Study on the Influence of Gas Desorption Characteristics under High-Pressure Fluid Fracturing of Deep Coal

Abstract

In order to study the influence law of gas desorption accumulation and emission characteristics under hydraulic fracturing, this experiment uses coal-rock adsorption–desorption test equipment to carry out isothermal desorption tests of water-bearing coal under various stress paths. The experimental object is anthracite from Four Seasons Chun coal mine in Guizhou Province. In this experiment, the influence law of the desorption emission characteristics of coal under different stresses is analyzed. Research shows that the stress directly affects the gas desorption of coal and plays a decisive role in the gas desorption and emission characteristics of water-bearing coal in the stress-affected zone. Under equivalent gas adsorption of water-bearing coal, the total accumulated gas desorption displayed by coal increases with the increase in stress under certain conditions and the increase rate slows down with the time; coal samples differing in moisture content are subjected to various stress paths, leading to the difference in the total gas desorption. The total accumulated gas desorption displayed by coal with higher moisture content is generally smaller than that with lower moisture content. Through field observation, a zone with high accumulated gas desorption is formed in the stress-affected zone beyond the radius of effective fracture influence, generating an imbalance of gas desorption and emission. The study results are of theoretical and practical engineering significance for the prevention and control of stress-induced disasters and gas disasters in deep coal seams.

1. Introduction

The supply of coal resources still occupies an important position in China’s energy system. The horizontal depth of coal mining has been increasing due to the massive exploitation of shallow coal resources over the years. In recent years, the output ratio of deep coal resources has increased, accompanied by more complicated deep stress environments, geological conditions, and gas occurrence. As the mining depth increases, the risk of dynamic disasters of coal and gas outburst is aggravated, and the gas pressure and gas content in coal body increase accordingly. Given the complicated deep stress environment and geological conditions, the proportions of soft coal and broken coal increase, and it is more difficult to drill holes and extract gas from outburst coal seams, which directly leads to more frequent gas ultra-limits, becoming the main controlling factor restricting the safe production of coal mines.

Underground coal is in a state of stress balance before mining. After mining, the stress imbalance superimposed by the geological structure and the comprehensive stress of the mining disturbance exerts a significant impact on the process of gas adsorption–desorption–diffusion–seepage in coal seams [1,2]. If the coal body is severely disturbed by continuous high stress, a composite dynamic disaster of coal and gas outbursts and gas impacts will be triggered. To manage the main controlling factors of comprehensive stress-dominated gas disasters in deep coal bodies, pressure relief in deep coal has been realized using high-pressure hydraulic fracturing measures, and the characteristics of gas desorption and emission under fracturing have been studied to provide theoretical and practical experience for gas disaster prevention in deep coal.

For many years, the gas absorption/desorption laws of coal have been extensively examined by Chinese experts from such perspectives as coal moisture content [3,4], damage condition [5,6], stress environment [7,8,9,10,11], temperature [12,13], and coal particle structure [14,15]. Among these studies, gas absorption/desorption models and diffusion formulas have been proposed by some scholars, laying a theoretical foundation for characterizing the mechanism of coal and gas outburst risk. Stress exhibits certain influencing characteristics on gas adsorption/desorption. Li X. C. et al. [16] found that effective stress evidently affects the methane adsorption capacity. Lv X. F. et al. [17] obtained the relationship between pore pressure and the desorption amount through simulation. In order to clarify the influencing mechanism of high-pressure fluid fracturing stress on the desorption characteristics of water-bearing gas coal, the stress state of anthracite in Sijichun Coal Mine, which was taken as the study object, was approximated and simulated using different stress combinations under the action of high-pressure fluid. Then, water-bearing gas coal samples were subjected to isothermal desorption tests under different stress states, so as to expound the influencing mechanism of the zone of stress influence on the desorption and emission characteristics of water-bearing gas coal under high-pressure fluid fracturing. This may help us to understand the phenomenon of gas emission after fracturing, and also provide a theoretical basis for gas disaster management.

2. Test Samples and Methods

In the experiment, the self-improved triaxial coal-rock adsorption/desorption testing apparatus was used to carry out isothermal adsorption and desorption tests of water-bearing coal under various stress paths [18,19], as shown in Figure 1.

2.1. Preparation of Test Materials

The test samples were collected from the 1166 working face of coal seam 16# in Sijichun Coal Mine, Guizhou Province, with an average dip angle of 13° and an average thickness of 1.5 m. The original gas content in the working face area was 15.83 m³/t and the original gas pressure was 1.02 MPa. There was no large fault geological structure in the working face area of this coal seam, and the coal body strike structure was stable. The coal body of this working face was selected as the material for preparing Φ50 mm × 100 mm standard coal samples. Considering the stress balance between the upper and lower cylinders during sample loading, the upper and lower cylinders of the coal samples were polished to make their surfaces smooth. The coal samples are shown in Figure 2.

