Experimental Research on the Effect of Ultrasonic Waves on the Adsorption, Desorption, and Seepage Characteristics of Shale Gas.

Shale gas reservoirs are tight reservoirs with ultralow porosity and ultralow permeability, and their matrix pores are mostly nanoscale. In addition, matrix particles and organic pore surfaces adsorb shale gas. These problems cause the production per well of shale gas to be lower than that of conventional natural gas. The use of hydraulic fracturing technology to exploit shale gas can achieve a good production increase effect. However, using this technology has some limitations caused by technical characteristics and geological conditions. Therefore, new technologies for shale gas exploitation need to be explored. In this study, we propose a method to improve the flow characteristics of shale gas by using ultrasonic waves to increase shale gas production and perform experimental tests to research the actual effect of this method. The lithology, mineral composition, pore structure, specific surface area, and pore size distribution of shale samples are tested. Then, the attenuation characteristics of ultrasonic waves propagating in shale are analyzed. Finally, the effect of ultrasonic waves on the adsorption, desorption, and seepage of shale gas is explored. Results show that the Langmuir adsorption isotherm can describe the adsorption characteristics of shale gas under the action of ultrasonic waves. The gas adsorption constant decreases with increasing ultrasonic wave power. The ultrasonic waves accelerate the gas desorption rate, significantly increase the desorption volume, and prolong the time taken to reach desorption equilibrium. They also increase the permeability of shale gas, and the growth is proportional to the power of the ultrasonic waves. These results indicate that the permeability of shale gas has a power function relationship with the effective stress under ultrasonic waves.

Introduction

As a clean energy source, natural gas is considered a bridge to a low-carbon future. It can be used to transition from non-renewable energy sources, such as petroleum and coal, to renewable energy sources, such as solar, wind, and tidal energy. It can effectively reduce the generation of carbon dioxide and organic pollutants.[1−4] Natural gas is an environment-friendly energy source but not an inexhaustible energy source. With the continuous increase in global energy demand, conventional natural gas reserves have been unable to meet the rising energy consumption.[5−7] The emergence of shale gas is a rule changer in the current international energy market.[8−10] With the exploitation of unconventional gas reservoirs represented by shale gas, some countries no longer rely on imported energy. According to the data released by the U.S. Energy Information Administration, the U.S. has now shifted from a natural gas-importing country to an exporting country relying on shale gas exploitation.[11,12] As early as 2012, U.S. shale gas production accounted for 34% of the total natural gas production. The entire production of shale gas exceeds 6800 × 108 m3.[13,14] Shale gas production is expected to account for 46% of the total natural gas production in the U.S. by 2035.[15] The successful commercial exploitation of shale gas in the U.S. has aroused widespread concern for shale gas exploitation in various countries worldwide. In addition to North America, China, Poland, Germany, India, and Australia have also started work related to shale gas exploitation.[16−18]

Shale is a type of sedimentary rock containing many organic compounds. Shale gas is an unconventional natural gas generated by bacteria and geochemistry during the burial of organic compounds.[19−21] The matrix pores of shale gas reservoirs are mostly nanoscale, belonging to a tight reservoir with ultralow porosity and ultralow permeability.[22,23] The characteristics of shale gas reservoirs are also the main reason for the apparent difference between the storage form and seepage method of shale gas and conventional natural gas. Only a small percent of shale gas is present in kerogen, asphaltenes, liquid hydrocarbons, and formation water in a dissolved state.[24−26] Most shale gas is present in the reservoir in a free state and an adsorbed state.[27,28] Free gas mainly exists in microfractures, such as inorganic and organic pores, and adsorbed gas primarily adsorbs on the surface of matrix particles and organic pores.[29,30]Figure shows the actual shale gas exploitation situation in North America; the adsorbed gas content is generally 20–85%, and the free gas content is generally 15–80%.[31,32] Owing to the very stable nature of the adsorbed gas and the poor physical properties of shale reservoirs, the gas adsorbed in reservoir pores can only desorb at a relatively slow rate.

Figure 1

Statistics of free and adsorbed gas contents during shale gas exploitation in North America.

