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Experimental study on gas desorption characteristics for different coal particle sizes and adsorption pressures under the action of pressured water and superheated steam
Journal of Petroleum Science and Engineering 179 (2019) 948–957
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Experimental study on gas desorption characteristics for different coal particle sizes and adsorption pressures under the action of pressured water and superheated steam
T
Dong Zhaoa,b,c,∗, Chao Zhanga,b,c, Hao Chena,b,c, Zengchao Fengb,∗∗ a
Department of Safety Science and Engineering, College of Safety and Emergency Management Engineering, Taiyuan University of Technology, Taiyuan, PR China Key-Laboratory of In-situ Property-improving Mining of Ministry of Education, Taiyuan University of Technology, Taiyuan, PR China c Graduate Student Education and Innovation Center in Coal Mine Safety of Shanxi Province, Taiyuan University of Technology, Taiyuan, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Adsorption and desorption Coal particle size Pressured water Superheated steam
Gas (i.e. methane and CO2) ad-/desorption characteristics are essential for unconventional gas production, and they are related to many factors such as coal particle size, adsorption pressure, external moisture and heat. In order to investigate the effect of these factors, as well as those arising from pressured water and superheated steam on gas desorption, a series of experiments were conducted. The experimental equipment designed by our research group could be used to supply pressured water and superheated steam during the gas ad-/desorption process. Our results were: 1) High-pressure water injection after adsorption resulted in a reduced gas desorption capacity. With an increase in the equilibrium adsorption pressure and water injection pressure, the desorption capacity of coal increased and decreased, respectively. 2) The desorption capacity of the residual adsorbed gas in coal increased with steam action, and it eliminated the influence of low gas adsorption pressure and highpressure water injection on gas desorption. 3) The gas desorption velocity increased with steam action, which was reflected by a decrease in the critical value of time effect. The coal particle size had a certain influence on the desorption of the adsorbed gas. Finally, with an increase in the water injection pressure, the influence of coal particle size became larger. However, the desorption under steam was less affected by the coal particle size. These results may have significant implications for energy exploitation and utilization.
1. Introduction Coalbed methane (CBM) is self-generating and self-storing, if the unconventional, natural gas that is mainly deposited in coal seams is in an adsorbed state. It can be used as a source of energy and raw chemical materials for human use. At the same time, coalbed methane can be used to achieve economic and social benefits in terms of reducing the amount of coal mine gas disasters, improving coal mine safety production, and providing for environmental protection (Zhou et al., 2016). Through the influence of the two-phase gas-water permeability of different coal ranks, and the influence of wettability and overburden pressure on its relative permeability, fissure-based seepage can be obtained (Shi and Durucan, 2005; Durucan et al., 2013). The relative permeability curve exhibits a linear, narrow feature with pore-based percolation, and the curve is nonlinear and has a wide saturation range
(Rahmanian et al., 2010). At the same time, the ratio of the effective coalbed methane relative permeability to absolute permeability increases with coal rank, meaning that in high-rank coals, the gas-flow effect is more pronounced than for low-rank coals (Shen et al., 2011). A new dynamic model can accurately describe the combined effects of coal matrix shrinkage effects, effective stress effects and the Klingenberg effect of coalbed methane reservoir gas. In a two-phase flow experiment, gas breakthrough experiments were carried out by applying different gas pressures to the saturated coal samples, and pressure changes were observed, where the flows of different types of gases through the saturated coal matrix are obtained (Han et al., 2010; Zhao et al., 2014). The gas–water mutual displacement process was observed using NMRI (Nuclear Magnetic Resonance Imaging) technology, and the inconsistency and dominant displacement path of the water-flood front were obtained (Pan et al., 2008). The above study was confirmed by simulation, and that the CBM mining process in the formation is a
∗
Corresponding author. Department of Safety Science and Engineering, College of Safety and Emergency Management Engineering, Taiyuan University of Technology, Taiyuan, PR China. ∗∗ Corresponding author. Key-Laboratory of In-situ Property-improving Mining of Ministry of Education, Taiyuan University of Technology, Taiyuan, PR China. E-mail addresses: [email protected] (D. Zhao), [email protected] (Z. Feng). https://doi.org/10.1016/j.petrol.2019.05.027 Received 12 February 2019; Received in revised form 19 April 2019; Accepted 7 May 2019 Available online 08 May 2019 0920-4105/ © 2019 Elsevier B.V. All rights reserved.