The fabrication process of samples differing in moisture content was as follows [20,21]:

(1)

The processed samples in Figure 2 were dehydrated and dried in a drying box and weighed (m₀).

(2)

The dry coal samples were completely soaked in a closed container filled with distilled water for 3 h.

(3)

The coal samples were taken out and their soaking mass m_s was measured. The moisture content w_s of the coal samples is fixed as follows:

$$w_s=\frac{m_s-m_0}{m_0}\times 100\%$$

(1)

(4)

The soaked samples were placed in the drying box and weighed every 15 min. The weighing time could be adjusted according to the specific needs until their mass reached the required value m_p. The coal samples were taken out, placed in a constant-temperature and -humidity box, and weighed at regular intervals until the mass m_u of the samples remained unchanged. The moisture content w_u of the coal samples is expressed as follows:

$$w_u=\frac{m_u-m_0}{m_0}\times 100\%$$

(2)

(5)

Steps (2) and (4) were repeated to acquire the moisture content of different coal samples.

(6)

The coal samples were divided into two groups in the experiment, with a moisture content of about 1.15% and 3.24%, respectively.

2.2. Experimental Method

To achieve the purpose of isothermal desorption tests of coal with different water-bearing states (gas-bearing coal) under various stress paths, a gas adsorption equilibrium was achieved in the experiment using a pressure of 2 MPa. Specifically, the coal samples were tested under a confining pressure of 3 MPa and an axial pressure of 0, 4, 8, 12 and 16 MPa, respectively. The gas desorption of coal samples with different water-bearing states under multiple stress paths was measured, and the desorption laws of loaded coal samples under different conditions were analyzed. The specific test steps were as follows: (1) the valves and switches of the system equipment were all closed, the coal samples were fixed in a stress chamber and subjected to the confining pressure of 3 MPa, and then the vacuum pump, valve 3, and valve 4 were opened for vacuum degassing of the coal samples. (2) After degassing was completed, valve 1 of the CH₄ gas tank was opened to charge the gas into the temperature-controlling and pressure-regulating device, and the temperature was regulated at 20 °C. (3) At this time, the stress chamber was kept at a constant temperature of 20 °C, and the temperature-controlling and pressure-regulating device and valve 3 were opened to charge the gas into the stress chamber. When the coal samples adsorbed gas, a gas pressure change was recorded. (4) After the reading of the pressure gauge became stable and adsorption was completed, the set axial pressure was applied to the coal samples, valve 5 was opened so that the free gas in the stress chamber was discharged, and when the pressure reading of the instrument was 0, the pressure was considered to identical to atmospheric pressure. In this case, the water-bearing gas coal samples under different stress states were subjected to isothermal desorption tests using a gas metering device, and the gas content was recorded.

3. Gas Desorption Laws of Coal with Different Water-Bearing States under Various Stress Paths

Under the stress produced by high-pressure fluid presplitting and permeability enhancement, water-bearing coal in the gas atmosphere was subjected to an ever-changing stress load, generating a significant impact on coal adsorption capacity, inevitably leading to a change in the adsorption–desorption state of coal after being subjected to different stresses. To study the desorption and emission laws of water-bearing coal under different stresses in the gas atmosphere, the gas desorption amount was analyzed by combining the above test methods.

3.1. Influencing Analysis of Various Stress Paths on Gas Desorption from Water-Bearing Coal Samples

During the test, the influence of stress on the gas desorption displayed by water-bearing coal could be analyzed using the accumulated gas desorption. The isothermal adsorption characteristics of coal samples could be described using the Langmuir equation, namely:

$$Q=\frac{abP}{1+bP}$$

(3)

where Q is the gas adsorption capacity with the unit of mL/g; a is the maximum adsorption capacity with the unit of mL/g; b is the Langmuir constant with the unit of Pa⁻¹; and P is the pore pressure with the unit of Pa.

Combined with the above formula, the accumulated gas desorption from the water-bearing coal body under various stress paths was analyzed by using a Langmuir-type relational expression of gas desorption with time [22]:

$$Q_t=\frac{AB\sqrt{t}}{1+B\sqrt{t}}$$

(4)

where Q is the accumulated gas desorption at time t with the unit of mL/g; t is desorption time with the unit of s; A is the limiting gas desorption with the unit of mL/g; and B is the desorption rate constant.