The seepage mechanism of shale gas is much more complicated than those of conventional natural gas reservoirs. The matrix pores and microfractures of shale gas reservoirs contain free gas and many adsorbed gases, as shown in Figure . The adsorbed gas and nanoscale pores complicate the seepage mechanism of shale gas. Scholars generally believed that the seepage process of shale gas has three stages:[33−35] (1) under the action of pressure difference, the shale gas is adsorbed on the surface of matrix pores and microfractures begin to desorb; (2) under the concentration difference effect, shale gas diffuses from matrix pores and microfractures to macropores or fractures; and (3) under the action of flow potential, shale gas seeps into the wellbore from macropores or fractures. The exploitation of shale gas reservoirs, given their particular occurrence form and complex seepage mechanism, usually needs to be supplemented by corresponding measures to increase production.

Figure 2

Schematic of shale gas flowing in the reservoir.

Hydraulic fracturing is a widely used and mature production increase measure for shale gas exploitation. Although this technology can bring a good production increase effect, many scholars have put forward some thought-provoking views on the drawbacks of applying it to shale gas exploitation. First, hydraulic fracturing increases production by increasing the number and extension of fluid passages between the formation and the wellbore. Microseisms inevitably occur during the operation and may also induce moderate earthquakes.[36,37] Tufts University scholars reported that most of the seismic activities caused by hydraulic fracturing are small in scale and will not affect human life. However, an injection depth greater than 1000 m may induce destructive earthquakes.[38] Data from the U.S. Geological Survey show that the strongest earthquake caused by hydraulic fracturing to date is the 5.8-magnitude earthquake in Oklahoma in 2016. Four earthquakes above 5.0 magnitude in the state are also related to hydraulic fracturing. Several surrounding states have also experienced more than 4.5–5.0 earthquakes induced by hydraulic fracturing.[39,40] Second, if the fractures produced by hydraulic fracturing extend to the water layer, part of the fracturing fluid fuses with the formation water. The water-based fracturing fluid used in hydraulic fracturing is composed of water, proppant, and chemical additives, and the proportion of chemical additives is between 0.01 and 0.05%.[41,42] The constituent components of chemical additives contain various toxic substances, such as carcinogens, benzene, xylene, toluene, and formaldehyde.[43,44] Although the proportion of toxic substances in a fracturing fluid is low, the total amount of toxic substances is not low because a large amount of fracturing fluid poses a severe threat to human health from drinking water. Third, the vast majority of shale gas exploitation areas are located in mountainous regions, and water resources are scarce,[45,46] which is not conducive to developing large-scale hydraulic fracturing operations and equipment deployment. Large-scale hydraulic fracturing operations for shale gas exploitation extend the construction period and increase the exploitation costs. From the above viewpoints, the development of a safe, environment-friendly, and easy-to-operate shale gas exploitation technology is in line with the current needs of shale gas exploitation.

Ultrasonic waves are high-frequency, high-energy mechanical waves that can produce mechanical vibration, cavitation, and thermal effects.[47,48] These three effects can trigger the expansion, compression, and vibration of microscopic molecules. Considering this feature of ultrasonic waves, scholars in the United States and the former Soviet Union have proposed using ultrasonic waves to increase the production of oil reservoirs. Ultrasonic waves have two main functions. One is to use the ultrasonic wave energy to change certain material states.[49,50] This type of ultrasonic wave is generally called “power ultrasound.” For example, ultrasonic cleaning, ultrasonic oil exploitation, and ultrasonic processing are power ultrasound applications in real life or production. The other is to use ultrasonic waves to collect information, especially that inside the material, such as ultrasonic flaw detection and ultrasonic diagnosis.[51−53] The first function of ultrasonic waves mainly increases oil reservoir production. Drawing lessons from the idea of using ultrasonic waves to increase production in oil reservoirs, ultrasonic waves can theoretically be applied to increase production in shale gas reservoirs. From the microscopic effect perspective, ultrasonic waves can increase shale gas production through mechanical vibration and thermal effects. The mechanism of mechanical vibrations is as follows. Ultrasonic waves that propagate in shale reservoirs cause small rock particles to vibrate. The particles on the microfracture surface of the reservoirs are not uniformly stressed because of movement, resulting in acceleration and loosening. The pore size of the microfractures gradually increases. Then, the mineral composition inside the bedrock is different, and the ultrasonic waves break the small-hardness particles because of vibration and form new fractures. Finally, high-frequency ultrasonic waves make shale gas molecules vibrate at high frequencies from the molecular movement perspective. The gas molecules adsorbed on the surface of matrix pores and microfractures are subjected to irregular forces and gradually remove interface constraints, producing free gas. The mechanism of thermal effects is as follows. Shale gas molecules absorb the high energy of high-frequency ultrasonic waves and then produce significant thermal effects, which increase the Brownian motion, kinetic energy, and vibration amplitude of shale gas molecules. Therefore, the adsorbed gas gradually departs from the matrix pores and microfracture surface to become free gas.