Journal of Petroleum Science and Engineering 179 (2019) 948–957
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complex multiphase fluid flow interaction process. Gas desorption from coal in various conditions is affected by numerous factors including gas content from underground boreholes, coalbed permeability, initial gas desorption properties, and coal strength (Tang et al., 2015; Zhao et al., 2017; Li et al., 2017; Zou et al., 2017). Temperature is an important factor that affects the desorption and transport of adsorbed gases, where many studies have shown that increasing the temperature can effectively promote the desorption velocity and desorption capacity of adsorbed gases in coal (Yang et al., 2008; Zhao et al., 2011, 2012, 2018a, 2018b). Recently, thermal injection has been used as a new technology in gas exploitation which has been successfully applied to enhance coalbed methane recovery. They are used in shale gas, CBM, numerical simulations and so on (Wang et al., 2015; Song et al., 2015; Lin et al., 2015; Shahtalebi et al., 2016). At the same time, water has a large specific heat capacity, and per unit mass it can carry more heat; hence gaseous water in the form of steam is an ideal working medium. The structure and distribution are the most important factors for the adsorption and desorption of gas in coal. Some elegant methods including nuclear magnetic resonance, scanning electron microscopy and X-ray nano computed tomography were used in the study of pore structure. They revealed that the contents of adsorption gas and free gas in the multiscale pore structure increase with the increase of gas pressure (Huang and Zhao, 2017). When using hydraulic or gaseous fracturing technology, the coal mass might change to different particle sizes, where each coal block might have different gas desorption features. In this paper, water vapor was used as a heating medium, and the interaction between water and gas in coal was considered. The adsorbed coal after being subjected to high-pressure water was taken as the research object to study the enhanced desorption characteristics of adsorbed gas under steam. We used different sized coal samples in this study, where the five different coal particle sizes had different adsorption and desorption characteristics under external effects. In real CBM wells, there are different sizes of coal blocks, and they display different desorption features for gas exploitation. Using an experimental model, where we considered different sized coal samples, the gas desorption characteristics for different coal particle sizes, and adsorption pressures under the action of pressured water and superheated steam, were revealed.
injection device, a drying box, a vacuum pump, a weighing device with an accuracy of 0.01 g, and a gas collecting device. The thermostatic control cabinet allowed the experiment to be carried out at a set temperature, which minimized the effects of temperature fluctuations. The experimental system is shown in Fig. 2. The parameters of main parts are as follows: (1) Adsorption tank and reference tank: The upper limit of pressure that the adsorption tank and reference tank could reach 30 MPa, which met the standard of high-pressure test containers. (2) Constant temperature system: The temperature control cabinet had a precision greater than ± 0.5 °C. (3) Gas content and pressure measurement system: A pressure gauge with an accuracy of 0.001 MPa was used. Volumetric method was used to calculate gas ad-/desorption content of coal, the pressure drop during gas injection was adopted to calculate gas content in adsorption tank. The pressure of adsorption tank was used to calculate free gas in it, and the other is adsorption gas. (4) Vacuum system: The vacuum was 8 Pa and was set using a vacuum pump that met those requirements. (5) Gas collection system: It is used to measure the content of gas desorption. The desorbed gas was collected using a drainage method, in real time, by using gas to drive the water into a closed container. (6) Superheated steam-generation system: The steam-generation system was composed of an automatic metering water injection pump and a three-stage heating chamber, which guaranteed an output steam temperature of ∼150 °C (Zhao et al., 2016). We used a constant steam speed of 4 g/min in this study, which was controlled using an automatic metering water injection pump. 2.2. Experimental procedures After vacuuming and loading the sample, gas was introduced into the