Under constant temperature, the accumulated gas desorption displayed by water-bearing coal under multiple stress paths was measured and fitted as per Formula (4) to acquire the limiting gas desorption under different test conditions. The water-bearing coal desorption curves and the change curves of the limiting gas desorption under multiple stress paths were drawn, as displayed in Figure 3, Figure 4, Figure 5 and Figure 6.

By analyzing Figure 3 and Figure 4, it can be seen that (1) the coal samples differing in moisture content were subjected to different stress paths, which also led to the difference in the corresponding gas desorption process. Within 0–3 min in the initial test stage, the stress borne by the coal samples gradually increased, accompanied by the overlapping or crossing of desorption curves under each stress path, but under equivalent gas adsorption by coal samples, the total desorption displayed by coal samples grew with the stress within 60 min. (2) The coal samples with different moisture contents were under the action of multiple stress paths, and the total gas adsorption produced by each stress path was different, but the variation trend of gas desorption was largely the same. In addition, the gas desorption displayed by the coal sample with higher moisture content was generally smaller than that with a lower moisture content under different stress paths.

It can be seen in Figure 5 and Figure 6 that when the axial stress borne by the coal samples with two different moisture contents showed hierarchical growth (0, 4, 8, 12, and 16 MPa), the limiting desorption displayed by coal samples was fitted, and then the limiting desorption displayed by the coal sample with a moisture content of 1.15% was about 16.12–21.33 mL/g, with an increase of 5.21 mL/g. The limiting desorption of the coal sample with a moisture content of 3.24% was about 14.81–19.34 mL/g, increasing by 4.53 mL/g. The desorption displayed by coal samples was obviously promoted by the hierarchical growth of the axial stress borne by coal samples. However, the increase amplitude of the limiting desorption declined due to the increase in the moisture content of coal samples.

3.2. Influencing Analysis of Stress on Gas Desorption and Emission from Water-Bearing Coal Samples

As a heterogeneous porous solid medium, coal can be characterized as a solid particle skeleton structure, and the forced compression of coal can be regarded as the combination of pore volume reduction and solid particle skeleton volume reduction [23]. The high-pressure fluid presplitting and permeability enhancement process is adopted for deep coal. In addition to ground stress, coal is also affected by the change in the high-pressure fluid stress, which directly influences the gas occurrence in coal and the gas desorption and emission.

Through the above tests, it can be observed that the stress promoted the gas desorption of water-bearing coal, and the total accumulated gas desorption observed in water-bearing coal also increased under various stress paths when the stress grew. The influence of gas desorption and emission could be further analyzed as follows:

(1)

There is a multi-stage pore system in the coal body, in which micro-pores filled with gas are characterized as “a gas storage molecular system”, representing a strong gas adsorption zone; under the stress action, the particle skeleton structure and the pore volume of coal are compressed, the gas adsorption equilibrium is destroyed, and a large amount of free gas is desorbed by the pore diffusion system, representing a weak gas adsorption zone; a connective channel exists between the pore system and the seepage system, and the compressed gas molecules are quickly discharged, which leads to the increase in the total accumulated gas desorption from coal.

(2)

The test coal is anthracite, which has a high degree of micropore development, and the larger the specific surface area of coal, the greater the gas adsorption capacity [24,25]. The coal is infiltrated by high-pressure fluid stress, which results in the competitive adsorption and desorption of water and gas molecules in the pore diffusion system in the weak pore adsorption zone. Under the high-pressure fluid stress, however, the gas desorption capacity is weakened, a large number of water molecules infiltrate the pore diffusion system to displace gas molecules in a specific stage, and water molecules block the connective channel between the pore diffusion system and the seepage system, resulting in the difference in gas emission in this zone.

(3)

The stress on coal reduces the connective channel between the pore diffusion system and the seepage system, and changes the gas occurrence and gas migration resistance, as shown in Figure 7 (water molecules are blue, gas molecules are colorless). Pore channel A of the water-bearing coal is the channel size before the stress is applied (solid line part), and A₁ and A₂ are the channel sizes after the stress is applied (dotted line part). Due to the blockage of water molecules and the narrowing of gas migration channels, the pore pressure increases, which increases the pressure difference between gas channels.