At present, most studies on the combination of ultrasonic waves and shale are focused on the propagation of ultrasonic waves in shale.[54−59] The primary purpose is to use the changing law of ultrasonic signals to determine the physical properties of shale reservoirs to provide a basis for exploitation operations rather than use ultrasonic waves to increase shale gas production. The present research aimed to verify the feasibility of using ultrasonic waves to increase shale gas production and to lay a theoretical foundation for the transformation of this idea into practical applications. Shale microstructure was analyzed in detail using a polarizing microscope, an X-ray diffractometer, an environmental scanning electron microscope, and a surface area and pore size analyzer. Then, the attenuation characteristics of ultrasonic waves when propagating in shale were tested and used as a basis for subsequent experimental research and theoretical analysis. Last, changes in the shale gas adsorption capacity, desorption capacity, and permeability of shale gas under the action of ultrasonic waves were determined. The feasibility of using ultrasonic waves to increase the production of shale gas was verified.

Shale Microstructure Analysis

Experimental Materials and Equipments

Shale samples were collected from the Weiyuan shale gas-producing field in Neijiang City, Sichuan Province, China, with a core depth of 2613–3208 m and a formation temperature distribution of 83.62–102.66 °C. The samples were processed by drilling the plunger sample perpendicular to the shale bedding direction. The maximum nonparallelism between the two ends of the shale sample did not exceed 0.05 mm. The end surface was perpendicular to the axis, and the maximum deviation did not exceed 0.25°. The basic physical parameters of the shale samples are listed in Table .

Table 1

Basic Physical Parameters of Shale Samples

The microstructure of the shale sample was observed using different equipment, which are listed in Table .

Table 2

Equipment Used in the Experiment

Experimental Methods

Lithology Test

WX-1–WX-8 shale samples were prepared into chip samples. The lithology of the samples was identified using the polarizing microscope model OLYMPUS BH-2 at a room temperature of 27 °C and a humidity of 35%.

Mineral Composition Test

Shale samples were prepared into a certain amount of powder. The mineral composition of the powder was identified using the X-ray diffractometer model X Pert PRO MPD at a room temperature of 27 °C and a humidity of 35%.

Pore Structure Test

The pore structure of the shale samples was observed using the scanning electron microscope model Quanta 450.

Specific Surface Area and Pore Size Distribution Test

The specific surface area and pore size distribution tests were performed using the adsorption method. At the temperature of liquid nitrogen, nitrogen was adsorbed on the sample surface by pure physical adsorption. When the temperature returned to room temperature, the adsorbed nitrogen was desorbed. The specific surface area and pore size distribution of the samples were calculated using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively. The main methods in the experiment are as follows:

(1) Degassing process. Shale samples were prepared into powders of 20–80 mesh. Before the adsorption experiment, the shale samples were degassed to remove the physical adsorption material on the adsorbent surface. The degassing treatment was carried out within the temperature and pressure ranges that do not affect the adsorbent properties. For shale, the degassing temperature is usually 80 °C. The degassing process was deemed completed when the residual gas pressure reached a stable value.

(2) Measurement of free space. During the test, the manifold was filled with helium until the pressure reached approximately 0.025 MPa. After the helium was in thermal equilibrium, the manifold equilibrium pressure P1 and temperature T1 were recorded. The sample tube was filled with helium in the manifold. When it reached a new equilibrium, the pressure P2 and temperature T2 were recorded. The above steps were repeated to measure the pressure Pi and temperature Ti. After the test, a vacuum process was used to remove the helium in the sample tube. On the basis of helium pressure and temperature data, the free space volume of the shale samples can be determined using eq where Vmaf is the volume of the manifold, m3; Vfs is the volume of the free space, m3; and Tstd is the standard temperature, 273.15 K.

(3) Determination of saturated vapor pressure. The saturated pressure tube was filled with nitrogen, and the phase state and pressure changes in the saturated pressure tube were observed at the same time. When the pressure reaches the saturated vapor pressure, nitrogen will condense in the saturated pressure tube, and the equilibrium pressure at this time is the saturated vapor pressure P0.

(4) Determination of the isotherm adsorption and desorption curves. In the experiment, nitrogen was filled into the sample tube gradually to increase the nitrogen pressure of the shale samples. The stable pressure P was recorded when the adsorption equilibrium was reached. The sample tube was continued to be filled with nitrogen according to a certain amount of gas, and the corresponding equilibrium pressure of each gas volume was recorded until the maximum equilibrium pressure was reached.