device to start the adsorption process until the process balanced. The experimental process was then divided into three stages. The first experiment was a conventional adsorption/desorption experiment, where the air tightness of the experimental system was determined. The system was at the constant temperature of 25 °C. Next, the gas pressures were in four different stages, they are 0.5 MPa, 0.75 MPa, 1.0 MPa and 1.25 MPa. Using the gas-storage equipment, the sample coal was adsorbed at a given gas pressure for about 24 h until equilibrium was reached. Changes in pressure that occurred during the process were recorded by a digital pressure gauge, and equilibrium was reached when the rate of change in the pressure was < 0.002 MPa/h. During the desorption process, after reaching equilibrium, the gas collection system was connected, and the desorption process was carried out under atmospheric pressure. The desorption process lasted for approximately 24 h until equilibrium was reached (assumed to be when the desorption velocity was < 9.6 × 10−2 mL/(g × s)). We used the drainage method described in Section 2.1 to measure the desorption capacity in real time. The second experiment used steam-injected adsorption–desorption, where the adsorption process was the same as that in the first stage. After adsorption equilibrium was reached, the steam-injection system was switched on, and desorption of the sample started under atmospheric pressure until the desorption reached equilibrium. The temperature of superheated steam was 150 °C and the pressure of it was in atmospheric pressure. The third stage of the experimental process used adsorption, water injection, conventional desorption, and then steam desorption. In the water-injection experiment, after the sample had adsorbed and reached equilibrium, the water-injection system was turned on, and water was injected from the bottom of the adsorption tank at a set water pressure until the top water outlet was turned on. Then, the water injection valve was closed and the pressure was regulated for 24 h. The pressure was
2. Materials and method 2.1. Sample preparation and experimental system Five different coal samples particle sizes (Φ50 mm × 100 mm (50 mm in diameter and 100 mm in height), 11 mm, 6 mm, 3 mm, and 1 mm, respectively) were prepared for the experiments, which were taken from Sanyuan Zhongneng Coal Mine Co., Ltd in Shanxi Province, China. The coal rank was lean coal. After each piece of raw sample coal was transported from the first-line coal production area, it was immediately sealed with wax and then taken back to the lab. Since the raw material did not meet the requirements of our experiments, each piece was sanded and trimmed so as not to damage its structural features. The mass of the Φ50 mm × 100 mm columnar coal sample and their surfaces were smooth, and for the test 256.42 g was used, while the other four coal samples sizes (11 mm, 6 mm, 3 mm, and 1 mm) were used at 100 g each (the sample size error was less than ± 0.5 mm), as shown in Fig. 1. The adsorbed gas used in this study was CO2, which had a purity of 99.9%. We used CO2 instead of methane, because CO2 has more distinct features than methane in terms of adsorption and desorption. Methane and CO2 both display similar and important gaseous geology in real coal seams. For example, to prevent coal and gas outburst disasters in coal mines, both methane and CO2 are outburst gaseous medium found in inner coalbeds. The test system included an adsorption tank, a high-precision pressure gauge, a constant-temperature control cabinet, a water949
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Fig. 1. Coal samples with different particle sizes.
decrease in the size of the sample. The desorption percentage of the 1 mm sample at 1440 min was ∼3% greater than that of the 50 mm sample, but at 360 min, the former was ∼12% larger than the latter, indicating the effect of coal particle size on the desorption velocity. The smaller the coal sample size was, the shorter the time needed for the desorption to reach equilibrium. However, in general, the particle size had little effect on the final desorption percentage. Fig. 5 shows the final adsorption capacity as a function of gas equilibrium adsorption pressure for different coal particle sizes. It can be seen that the final adsorption capacity gradually increased with an increase in the equilibrium adsorption pressure, and the velocity of change gradually decreased until it became stable.