4. Gas Measurement and Analysis under Field Stress Action

To investigate the influence of high-pressure fluid stress borne by coal on gas desorption, the hydraulic presplitting process test was implemented in the haulage roadway of the 1166 working face in Sijichun Coal Mine, Guizhou Province. There was no fault geological structure in the test area, and the coal seam was stable. Fracturing holes and observation holes (1#–5#) were drilled to investigate the gas pressure, gas content, gas concentration, and single-hole pure gas extraction, as displayed in Figure 8. The observation holes were used to measure the gas pressure.

Before the hydraulic presplitting process was implemented, the fracturing holes and observation holes (1#–5#) were constructed first, and the observation holes were simultaneously used for the measurement of the gas pressure and gas extraction. During this period, the actual pressure range of hydraulic fracturing was 0–34 MPa, and the pressure was stabilized at 32–34 MPa after pressure was initially applied. When the process measure was completed, the total amount of water injected was 198.6 m³.

According to the previous hydraulic presplitting test in this mine, the radius of the effective fracture influence was 22 m. The previous test area belonged to the same geological unit and coal seam with the existing test area, and the coal seam occurrence was steady without geological structures, so the radius of effective fracture influence was set to 22 m during hydraulic presplitting in this test area. Beyond this distance was a zone where the extension of the hydraulic fracture ended and the high-pressure fluid stress exerted its action. The gas measurement in the stress-affected zone was analyzed as follows.

(1)

Analysis of gas pressure in the stress-affected zone

The observation holes (1#–5#) were constructed simultaneously with fracturing holes. After that, the gas pressure changes before and after hydraulic presplitting were observed, as shown in Figure 9.

Because observation hole 1# was located at the radius of the effective fracture influence of 22 m and on the edge of the effective fracture influence, a big gap was observed between observation hole 1# and observation holes 2#–5# in terms of gas pressure. As can be seen from the change curve in Figure 9, the gas pressure in the observation holes (1#–5#) before the stress affected the test area was 9.8–10.5 MPa, and the gas pressure changed only by 0.7 MPa. Under the stress action, the gas pressure was 6.46–18.2 MPa, and the maximum gas pressure exceeded 7.7 MPa before the stress action.

(2)

Concentration analysis of gas extracted in the stress-affected zone

The gas concentration changes in the observation holes (1#–5#) in the stress-affected zone are exhibited in Figure 10.

Observation hole 1# was located on the edge of the radius of the effective fracture influence, and the gas concentration ranged from 35.1% to 40.6%. The gas concentration in observation holes 2#–5# was 77.4–90.4%. Therein, the highest gas concentration in observation hole 2# was 86.6–90.4%, always being higher than that in observation hole 1#.

(3)

Analysis of pure gas extraction in the stress-affected zone

The changes in the pure gas extraction in the observation holes (1#–5#) in the stress-affected zone are displayed in Figure 11.

The pure gas extraction from observation hole 1# was 0.51–0.55 m³/min, which was higher than that from the other observation holes. As the distance from the observation hole to the fracturing hole increases, the pure gas extraction presents a progressive declining trend.

To sum up, after the implementation of the hydraulic presplitting and permeability enhancement process, the coal body was in the stress-affected zone, and under certain conditions, the total accumulated desorption displayed by the coal body infiltrated by high-pressure water increases with the increase in stress, and the increase rate slows down with the time. This is because the adsorption capacity of gas molecules is reduced and the pore volume is compressed due to the competitive adsorption of water molecules and gas molecules in the coal body, and the multi-level gas connective channels in the coal body are also compressed, which inhibits the seepage and migration of gas and increases the gas pressure. Within 1 week after gas extraction, the gas concentration was high, the gas extraction was considerable, and the pure gas extraction was much greater than the pure gas amount without the implementation of the hydraulic fracturing process. It can be found by analyzing the process that the gas desorption and emission from the coal body were affected by multiple factors. The gas desorption displayed by coal could be promoted by the hydraulic presplitting process. Since the permeability of the coal body in the stress-affected zone was not enhanced, most gas connective channels were not connected, and a zone with high accumulated gas desorption was formed within a certain range beyond the radius of the effective fracture influence during hydraulic fracturing.

5. Conclusions

(1) The ground stress and high-pressure water stress directly affect the desorption ability of gas-bearing coal during gas control in the deep coal body, which determines the gas emission characteristics of the coal seam in the stress-affected zone.

(2) Because coal samples with different moisture contents are affected by various stress paths, the total amount of gas desorption produced by each stress path is different, but the change trend of gas desorption is roughly the same.

(3) Most of the gas flow channels in the stress-affected area are not connected, and a “high-volume area” of gas desorption accumulation is formed in a certain area outside the influence radius of the effective crack of water pre-escalation, resulting in the imbalance of gas desorption and emission.