(5) The BET method was used to select data points with a relative pressure between 0.05 and 0.35 from the isotherm adsorption and desorption curves to calculate the specific surface area. The BJH method was used to calculate the pore size distribution.

Shale Microstructure

Lithology

As shown in Figure , the lithologies of the shale samples used in this study are as follows:

Figure 3

Lithological analysis of the shale samples.

WX-1 is argillaceous shale. The shale bedding of this sample is relatively developed and perpendicular to the primary stratification. Unfilled microfractures are distributed in the direction of the shale bedding.

WX-2 is silty shale. This sample contains mudstone and argillaceous fine-grained siltstone with a distinct interface in between. An unfilled microfracture can be found in the mudstone part.

WX-3 is grayish-black shale. This sample contains silt-bearing clay shale. The shale bedding is relatively developed, and it is perpendicular to the main bedding. Unfilled microfractures are distributed in the direction of the shale bedding.

WX-4 is argillaceous shale. This sample is distributed with a small amount of fine silt-like feldspar and quartz and contains a small number of pyrite pellets in a nest-like distribution.

WX-5 is argillaceous shale. This sample contains a small number of pyrite pellets in a nest-like distribution. A small amount of incomplete shale bedding is distributed in the direction of the primary stratification.

WX-6 is carbonaceous shale. This sample has many dry shrinkage fractures distributed in nearly vertical stratification. Unfilled microfractures are distributed in the direction of the stratification.

WX-7 is carbonaceous argillaceous shale. This sample contains carbonaceous and argillaceous materials that are unevenly mixed. Structural fractures are relatively developed, and a few serpentine structural dissolution fractures are filled mainly by calcite. Siliceous and carbonaceous materials fill a small amount.

WX-8 is carbonaceous argillaceous shale. This sample contains carbonaceous and argillaceous and contains a small amount of fine silt. Pyrite is distributed sporadically or in bands. This sample also contains a structural fracture filled with calcite and silica and several unfilled microfractures.

Mineral Composition

As shown in Table , the main mineral components of the shale samples used in the experiment are quartz (26.8–62.9%), plagioclase (6.2–11.2%), pyrite (1.4–3.0%), and clay minerals (21.7–63.1%). The contents of quartz and clay minerals are relatively large, whereas those of plagioclase and pyrite are relatively small. The content of other minerals is small or undetectable.

Table 3

Mineral Composition of the Shale Samplesa

	relative content of clay minerals (%)								quantitative analysis of whole rock (%)
sample no.	S	C	I	K	I/S	C/S	I/S %S	C/S %S	clay minerals	quartz	plagioclase	calcite	pyrite	proxene
WX-1	*	37	34	*	29	*	5	*	21.7	62.9	10.8	4.6	*	*
WX-2	*	31	46	*	23	*	5	*	63.1	26.8	8.4	*	1.7	*
WX-3	*	46	32	*	22	*	5	*	44.0	44.8	11.2	*	*	*
WX-4	*	36	41	*	23	*	5	*	49.3	31.2	10.6	*	2.4	6.5
WX-5	*	38	41	*	21	*	5	*	48.3	39.3	10.9	*	1.5	*
WX-6	*	35	38	*	27	*	5	*	57.0	29.5	10.5	*	3.0	*
WX-7	*	40	27	*	33	*	5	*	42.7	44.6	10.7	*	2.0	*
WX-8	*	41	38	*	21	*	5	*	50.5	37.1	8.5	1.8	2.1	*

S: smectite; C: chlorite; I: illite; K: kaolinite; I/S: illite/smectite; C/S: chlorite/smectite; %S: interlayer ratio; *: undetected.

Pore Structure

The scanning results in Figure show that the pore structures of the shale samples used in the experiment are mainly matrix pores and fracture pores.

Figure 4

Pore structure of the shale samples.

Specific Surface Area and Pore Size Distribution

The nitrogen adsorption test was performed on eight shale samples, and the obtained isotherm adsorption and desorption curves are shown in Figure . BET results reveal that the specific surface area of the shale sample is 11.636–30.751 m2/g, with an average value of 17.760 m2/g.

Figure 5

Isotherm adsorption and desorption curves for the shale samples.