documented with a recording device. The conventional desorption procedure was the same as described in the first stage, but the only difference was that the desorption process occurred after the injection of high-pressure water. During the subsequent steam-injected desorption process, the desorption equilibrium was determined in the same manner as the first stage. 3. Experimental results 3.1. Gas adsorption-desorption characteristics under natural conditions As shown in Fig. 3, and taking a pressure of 1.0 MPa as an example, the adsorption amount of the different sized coal samples was similar with time (Φ50 mm × 100 mm was marked as 50 mm). The initial adsorption velocity was initially rapid, and then gradually decreased, and once it was close to reaching equilibrium, the quasi-equilibrium state lasted for a long time. The gas adsorption in coal was a simple gasphase flow process, which was related to the size of the adsorbate. This implied that the gas adsorption time to equilibrium decreased with a decrease in the coal sample size, and the gas adsorption capacity gradually increased at the same time. This was because, as the size became smaller, more adsorbate molecules were able to diffuse through the coal. The decrease in the distance used for adsorption was reflected in the adsorption time, where the smaller the size, the shorter the time taken for adsorption. The final desorption percentage was calculated using equation (1):
ηmax = VD/ VA
3.2. Enhanced desorption characteristics of adsorbed gases under steam In the state of natural desorption, gas desorption under atmospheric pressure, the desorption percentage of coal samples with different particle sizes was compared with that of coal samples with different equilibrium adsorption pressures, as shown in Fig. 6. In order to compare the effect of natural desorption and steam desorption more directly, a surface diagram is shown in Fig. 7. As shown in Fig. 6, for coal samples of the same particle size, the desorption percentage increased with an increase in the equilibrium adsorption pressure; while under the same equilibrium adsorption pressure, the desorption percentage did not change with varying coal particle size. When the equilibrium adsorption pressure was 1.25 MPa, the desorption percentage during the natural state reached 70%; however, when the equilibrium adsorption pressure was 0.5 MPa, the desorption percentage during the natural state was only about 35%, about half of that at the 1.25 MPa state. This may be because gases under high equilibrium adsorption pressures have greater internal energies and are more likely to diffuse and desorb. As shown in Fig. 7, the desorption percentage of the coal samples
(1)
where, VA is the adsorption capacity in units of mL/g; VD is the desorption capacity in units of mL/g; and ηmax is the final desorption percentage. As shown in Fig. 4, the desorption velocity [the desorption capacity per unit time, in units of mL/(g × min)] increased slightly with a 950
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Fig. 2. The experimental system.
3.3. Desorption characteristics during the natural state after being subjected to high-pressure water
increased significantly after steam action, steam is water vapor in superheated state, where the desorption percentages increased by more than 80%. For coal samples with an equilibrium adsorption pressure of 0.5 MPa, the desorption percentages increased to twice their initial values. At 1.25 MPa, the desorption percentages increased by about 15%. These results showed that steam significantly increased the desorption energy of the adsorbed gases, and promoted the desorption of residually adsorbed gases under low-gas pressures. The results of the high gas pressure experiment showed that the enhanced desorption effect under steam was limited because of the large initial internal energy of the gas. Therefore, steam significantly improved the desorption capacity of residually adsorbed gases in coal. For coal reservoirs with low adsorption pressures, the strengthening effect was more obvious.
Based on the gas equilibrium adsorption pressure, we selected three different water injection pressures: (1) isobaric water injection, where the water injection pressure was equal to the gas equilibrium adsorption pressure; (2) triple water injection, where the water injection pressure was three times larger than the gas equilibrium adsorption pressure; (3) nine-fold water injection, where the water injection pressure was nine times larger than the gas equilibrium adsorption pressure. The purpose of this experiment was to study the effects of different coal particle sizes, different equilibrium adsorption pressures, and adsorption gases with different desorption characteristics after being subjected to a stage of sustained water pressure. Fig. 8A–C shows bar graphs of the gas desorption characteristics of isobaric water injection, triple water injection, and nine-fold water injection, respectively, as a function of different coal particle sizes and
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Fig. 3. Adsorption capacity as a function of time for different coal particle sizes at 1.0 MPa gas equilibrium adsorption pressure.
compared with the natural state, according to different particle sizes and adsorption pressures. The pressure was maintained at equilibrium, and surface diagrams are shown in Fig. 9A–C. As shown in Fig. 9, after steam injection, the residual adsorptive gas after high pressure water was obviously desorbed, and the desorption percentage increased by about 80%. At an equilibrium adsorption pressure of 0.5 MPa, the desorption percentages of isobaric water injection, triple water injection, and nine-fold water injection increased from 18%–25%, 14%–22%, and 11%–15% to 79%–80%, 78%–79%, and 77%–78%, respectively. At an equilibrium adsorption pressure of 1.25 MPa, their respective desorption percentages increased from 54%–62%, 45%–54%, and 27%–33% to 83%−84%, 81%–83%, and 80%–81%, respectively. The results of our study confirmed that the desorption percentage after steam strengthening was less affected by reservoir pressure, and the desorption percentage at 1.25 MPa reservoir pressure after steam was only 1.04 times that at 0.5 MPa. It could be concluded that for a low-pressure reservoir coal seam, the coalbed gas mining efficiency of conventional mining method was relatively low and that the mining efficiency caused by the combined two-phase gas–water flow was extremely low, while the use of steam significantly improved coalbed gas mining efficiency.
