Experimental Study on Coal Seam Gas Desorption Characteristics Caused by Moisture under Stepwise Depressurization

Abstract

Expansion energy is the main factor of coal and gas outbursts, and the gas desorption around the outburst hole is developed in variable pressure conditions. While studying the impact of moisture on gas desorption characteristics, atmospheric pressure desorption is usually used, but its characteristics under variable pressure conditions have not been thoroughly investigated. In this study, typical outburst coal samples with different water contents from the Jincheng mining area of China were selected as research objects, and the effects of water on gas displacement, desorption, desorption rate, and gas desorption index (K₁) of drilling cuttings under step-by-step depressurization were analyzed by means of stepwise depressurization and atmospheric desorption experiments. The research conclusions suggest that (1) the amount of gas replacement, which augments rapidly during the inception, increases with the growth of water content under the experimental conditions, and then the rate decreases; (2) the gas desorption falls gradually at different depressurization stages when the humidity is constant, while the total desorption and the drop amplitude taper with the increasing water content; (3) the additional water enhances the desorption rate significantly only at the initial stage, but scarcely has an impact later on; and (4) the value of the drilling cuttings’ gas desorption index (K₁) shows a downward trend with the developing humidity in each stage of stepwise depressurization desorption. We take humidity as a variable to simulate the desorption process of coal gas around the hole when coal and gas outbursts occur in the laboratory and study the influence of water on the desorption characteristics under desorption conditions of stepwise depressurization. This provides a reference for the purpose of studying the mechanism of coal and gas outbursts from the perspective of energy.

1. Introduction

Coal mining often suffers various threats, such as gas, fire, mine pressure, etc. Among these, gas accidents are the main factor affecting China’s coal mine safety because of their high frequency, the casualties, and the serious damage caused [1]. Coal and gas outbursts can destroy underground equipment and the ventilation system, and can even induce secondary accidents, which not only endanger the lives and safety of workers, but also cause huge economic losses. With the increasing mining depth, gas occurrence and mining conditions will become more complex [2], while the intensity and scale of coal and gas outbursts will also worsen greatly. In order to eliminate or reduce the gas disaster in mining, hydraulic slotting, punching, fracturing, coal seam high pressure water injection, and other measures have been widely used [3,4,5].

Hydraulic slotting and punching adopt drilling and cutting equipment, which uses the power of high-pressure water to cut and scour coal seams in the borehole for the purpose of improving the permeability of coal seams. Compared with the ordinary method, ultra-high-pressure water jet technology can greatly improve the permeability of coal seams around slotted drilling in soft coal seams [6]. However, traditional hydraulic punching technology has the disadvantages of low efficiency, high water consumption, and difficulty in ensuring the hole size. The new technology, assisted by water jet mechanical cutting, is much higher than hydraulic flushing, which can elevate the effectiveness of drainage and reduce coal mining costs [7]. In addition, the axial and radial stress distribution of high-pressure water jets, as well as the energy accumulation effect of nozzles, are key factors that affect the cutting efficiency, directly impacting the attenuation of jet velocity and the strength [8]. The geomechanical model based on the plastic volumetric strain can effectively describe the airflow in the original coal seam and the disturbed coal seam. Under the guidance of theoretical research, the gas injection displacement technology is resultful in eliminating the stress concentration caused by hydraulic punching and in promoting the desorption of residual methane in the stress concentration area [9,10]. This technology and process are being widely applied for diverse coal seams.

Hydraulic fracturing is the use of high-pressure water to integrally generate fracturing in coal seams as gas migration channels, as well as improving the moisture and enhancing the gas drainage effect. The average length of microcracks and the expansion of fracture porosity are the most effective characterizations of coal seam permeability. Therefore, the competitive initiation and expansion of coal seam cracks are the fundamental goals of this technology [11,12]. Special fracturing techniques, represented by pulse fracturing, can significantly improve the gas displacement and natural desorption under different hydraulic fracturing methods [13]. In practical applications, the scope of hydraulic fracturing is often defined through stress transfer, and hydraulic perforation and hydraulic fracturing are usually combined to improve the efficiency of coal seam gas extraction [14,15]. Hydraulic measures are commonly used, and many scholars have carried out extensive work on them, but most research simply expands upon the normal pressure desorption law.