The pore volume and pore size distribution of the shale samples were calculated using the BJH method, and the calculation results are shown in Figure . The pore volume of the shale samples is 0.006–0.0136 mL/g, the average value is 0.0094 mL/g, and the average pore size is 3.56–9.35 nm.

Figure 6

Calculation results for the pore size distribution.

Attenuation of Ultrasonic Waves in Shale

Experimental Equipment

Similar to audible sound waves, ultrasonic waves can only propagate in elastic media. They cannot propagate in a vacuum, and they also have the problem of energy attenuation during propagation.[60,61] Therefore, the characteristics of ultrasonic waves in shale must be determined first before using them to increase shale gas production.

Figure shows a self-designed and assembled ultrasonic attenuation test equipment, which mainly includes an ultrasonic generator, a piezoelectric ceramic transducer, a virtual oscilloscope, and a computer.

Figure 7

Self-designed and assembled ultrasonic attenuation test equipment.

In this system, the ultrasonic generator can send ultrasonic signals during experimental testing and adjust the frequency of ultrasonic waves. Piezoelectric ceramic transducers are mainly used to receive the transmitted and reflected ultrasonic signals and convert them into electrical signals. The virtual oscilloscope cooperates with the software on the computer to digitally process the electrical signal, thereby obtaining the changing law of the electrical signal.

Experimental Principle

The principle diagram of the ultrasonic attenuation test is shown in Figure .

Figure 8

Principle diagram of the ultrasonic attenuation test.

The 0 point in Figure is the location of the ultrasonic sound source. The ultrasonic generator sends out the ultrasonic waves, and the wave equation when propagating to the ultrasonic receiver is shown in eq When the ultrasonic receiver captures the ultrasonic waves and reflects them, the wave equation is shown in eq where A0 is the initial amplitude of the ultrasonic waves, m; R is the reflection coefficient, dimensionless; γ is the propagation coefficient of the ultrasonic waves, dimensionless; ω is the frequency of the ultrasonic waves, Hz; and t is the propagation time, s.

In the interval between the ultrasonic incident point and the receiving point, the incident wave and the reflected wave superimpose, and the wave equation of the composite wave generated after superposition is shown in eq Each point of the composite wave presents simple harmonic vibration, and its amplitude distribution can be calculated via eq where α is the attenuation coefficient of the ultrasonic wave, dimensionless.

If the ultrasonic receiver is used as the reflecting surface, the composite wave amplitude received by the ultrasonic receiver can be calculated via eq The ultrasonic generator and receiver are made of the same material; thuswhere U is the voltage value displayed by the virtual oscilloscope, mv; and U0 is the voltage value output by the ultrasonic generator, mv.

Assuming that the voltage value U is displayed by the virtual oscilloscope when the ultrasonic receiver is at any peak position x, the functional relationship between U and U0 is shown in eq The position coordinate x and voltage value U were measured at multiple peaks, and the results were recorded in the experiment table. The experimental data were fit to obtain the attenuation coefficient of ultrasonic α.

Reliability Testing of Equipment

The ultrasonic attenuation in the air was tested using the self-designed and assembled experimental equipment to verify its reliability. The frequency of the ultrasonic waves used in the experiment was 24 kHz, and the temperature was 27 °C. The peak voltage of the oscilloscope and the corresponding position of the piezoelectric ceramic transducer during the experiment were recorded. Figure shows the experimental results.

Figure 9

Test result of ultrasonic attenuation in air.

When the power of ultrasonic waves is 50 W, the attenuation coefficient in the air obtained by fitting is 0.0197 dB/m. When the ultrasonic power is 110 W, the value is 0.0318 dB/m. These results indicate that the ultrasonic attenuation coefficient is related to its power; the greater the power, the greater the attenuation coefficient of the ultrasonic waves. The average value of the two test results is 0.0258 dB/m. This data is consistent with the 0.02 dB/m used in most literature.[62−64] Thus, the equipment used in this experiment is reliable.

Attenuation of Ultrasonic Waves in Shale

The same method was used to test the attenuation characteristics of the ultrasonic waves in shale, and the test results are shown in Figure .

Figure 10

Test result of ultrasonic attenuation in shale.

When the power of the ultrasonic waves is 50 W, the attenuation coefficient in the shale obtained by fitting is 0.0764 dB/m. When the ultrasonic power is 110 W, the attenuation coefficient is 0.0807 dB/m. The average value of the two test results is 0.07 dB/m.

After obtaining the attenuation coefficient of ultrasonic waves, the effective propagation distance of the ultrasonic waves in different media is calculated using eq .where J0 is the initial sound intensity, dB/mm; and l is the propagation distance, mm.