equilibrium adsorption pressures. It can be seen that the influence of water injection on the desorption capacity of the coal samples with different particle sizes was different, where the desorption percentage decreased with an increase in the coal particle size. For different equilibrium adsorption pressures, water injection had a different influence on the desorption properties of the coal samples. The gas desorption capacity arising after a lower adsorption pressure was much more affected by water injection than that arising after higher adsorption pressure. When the adsorption equilibrium pressure was 0.5 MPa, the desorption percentages of isobaric water injection, triple water injection, and nine-fold water injection were between 18%–25%, 14%–22%, and 11%–15%, respectively. When the adsorption equilibrium pressure was 1.25 MPa, their respective desorption percentages were between 54%–62%, 45%–54%, and 27%–33%, respectively. With an increase of water injection pressure, the gas desorption percentage at the same particle size and equilibrium adsorption pressure gradually decreased. As shown in the bar graphs from Fig. 8A–C, the larger the water injection pressure was, the more obvious its effect on the desorption. 3.4. The desorption characteristics of steam enhanced by high pressure water As described in Section 3.3, the desorption percentage of gas after being treated with high-pressure water was greatly reduced. In order to explain the effect of steam-enhanced desorption, steam was injected into the adsorbed sample under the action of high-pressure water, and
Fig. 4. Desorption percentage as a function of time for different coal particle sizes at 1.0 MPa gas equilibrium adsorption pressure. 952
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Fig. 5. Final adsorption capacity as a function of gas equilibrium adsorption pressure for different coal particle sizes.
Fig. 6. Final desorption percentage as a function of gas equilibrium adsorption pressure for different coal particle sizes at the natural desorption stage.
Fig. 7. Surface diagrams of desorption percentage as a function of gas equilibrium adsorption pressure for different coal particle sizes in comparison with the natural desorption (lower panel) and steam enhanced stages (upper panel).
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Fig. 8. Desorption percentage as a function of gas equilibrium adsorption pressure for different coal particle sizes after water injection at the natural desorption stage.
4. Discussion
the adsorption of coal after applied to coal and gas desorption gas internal migration, based on the thermal extraction is a kind of effective method in low permeability CBM exploration. However, when using superheated steam or water as medium of heat injection, desorption and migration of coalbed methane are influenced by the steam or water. After water is injected into a coal seam, the water-saturation degree
4.1. Mechanism of inhibited water injection desorption and steam-enhanced desorption Steam forms by a certain quantity of heat, and can be increased, and 954
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Fig. 9. Surface diagrams of the desorption percentage as a function of gas equilibrium adsorption pressure for different coal particle sizes after water injection, in comparison with natural desorption (lower panel) and steam-enhanced desorption (upper panel).
gas molecules. There are a large number of pores in coal, which are connected via fissures, some of them are affected by extra fluid flow. As coal physically adsorbs gas and water molecules, after the coal is injected with water, water molecules are absorbed into the pores. Because water molecules block the gas-seepage channels, there is only a certain