This study focuses on the influence of moisture on the desorption law of coal seam gas, which is also affected by many factors such as temperature, pressure, pore structure, particle size, etc. Coal seams with different degrees of metamorphism have distinct adsorption and desorption modes, and the water resistance effect inhibits the desorption of coalbed methane [16,17]. By monitoring the gas pressure and temperature of deformed coal and primary coal, and comparing methane diffusion in water-bearing coal and dry coal using low-field nuclear magnetic resonance, the evolution law of key gas parameters during desorption process can be established [18,19,20]. The characterization of the pore morphology reflects the influence of water injection on the pore properties of coal, which manifests as the desorption characteristics of gas in coal under the coupling conditions of water and pressure [21,22,23], and the study of the gas–water distribution model verifies the adsorption and desorption mechanisms of gas [24]. Most research does not correspond to the variable pressure environment in which outbursts occur, although the influence of water on the gas desorption law under normal pressure has been relatively sufficient. The gas desorption index of drill cuttings is the key parameter for predicting the risk of coal and gas outbursts. Therefore, the study of water’s effect on the gas desorption characteristics of drill cuttings is of great significance.

The sensitivity of the index is the key to the prediction of outburst. The cutting quantity index S, the initial velocity index q, and the gas desorption index K₁ are commonly used for predicting coal and gas outbursts [25,26,27]. Among them, the index K₁ is the most widely applied, and achieves the greatest impact in terms of the coal seam gas desorption characteristics. In addition, it is closely related to the consistent coefficients and particle sizes of coal [26,27]. Laboratory research is usually based on desorption characteristics under atmospheric pressure, but the above research also did not consider the pressure-vibrating environments in which outbursts occur.

Although relatively sufficient research efforts have been made, including those with respect to the characteristics of gas desorption and hydraulic technology, which mainly focus on the atmospheric pressure desorption and do not effectively describe the objective reality to some extent. In this research, the stepwise depressurization desorption conditions were simulated innovatively in laboratory, which is consistent with the mining practice that the gas desorption around the hole usually develops under variable pressure when coal and gas outbursts occur. During this experiment, coal samples with different water contents were selected in order for us to carry out stepwise depressurization and atmospheric pressure desorption, respectively, aiming to investigate the influence of moisture on gas replacement, to analyze the change rule of gas desorption amount and rate in coal samples at a distinct pressure drop stage, and to quantitatively study the influence of coal samples with different water contents on the gas desorption index (K₁) at each corresponding stage. The experiment simulates the desorption process of gas around the hole when coal and gas outbursts occur, and can provide a reference for the improvement of hydraulic technology and the study of the mechanism of coal and gas outbursts from the perspective of energy.

2. Materials and Methods

2.1. Coal Sample Collection and Preparation

Qinshui Basin is located in the southeast of Shanxi Province, China, where the Jincheng mining area is one of the core coal areas, mainly mining the No. 3 coal seam, which is hard and characterized by high metamorphism. With the growing mining depth, the gas pressure and gas content of the coal gradually escalate, and the gas grades of mines in this region overwhelmingly denote high gas and outburst risks. The area has a long mining history, which has experienced three stages: low gas, high gas, and coal and gas outburst. This feature is not only representative of the Qinshui Basin, but is also typical in most mines with underground mining in China. This is the reason that the No. 3 coal seam was selected as the experimental coal sample in this study. When preparing a granular coal sample, a complete coal wall is selected as the sampling location. First, the sampling surface is cleaned, then a chisel is used to mark the boundary, canvas or an iron sheet is placed on the working surface, and then gap sampling is conducted. After a whole block of raw coal is taken from the freshly exposed coal wall according to the regulations, the collected lump coal is broken in laboratory and 1~3 mm granular coal is screened out, then placed into the drying box and sealed for preservation. The prepared granular coal sample is shown in Figure 1.

2.2. Basic Parameters of Coal Samples

Basic parameters can reflect the adsorption characteristics and metamorphism of a coal seam, including its adsorption constants, proximate analysis, density, and porosity. A certain amount (about 3 kg) of the lump coal sample was selected to be crushed, and the coal with particle sizes of 0.2 mm to 0.25 mm was screened out and dried for at least 5 h. Then, the adsorption constant was determined using the high-pressure capacity method in laboratory. Granular coal with particle sizes less than 0.2 mm was selected for the samples, and was analyzed according to “proximate analysis of coal (GB/T212-2008)”. After the lump coal was crushed and screened, we chose to temporarily divide the sample into four coal sample tanks before measurement, with the purpose of preventing moisture from affecting the measurement results. The results are shown in

Table 1. Measurement results of basic parameters of coal samples.

