fracture propagation was analyzed by using the self-developed true triaxial hydraulic fracturing experimental system, and the mechanism of clean fracturing fluid on coal fracture propagation was discussed. The following conclusions have been drawn.

  1. (1)

    With low viscosity or no horizontal ground stress difference, the coal samples produced small and more complicated fractures whose width were narrow, and the AE event points were more dispersed. With the increase of viscosity or horizontal ground stress difference, the fracture form tended to be single, the fractures width increased obviously, and the distribution of AE event points was concentrated.

  2. (2)

    The AE energy gradually decreases with the increase of clean fracturing fluid viscosity and increases with the increase of the horizontal in-situ stress difference. The more fractures are generated, the higher the AE energy is.

  3. (3)

    As the viscosity of the clean fracturing fluid increases, the initiation pressure gradually increases, but the increase gradually slows down, and the pressure drop gradually increases. With the increase of horizontal in-situ stress difference, the initiation pressure decreases gradually, and the pressure drop decreases gradually.

  4. (4)

    Low-viscosity clean fracturing fluid has strong elasticity and is easier to be compressed to reach the tip of fractures, resulting in more complex fractures. The high viscosity clean fracturing fluid has poor elasticity and should not be compressed to enter the tiny fractures, and the fractures are relatively single.