increases. From a microscopic point of view, the coal absorbs water because the surface of the adsorbent attracts water molecules, which leads to multi-layered adsorption. The way in which the adsorbent adsorbs the coalbed methane is also a physical adsorption process, which is the result of the pores of the adsorbent and the pores attracting 955
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4.2. Analysis of the time effect of steam-strengthening desorption
Table 1 Time effect on the natural desorption stage after water injection for the coal sample particle size of Φ50 mm × 100 mm. Sample particle size
Desorption condition
Φ50 mm × 100 mm
Natural Isobaric water injection Triple water injection Nine-fold water injection
The time effect of desorption is crucial to the efficiency of coalbed methane production, and can directly reflect the level of mining efficiency and operating time. According to Airey's desorption characteristics of broken coal with time, the correlation between the desorption percentage and time is summarized by equation (2) (Airey, 1968):
R2
Simulation results
t 0.9433 ⎫ ⎤ 569 ⎦⎬
0.9928
t 0.8831 ⎫ ⎤ 822 ⎦⎬
0.9898
t 0.892 ⎫ ⎤ 880 ⎦⎬
0.9914
t 0.902 ⎫ ⎤ 949 ⎦⎬
0.9917
η = ηmax ⎧1 − exp ⎡− ⎨ ⎣ ⎩
( )
η = ηmax ⎧1 − exp ⎡− ⎨ ⎣ ⎩
( )
η = ηmax ⎧1 − exp ⎡− ⎨ ⎣ ⎩
( )
η = ηmax ⎧1 − exp ⎡− ⎨ ⎣ ⎩
( )
⎭ n
t η = ηmax ⎧1 − exp ⎡−⎛ ⎞ ⎤ ⎫ ⎢ ⎝ t0 ⎠ ⎥ ⎬ ⎨ ⎦⎭ ⎣ ⎩
⎭
⎜
⎭
Desorption condition
⎭
Φ50 mm × 100 mm
Natural Isobaric water injection Triple water injection Nine-fold water injection
R2
Simulation results
t 0.9758 ⎫ ⎤ 320 ⎦⎬
0.9997
t 1.1123 ⎫ ⎤ 345 ⎦⎬
0.9939
t 1.1141 ⎫ ⎤ 364 ⎦⎬
0.9875
t 1.1147 ⎫ ⎤ 368 ⎦⎬
0.9862
η = ηmax ⎧1 − exp ⎡− ⎨ ⎣ ⎩
( )
η = ηmax ⎧1 − exp ⎡− ⎨ ⎣ ⎩
( )
η = ηmax ⎧1 − exp ⎡− ⎨ ⎣ ⎩
( )
η = ηmax ⎧1 − exp ⎡− ⎨ ⎣ ⎩
( )
⎭
⎭
⎭
⎭
Table 3 Theoretical desorption percentage as a function of coal particle size for different adsorption pressures from the four desorption conditions. Desorption condition
Natural
Isobaric water injection
Triple water injection
Nine-fold water injection
Sample particle size (mm)
Gas equilibrium adsorption pressure (MPa) 0.5
0.75
1.0
1.25
1 3 6 11 Φ50 × 100 1 3 6 11 Φ50 × 100 1 3 6 11 Φ50 × 100 1 3 6 11 Φ50 × 100
100% 98.95% 98.07% 97.64% 97.14% 100% 98.49% 88.27% 81.43% 72.88% 100% 97.31% 75.11% 68.74% 62.24% 100% 90.02% 81.69% 75.02% 70.85%
100% 97.90% 95.45% 94.44% 93.55% 100% 98.61% 94.22% 88.04% 82.85% 100% 96.92% 82.00% 72.97% 66.98% 100% 90.83% 86.59% 74.73% 69.64%
100% 99.02% 98.80% 97.03% 96.97% 100% 97.77% 94.51% 90.77% 87.14% 100% 97.67% 89.30% 85.59% 82.87% 100% 91.47% 89.00% 83.03% 80.71%
100% 99.52% 98.81% 96.93% 96.04% 100% 99.06% 95.29% 90.79% 86.66% 100% 99.08% 90.37% 85.70% 82.42% 100% 95.79% 92.59% 86.04% 82.93%
(2)
where η is the real desorption percentage, n and t0 are the critical values of time effect divergence and time effect respectively, which are related to the water injection pressure and the coal scale, and t is the time. The unit of t and t0 are both minute. Taking the sample of Φ50 mm × 100 mm as an example, the fitting results of natural desorption are shown in Table 1, and the fitting results of steam-enhanced desorption are shown in Table 2. According to the simulation results presented in Table 1, t0 gradually increased with an increase of water injection pressure, the natural desorption percentage was the smallest, the maximum was nine-fold water injection, and the value of n was quite small (0.90). This was because desorption became difficult with extra water pressure increasing. According to the fitting results shown in Table 2, under the action of steam, t0 was significantly lower (about ½) than during natural desorption, and the time effect of steam-enhanced desorption was not obvious with any increase in the water-injection pressure. The maximum value was approximately 1.15 times that of the minimum value. Although the value of n had changed, the magnitude of the change is still limited. It is influenced by extra high-pressure water injection, but no matter water pressure.