Test Project	Moisture Mad (%)	Ash Ad (%)	Volatile Vdaf (%)	a (m3/t)	b (MPa−1)	True Density (g/cm3)	Apparent Density (g/cm3)	Porosity (%)
Test Results	1.50	9.47	4.03	40.97	1.03	1.58	1.47	6.99

, where “a, b” represents the adsorption constant.

2.3. Experimental Equipment

A set of devices that could meet the experimental requirements was built which was able to achieve high-pressure water injection into coal samples, keep the experiment temperature constant, ensure that the pressure will be provided according to the design, and record the gas pressure and desorption data in real time during the entire process. The principle of the device is shown in Figure 2. The experimental device was mainly composed of a vacuum degassing unit, a high-pressure water injection unit, a high-pressure gas injection unit, an isothermal adsorption test unit, a back-pressure unit, an automatic metering unit, and a data acquisition unit. The physical device is shown in Figure 3.

(1)

Vacuum degassing unit

Vacuum degassing of the pipelines as well as the coal sample tanks must be carried out to reduce the influence of systematic errors on the results before the experiment.

(2)

High-pressure water injection unit

The device is the core unit of the high-pressure water injection adsorption and desorption system, the function of which is to inject high-pressure water into coal samples after gas adsorption balance is reached, and the volume of injection can be adjusted according to the needs of the experiment, as shown in Figure 4.

(3)

High-pressure gas injection unit

This unit stores and supplies high-pressure gas for the experiments.

(4)

Isothermal adsorption test unit

By regulating the temperature of the water bath, the device can ensure the constant temperature of the coal during the adsorption and desorption experiment, so as to eliminate the influence of temperature on the results.

(5)

Back pressure unit

By controlling the outlet pressure of experimental system, the device, which is composed of a constant pressure and speed pump, a back pressure valve, and a connecting pipeline, can set a stepwise depressurization path.

(6)

Automatic metering unit

The main function is to collect the desorbed gas and record the gas desorption amount in real time when gas is replaced by water or coal sample desorption after water injection.

(7)

Data acquisition unit

Data such as gas pressure and desorption amount from the coal sample tank are recorded by the sensor and saved as documents for experimental analysis.

2.4. Experimental Procedure

The following standards, though not an exhaustive list, should be followed during the experiment: “General provisions on procurement and management of the measuring instrument of water in organization of water using (GB24789-2009)” and “Safety in testing laboratories (GB/T27476)”.

(1)

Coal sample preparation and air tightness checking: Weigh an appropriate amount of the coal sample and place it in the drying box at 105 °C for 6 h to remove the moisture. After cooling, use an electronic balance to weigh out 60 g of the dry coal sample and put it into the coal sample tank, then check the tightness of the coal sample tank and pipeline.

(2)

Vacuum degassing: Place the coal sample tank into the constant temperature water bath device, adjust the temperature to 60 °C, run the vacuum pump, pump the coal sample tank and pipeline to the vacuum state, and maintain the vacuum state for more than 12 h.

(3)

Adsorption balance under constant temperature: adjust the temperature of the water bath device to 30 °C and regulate the valve so that the gas adsorption equilibrium pressure is stable at the set pressure.

(4)

Water injection and replacement: inject a set volume of water into the coal sample tank through the water injection unit, close the valve of injection device, and open the valve of the back-pressure one. Then, adjust the back pressure to the first adsorption equilibrium, open the automatic gas metering device, collect the replaced gas into the air collection bag, and record the replaced amount of gas.

(5)

Desorption under stepwise depressurization: Set the first level of desorption pressure, collect the free gas under this level with the gas-collecting bag, close the valve connecting the coal sample tank to the bag when the pressure of the coal sample tank reaches the first level of desorption, start the back-pressure device and the automatic gas metering device, keep the back-pressure constant for desorption, and record the desorption. Repeat the above operation to complete the planned stepwise depressurization desorption.

(6)

Determination of water added: After the desorption experiment, remove all the coal samples and weigh them. The difference between the measured quality of the coal sample and that of the dried one is the actual water injection.