The ultrasonic power was set to 50 W, the frequency to 24 kHz, the initial amplitude to 1 mv, and the initial sound intensity to 1 dB/mm. Table shows the attenuation degrees of the ultrasonic waves in air and shale calculated via eqs and 9.

Table 4

Attenuation Degrees of the Ultrasonic Waves in Air and Shale

	attenuate in the air			attenuate in the shale
distance (mm)	amplitude (mv)	intensity (dB/mm)	remarks	amplitude (mv)	intensity (dB/mm)	remarks
5	0.995	0.990		0.977	0.954
10	0.990	0.980		0.954	0.910
20	0.980	0.961		0.910	0.828
40	0.961	0.923		0.828	0.686
80	0.923	0.852		0.685	0.470	intensity attenuated by half
150	0.861	0.741		0.492	0.243	amplitude attenuated by half
300	0.819	0.670		0.389	0.151
450	0.741	0.549	intensity attenuated by half	0.242	0.059
600	0.670	0.449	amplitude attenuated by half	0.151	0.023
750	0.606	0.368		0.094	0.009
900	0.549	0.301		0.059	0.003
1050	0.496	0.246		0.037	0.001
1200	0.449	0.202		0.023	0.001

The calculation results in Table show that when the ultrasonic waves propagate 1.2 m in the shale, their amplitude attenuates to 2.3% of the initial value. The sound intensity attenuates to 20.2% of the initial value. The calculation results show that the effective propagation distance of ultrasonic waves with a frequency of 24 kHz and a power of 50 W in shale is about 1.2 m.

Effect of Ultrasonic Waves on Shale Gas

To prove that ultrasonic waves can increase shale gas production, we designed and assembled the experimental equipment by ourselves to test and compare the adsorption, desorption, and seepage characteristics of shale gas under different ultrasonic powers. Figure shows the experimental equipment.

Figure 11

Shale gas adsorption, desorption, and seepage test equipment under the action of ultrasonic waves.

The shale gas adsorption capacity measurement was performed using the volume method. The basic principle is that the gas in the shale reservoir is in free, adsorbed, and dissolved states, and the contents of the free and dissolved states are relatively small. Thus, the adsorbed state is mainly considered. The Langmuir equation and the gas state equation are used to calculate the gas adsorption capacity and Langmuir parameters. Equations –12 show the calculation equationswhere na is the adsorption after adsorption equilibrium, mol; nk is the total gas intake, mol; and np is the free gas volume after adsorption equilibrium, mol.where Pk is the pressure in the sample cylinder after the gas is injected, MPa; Pp is the pressure in the sample cylinder after adsorption equilibrium, MPa; Zk and Zp are the gas deviation coefficients, respectively, dimensionless; R is the gas constant, dimensionless; T is the temperature, K; and Vs is the volume of the sample cylinder after placing the sample, cm3.

The gas permeability is calculated by the conventional steady-state method using eq where Q is the gas seepage flow, cm3/s; Pa is the atmospheric pressure, MPa; L is the sample length, cm; P1 is the pressure at the gas inlet, MPa; P2 is the pressure at the outlet, MPa; A is the cross-sectional area of the sample, cm2; and μ is the viscosity coefficient of gas, Pa·s.

Shale Gas Adsorption Test

Relying on the above-mentioned experimental equipment, the following steps are used to test the adsorption characteristics of shale gas under the action of ultrasonic waves:

Place the shale sample to be tested in a vacuum drying oven at 80 °C for 8 h to dehydrate the shale sample completely.

Place the dehydrated shale sample in the adsorption cylinder, use the sample sealing sleeve to seal the adsorption cylinder, and place it in the pressure vessel.

Load the confining pressure, which should be 1.5–2 MPa higher than the adsorption pressure.

Degas the shale sample and measure the free space V1 of the adsorption cylinder.

Continue to degas for 2 h.

After complete degassing, apply a certain pressure of methane gas to the adsorption cylinder.

After reaching the adsorption equilibrium, measure the water’s volume falling in the burette V2; then V1–V2 is the adsorption volume of shale gas.

After the adsorption test without ultrasonic effect is over, fill in high-pressure methane and immediately turn on the ultrasonic generator.

Measure the amount of adsorption after reaching the adsorption equilibrium.