(7)

Data processing: Convert the desorption data obtained in the experiment into the volume under standard conditions (101,325 Pa, 0 °C). The conversion formula is as follows:

$$Q_t=\frac{P_0·273.2}{101324(273.2+T)}{Q}_{t^{\prime }}$$

(1)

where $Q_t$ is the gas desorption amount in a standard state, mL/g; $Q_{t^{\prime }}$ is the measured gas desorption amount at room temperature, mL/g; T is the laboratory temperature during the experiment, °C; and P₀ is the atmospheric pressure, Pa.

(8)

Cycle experiments: Set a different moisture level, repeat the steps above, and carry out the stepwise depressurization desorption experiment under distinct moisture and diverse adsorption equilibrium pressure conditions, respectively. Record the desorption of gas under each experimental condition.

3. Results and Discussion

3.1. Replacement of Coal Sample Gas by External Water

The adsorption equilibrium pressure was set as 2.5 MPa, and the moisture content was chosen as the variable in order to study the replacement of coal sample gas by added water. A piece of the sample was selected for the implementation of the adsorption equilibrium under 2.5 MPa, and then was desorbed under stepwise depressurization to obtain the free gas volume of dry sample. In addition, another sample was taken to inject high-pressure water according to the setting and achieve the re-adsorption balance. The back-pressure unit was used to carry out stepwise depressurization desorption of the water-bearing coal sample, and the free gas volume of the sample was obtained. The free gas volume, for both the dry and water-bearing coal samples, was converted into the volume under adsorption equilibrium pressure, and the volume difference between the water-bearing and the dry coal sample then represented the amount of replaced water. The results are shown in

Table 2. Gas displacement amounts at adsorption equilibrium pressure of 2.5 MPa.

Moisture/%	0.00	1.37	3.63	4.50	7.44
Replacement volume/(mL/g)	0.00	3.53	4.43	6.27	8.04
Adsorption equilibrium pressure after water injection/MPa	2.50	2.90	3.01	3.21	3.41

.

It can be seen from Table 2 that the gas pressure of the coal sample tank increased from 2.5 MPa to 3.41 MPa after water injection. The reason is that the free space of the tank lessened after water injection, and the pressure of a certain amount of gas ascended when the volume diminished, indicating that the water injection was able to increase the gas pressure of the coal seam. When the moisture content increased from 0% to 7.44%, the replacement amount reached 8.04 mL/g, illustrating that the moisture was able to replace part of the adsorbed gas in the coal sample and the replacement amount increased with the growth of the moisture content within a certain range. In the initial stage of increasing water, the replacement volume was augmented rapidly, and then the growth showed a downward trend, indicating that the influence of water on desorption mainly occurred in the early stage and that there may be a limitation of the replacement in that water can no longer replace the adsorbed gas in the coal as soon as it exceeds the limitation. In terms of the mechanism of the replacement, coal has different adsorption capacities for water and methane, which perform competitive adsorption, and coal has a stronger adsorption effect on water molecules, so part of the adsorbed gas is replaced during the competitive adsorption. This consequence shows that water has a displacement effect on methane, which demonstrates that the gas content can be reduced by water injection into the coal body, and the quantitative study will help us to understand the desorption characteristics of coal bed gas more comprehensively.

3.2. Gas Desorption Amount of Coal Samples with Different Moisture under Stepwise Depressurization Condition

The experiment was divided into 5 groups, with the moisture content of the coal samples being 0%, 1.37%, 3.63%, 4.56%, and 7.44%, respectively. The experiment, which was performed under step-by-step depressurization, included three aspects. (1) Under the same water content, the desorption characteristics of the five samples were analyzed by step-by-step depressurization. (2) Regarding the equilibrium pressure of adsorption as a variable, the change in the desorption amount was studied according to the condition of diverse moisture levels. (3) Under the same equilibrium pressure as that of adsorption, with water content being the variable, the influence of different external moisture levels on gas desorption in each stage of the stepwise depressurization desorption experiment was studied.

The desorption time was 120 min, and the 5 samples with water contents of 0%, 1.37%, 3.63%, 4.56%, and 7.44%, were, respectively, tested in a stepwise step-down desorption experiment. The desorption curve is shown in Figure 5, and the desorption data of each step-down stage are shown in

Table 3. Gas desorption capacity in different stages of depressurization and desorption with different moisture levels.

Gas Desorption Stage	Moisture 0%	Moisture 1.37%	Moisture 3.63%	Moisture 4.56%	Moisture 7.44%
2.5 MPa–2 MPa	2.365	2.129	1.920	1.764	1.448
2 MPa–1.5 MPa	2.588	2.329	2.205	2.087	1.725
1.5 MPa–1 MPa	2.994	2.545	2.386	2.272	2.158
1 MPa–0.5 MPa	3.709	3.153	2.909	2.831	2.624
0.5 MPa–0 Pa	5.563	4.728	3.891	3.446	2.999
Aggregate	17.219	14.884	13.311	12.400	10.954

.

From Figure 5 and Table 3, it can be seen that, according to the condition of constant moisture, when the added moisture was 0%, the gas desorption amount in the 0.5 MPa–0 Pa depressurization stage was 5.563 mL/g, and in the 2.5 MPa–2 MPa depressurization stage, it was 2.365 mL/g. When the added water content was 3.63%, the gas desorption in the above two stages was 3.891 mL/g and 1.92 mL/g, respectively. This shows that under the same water content conditions, the desorption levels at the high-pressure stage are less than those at the low-pressure stage when the desorption is carried out with the same pressure drop span. In the same pressure drop stage, the higher the water content of the coal sample, the smaller the gas desorption amount. Regarding the diverse moisture, when the adsorption equilibrium pressure fell from 2.5 MPa to 0.5 MPa in the dry state, the gas desorption amount descended from 17.219 m/g to 5.563 mL/g, a decrease of 67.7%. When the moisture was 3.63%, the gas desorption reduced from 13.311 mL/g to 3.891 mL/g, and the amplitude of the decrease was 70.7%. The gas desorption increased linearly with the rise in adsorption equilibrium pressure, indicating that the gas adsorption equilibrium pressure affected the level of gas desorption, and the decrease amplitude lowered with the increasing added water. When the adsorption balance pressure was fixed and the water content vibrated, the gas desorption amount decreased from 5.563 mL/g to 2.999 mL/g, a drop of 46.1%, after the added water increased from 0% to 7.44% under the adsorption balance pressure of 0.5 MPa. The pressure was 1 MPa and the added water increased from 0% to 7.44%, as before. The desorption capacity decreased from 9.272 mL/g to 5.624 mL/g, a drop of 39.3%. At 1.5 MPa and 2 MPa, the gas desorption rate decreased by 36.6% and 36.2%, respectively with the same applied water change. This experiment illustrates that at the initial stage of water injection, the decrease is the greatest, and with water injection, the decline gradually diminishes and tends to be stable, which indicates that there exists a range for the influence of water on gas desorption, and the influence weakens when the water content reaches a certain degree. Under low adsorption equilibrium pressure, water has a greater impact on gas desorption, and the desorption volume presents a larger decline. With the ascension of adsorption equilibrium pressure, the drop in desorption is inclined not to fluctuate, indicating that under low adsorption equilibrium pressure, the added water has a strong inhibition effect on gas desorption.

The research on the influence of moisture on the gas desorption characteristics has been quite sufficient, but most of them derive from the atmospheric desorption environment, which is not in line with the actual situation. The desorption, based on this, inevitably brings about significant errors which affect the reliability of research. Although we simulated the variable pressure condition, the samples only included highly metamorphic coal. In the future, more coal samples should be selected from different regions so as to make the results more meaningful.

3.3. Gas Desorption Rate of Coal Samples with Different Moisture Levels under Stepwise Depressurization

The desorption quantity reflects the total amount of gas that causes disasters, which is the characterization of disaster potential, while the desorption rate is the amount of gas released per unit of time. The greater the desorption rate, the more serious the disaster. In this experiment, the gas desorption rates of different desorption stages under different moisture levels were tested so as to explore the effect of added water on the desorption rate. The results are shown in Figure 6.