Shale Gas Desorption Test

Compared with the adsorption characteristics test, the desorption characteristics test of shale gas is relatively straightforward:

After reaching the adsorption equilibrium, open the high-pressure gas valve and connect the adsorption cylinder to the atmosphere for 20–30 s.

After exhausting all of the gas in the free space of the adsorption cylinder, close the high-pressure gas valve and start the desorption test.

Record the volume of water discharged in the graduated cylinder within a certain period and obtain the relationship curve between the desorption volume and time.

After the desorption test without ultrasonic effect, repeat the first two steps in the desorption test and readsorb the gas. When starting the desorption test, turn on the ultrasonic generator immediately and record the amount of desorption and time under the ultrasonic effect.

Shale Gas Seepage Test

The following steps are followed to test the seepage characteristics of shale gas under ultrasonic waves:

Seal the shale sample with a sample sealing sleeve, press both ends with pressure plates, and put it into a pressure vessel.

After installation, apply a certain confining pressure.

Turn on the vacuum pump for degassing for 1 h.

After the degassing is completed, close the return valve, open the intake valve, and pass in high-pressure methane gas for adsorption for 1 h.

Open the return valve and use the drainage method to measure the flow rate.

After the bubbles are continuously and stably discharged, record the volume and time of the drainage and measure three times under each condition to obtain the average value.

After the gas flow without the ultrasonic effect is stable, turn on the ultrasonic generator and measure the drainage volume and time under the ultrasonic effect.

Results and Discussion

Shale Gas Adsorption

Shale samples WX-1 and WX-2 were selected, and their adsorption characteristics were tested without and with the action of ultrasonic waves in accordance with the test method mentioned above. Figure shows the experimental test results of shale gas adsorption characteristics under the action of ultrasonic waves and the results of fitting using the Langmuir equation.

Figure 12

Experimental and fitting results of shale gas adsorption under ultrasonic waves.

Results show that the greater the ultrasonic power under the same pressure conditions, the smaller the adsorption capacity of shale gas. The fitting results of the experimental data using the Langmuir equation are excellent, indicating that the Langmuir isotherm adsorption curve can also describe the adsorption characteristics of shale gas under the action of ultrasonic waves.

Table shows the correlation coefficient when the Langmuir equation is used to fit the experimental data. As the ultrasonic power increases, the adsorption constant of the Langmuir equation shows a decreasing trend. The law mentioned above shows that the adsorption characteristics of shale gas are reduced under the action of ultrasonic waves.

Table 5

Adsorption Constant and Pressure Constant in Langmuir Equation under Ultrasonic Waves

sample no.	ultrasonic power (W)	adsorption constant a (cm3/g)	pressure constant b (1/MPa)	correlation coefficient R2
WX-1	0	3.915	0.265	0.998
	50	3.328	0.209	0.997
	110	2.683	0.212	0.999
WX-2	0	4.831	0.589	0.997
	50	4.053	0.661	0.996
	110	3.812	0.499	0.995

Shale Gas Desorption

The desorption characteristics of shale samples WX-1 and WX-2 were further analyzed without and with the action of ultrasonic waves. Figure shows the experimental test results of shale gas desorption characteristics under the action of ultrasonic waves.

Figure 13

Experimental results of shale gas desorption under ultrasonic waves.

With the increase in ultrasonic power, the shale gas desorption volume and desorption rate become more apparent. Under the action of ultrasonic waves, shale gas takes longer to reach the desorption equilibrium. Under the pressure of 2 MPa, the desorption volumes of shale gas under the action of 50 and 110 W ultrasonic waves reach 24 and 31%, respectively, indicating that ultrasonic waves can indeed improve the desorption effect of shale gas.

Shale Gas Seepage

Shale samples WX-3, WX-4, and WX-5 were selected, and their seepage characteristics without and with the action of ultrasonic waves were analyzed as described above. Figure shows the test results of the seepage characteristics of shale gas under ultrasonic waves.

Figure 14

Experimental results of shale gas seepage under ultrasonic waves.

Experimental data show that the permeability of shale gas is higher with the action of ultrasonic waves than without ultrasonic waves. As the ultrasonic power increases, the permeability also gradually increases, indicating that ultrasonic waves can improve the seepage characteristics of shale gas.

By fitting and analyzing the experimental data, we found that the permeability and effective stress of shale gas satisfy the power function relationship with or without ultrasonic waves. Table shows the fitting relationship between shale gas permeability and effective stress under ultrasonic waves. Under the same pore pressure conditions, the fitting parameters under the action of ultrasonic waves are more significant than those under the action of no ultrasonic waves. With the increase in ultrasonic power, the magnitude of increase of the fitting parameters also increases.