It can be seen from Figure 6 that in each stage of stepwise depressurization desorption, the gas desorption rate was fast in the first 10 min, then the inclination of the drop was obvious, indicating that the gas desorption intensity was high in the early desorption stage. After 30 min, the rates at different pressure drop stages were basically the same, gradually tending to be 0 mL/(g·min). In the first 10 min of desorption, moisture reduced the rate significantly, which decreased with the increase in moisture and was generally the same as the desorption rate after 30 min. We learned that moisture mainly affects the gas desorption in the initial stage (the first 10 min), and in the middle and later stages (after 30 min), the effects nearly fade away. Therefore, high-pressure water injection can be used to enhance the coal moisture in mining areas, and the inhibitory effect of water on desorption in the early stages should be utilized fully, thus reducing the difficulty of gas disaster prevention and control. Meanwhile, there exists a reasonable value of the moisture content that affects the gas desorption rate, so the water injection method should be used according to the specific properties of the coal seam.

Previous studies have mostly focused on the entire process, from desorption to completion, which cannot accurately guide disaster prevention and resource evaluation. Dividing desorption into several stages based on different pressure drops is more in line with the characteristics of disturbances of coal seams. This study explores the gas desorption characteristics under variable pressure conditions and preliminarily presents the characteristics of coal samples with distinct humidity around the desorption turning point. Due to the representativeness of the experimental coal samples, there is still room for improvement in this research. Later, a study on the rapid desorption stage can be strengthened in order to guide the development of coalbed methane resources more effectively.

3.4. The Gas Desorption Index of Drill (K₁) with Different Moisture Levels under Stepwise Depressurization Conditions

The value of the gas desorption index of drill cuttings (K₁) is an important index for predicting the risk of coal and gas outbursts, which is closely related to the amount of gas desorption, and its critical value is 0.5. For coal samples with different water contents, step-by-step depressurization desorption was carried out in the laboratory, and the index K₁ of drilling cuttings was measured. The influence of added water on the index in each stage was analyzed, as shown in Figure 7.

It can be seen from Figure 7 that when the added water increased from 0% to 7.44%, the K₁ of the 0.5 MPa–0 MPa depressurization stage descended from 0.942 mL/(g·min^1/2) to 0.317 mL/(g·min^1/2), with a decrease of 66.3%, and the K₁ values of the other pressure drop stages decreased by 50.6%, 50.8%, 45.2%, and 46.1%, respectively. In each depressurization stage of the desorption process, the gas desorption index decreased with the increase in moisture, and the index of the dry sample was the largest. When the three desorption stage, 1.5 MPa–1 MPa, 1 MPa–0.5 MPa, and 0.5 MPa–0 MPa, were followed, the gas desorption index value decreased with the increase in the water content with a negative exponential trend, indicating that in the early stage of water injection, the gas desorption index declines greatly and the descending step of the index gradually narrows down with the increasing external water, finally tending to be stable. In the desorption stages of 2.5 MPa–2 MPa and 2 MPa–1.5 MPa, the gas desorption index decreased linearly with the increase in moisture. This study shows that added water can reduce the K₁ index, thereby effectively bringing the risk of a coal seam outburst down. There is a reasonably matching value between the water content and the index. In fact, the water injection parameters are not fixed and unchanging, which means that coal seams with diverse metamorphic degrees in different areas correspond to various ones, so water injection parameters should be determined by the attributes of the target coal seam.

4. Conclusions

The desorption experiments were carried out under stepwise depressurization and atmospheric pressure, respectively, and the influence of humidity on gas desorption characteristics was explored. The main conclusions are as follows.

(1)

Added water can replace some of the adsorbed gas in coal samples. When the humidity rises from 0% to 7.44%, the replacement level reaches 8.04 mL/g. However, the increment shows a downtrend at different pressure drop stages, decreasing from 3.53 mL/g to 1.77 mL/g.

(2)

Added water can reduce the amount of gas desorption, and the desorption decreases from 17.219 mL/g to 10.954 mL/g under experimental conditions. When the adsorption equilibrium pressure changes from 0.5 MPa to 2.0 MPa, a reduction in amplitude occurs from 46.1% to 36.2%, and the range of desorption decreases gradually.

(3)

In the first 10 min, the desorption rate is faster and shows an obvious decreasing trend; after 30 min of desorption, the rate at different pressure drop stages is consistent and gradually approaches 0 mL/(g min). In the first 30 min, the rate decelerates, while after 30 min, the desorption rate remains basically the same under different humidity levels.

(4)

Water injection can reduce the risk of outburst. During each pressure drop stage, the indicator K₁ shows a decreasing trend. When the water content increases from 0% to 7.44%, the decrease in K₁ changes from 66.3% to 46.1%.