Table 6

Fitting Relationship between Shale Gas Permeability and Effective Stress under Ultrasonic Waves

sample no.	ultrasonic power (W)	fitting eq (Pp = 2.0 MPa)	correlation coefficient	fitting eq (Pp = 3.5 MPa)	correlation coefficient
WX-3	0	k = 0.02229σe–0.98026	0.995	k = 0.00715σe–0.66426	0.997
	50	k = 0.02437σe–0.9911	0.994	k = 0.00771σe–0.65258	0.998
	110	k = 0.02529σe–0.95651	0.993	k = 0.00995σe–0.73144	0.998
WX-4	0	k = 0.01808σe–0.69541	0.981	k = 0.00983σe–0.56223	0.998
	50	k = 0.01972σe–0.70045	0.991	k = 0.01124σe–0.58905	0.997
	110	k = 0.02122σe-0.68538	0.979	k = 0.01197σe–0.5632	0.995
WX-5	0	k = 0.01645σe–1.10139	0.984	k = 0.00568σe-0.74939	0.974
	50	k = 0.01804σe–1.01399	0.988	k = 0.00622σe–0.74939	0.980
	110	k = 0.01958σe–1.03399	0.988	k = 0.00675σe–0.74956	0.983

Results Analysis

Experimental results show that under the action of ultrasonic waves, the adsorption, desorption, and seepage characteristics of shale gas can be improved. These results are mainly due to the thermal effect and mechanical vibration effect of ultrasonic waves.

The thermal effect of ultrasonic waves can promote the desorption of methane and reduce the methane adsorption capacity of shale. When the temperature rises, the thermal movement of methane molecules accelerates and the activity increases, making it more difficult to be adsorbed by shale. At the same time, the adsorbed methane molecules gain kinetic energy under the action of rising temperature, their Brownian motion intensifies, and the energy of desorption increases such that the original adsorption equilibrium state develops to the desorption state. When a new equilibrium is reached, the number of methane molecules adsorbed on the shale surface will be less than the number of molecules previously adsorbed. Conversely, the ultrasonic wave is a type of energy radiation. When ultrasonic waves pass through methane-containing shale, the internal friction between shale particles caused by the viscosity of the shale absorbs a certain amount of ultrasonic energy. This part of the absorbed ultrasonic energy converts into heat energy, which increases the local temperature of the shale, which is conducive to reducing the amount of methane gas adsorbed by the shale, thereby promoting the desorption of methane.

Ultrasonic waves include two forms: longitudinal wave and transverse wave. Under the action of longitudinal waves, the propagation direction of the wave is consistent with the vibration direction of the shale particles, which can subject the elastic medium to tensile and compressive stresses, thereby causing the shale to produce alternating elongation and compression deformations. Under the action of transverse waves, the propagation direction of the waves is perpendicular to the vibration direction of the shale particles. When the shale is subjected to alternating shear forces, alternating shear deformations occur. Under the mechanical vibration effect of ultrasonic waves, shale is prone to breakage, creating new fracture networks, increasing the pores of shale and effectively increasing the permeability of shale gas.

Conclusions

We used experimental testing to study the microstructure of shale, the attenuation characteristics of ultrasonic waves in shale, and the effect of ultrasonic waves on the adsorption, desorption, and seepage of shale gas. Among them, the third research content is the core content of this paper. The first two research contents are the theoretical foundation for the smooth development of the third research work and a clear understanding of the research results. After completing a large number of experimental tests, our conclusions are as follows:

The Langmuir adsorption isotherm can describe the adsorption characteristics of shale gas under the action of ultrasonic waves. The gas adsorption constant in the Langmuir equation decreases as the power of ultrasonic waves is increased, indicating that the adsorption volume of shale gas is reduced under the action of ultrasonic waves.

Under the action of ultrasonic waves, the desorption volume and rate of shale gas become more apparent with the increase in ultrasonic power. The time to reach desorption equilibrium is extended, suggesting that ultrasonic waves can promote the desorption of shale gas.

Ultrasonic waves can increase the permeability of shale gas. The growth is proportional to the power of ultrasonic waves, indicating that ultrasonic waves can promote shale gas seepage.

The permeability and effective stress of shale gas satisfy the power function relationship with or without ultrasonic waves. Under the same pore pressure conditions, the fitting parameters under the action of ultrasonic waves are more significant than those without the action of ultrasonic waves.