Table 2 Time effect on steam enhanced desorption after water injection for the coal sample particle size of Φ50 mm × 100 mm. Sample particle size
⎟
4.3. Analysis of the effect of particle size scale on desorption Desorption of the adsorbed gas is affected by both gas diffusion and seepage, which are both related to the adsorbate size. By studying the desorption characteristics of five different sized samples, it was shown that the desorption capacity gradually decreased with an increase of the size of the sample, and the natural desorption and the injection effects showed different phenomena. However, the desorption capacity under the action of steam injection was minimally affected by the size, which was not discussed further in our study. In order to better understand why the desorption capacity was affected by the coal particle size, because the desorption percentage of 1 mm sample was the largest under the same conditions, the desorption percentage of the 1 mm sample was marked as 100%, and the desorption percentage of the other samples were all less than 1. The calculated results are shown in Table 3. As shown in Table 3, in the natural desorption state, the desorption was less affected by the size. The 50 mm coal sample had 93.6% of the desorption percentage as that of the 1 mm coal sample at 0.75 MPa. However, under the conditions of water pressure injection, the desorption was affected by the size. The desorption percentage of the 50 mm column coal was 72.9%–87.1% under different gas pressures, while under the triple water injection conditions, the effect of size on the desorption was further increased. The desorption percentage of the 50 mm coal sample was 62.2%–82.9% under different gas pressures, while under the condition of nine-fold water injection, the effect of size on desorption tended to be stable. The desorption percentage of the 50 mm coal sample was 69.6%–82.9% under different gas pressures. Therefore, when water exists, the coal size has a great influence on its desorption capacity. The desorption capacity may decrease with an increase of water injection pressure, but it tends to be stable when the pressure reaches a certain value.
scale and pore diameter that can let water molecules through. When water molecules enter and occupy in pore or fissure, they can reduce the connectivity with the originally interlinked pore structure of the adsorbate. This isolation effect makes the adsorbate gas closed in connected pores due to moisture absorption, and the high-pressure water results in pressure instead of connected pores of coal. The gas in a free state has changed into an adsorbed state due to the effect of water injection, gas desorption is prevented.
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5. Conclusions
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In our desorption experiments, coal samples with different particle sizes and different equilibrium adsorption pressures were used. We introduced steam to strengthen the coal desorption ability. In summary, our results show. (1) High-pressure water injection after coal sample adsorption equilibrium is reached will significantly reduce the desorption capacity of gas. With an increase in the water injection pressure, its desorption capacity gradually decreases, and with an increase in the equilibrium adsorption pressure, its desorption capacity will gradually increase under the same conditions. (2) Steam action can significantly improve the desorption capacity of the residual adsorbed gas in the coal and can eliminate the influence of low gas adsorption pressure and high-pressure water injection to its desorption capacity. Therefore, the steam enhanced desorption effect is more obvious for coal reservoirs with lower adsorption pressures. (3) The action of steam can accelerate the desorption velocity and percentage of gas, which is reflected in a decrease in the critical value of time effect. The coal particle size has an influence on the desorption of adsorbed gas. With an increase of water injection pressure, the influence of size becomes greater. However, desorption under steam is less affected by the size. Conflicts of interest The authors declare that there is no conflict of interest regarding the publication of this paper. Acknowledgement We sincerely thank the anonymous reviewers for improving the quality of this manuscript. This study was supported by the National Natural Science Foundation of China (Grant No. 51304142 and No. 21373146); the Program for Outstanding Innovation Teams of Higher Learning Institutions (Grant Year 2014), the Project of Applied Foundation Research (Grant No. 201801D121283 and 201801D221366), and the Education Reform Project about Graduate Students of Shanxi Province in China (Grant Year 2018). We thank International Science Editing for editing and checking the language of this manuscript, and sincerely thank Sanyuan Zhongneng Coal Mine Co., Ltd in Shanxi Province of China for providing raw coal samples. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.petrol.2019.05.027. References Airey, E.M., 1968. Gas emission from broken coal: an experimental and theoretical
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Experimental study on gas desorption characteristics for different coal particle sizes and adsorption pressures under the action of pressured water and superheated steam
T
Dong Zhaoa,b,c,∗, Chao Zhanga,b,c, Hao Chena,b,c, Zengchao Fengb,∗∗ a
Department of Safety Science and Engineering, College of Safety and Emergency Management Engineering, Taiyuan University of Technology, Taiyuan, PR China Key-Laboratory of In-situ Property-improving Mining of Ministry of Education, Taiyuan University of Technology, Taiyuan, PR China c Graduate Student Education and Innovation Center in Coal Mine Safety of Shanxi Province, Taiyuan University of Technology, Taiyuan, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Adsorption and desorption Coal particle size Pressured water Superheated steam
Gas (i.e. methane and CO2) ad-/desorption characteristics are essential for unconventional gas production, and they are related to many factors such as coal particle size, adsorption pressure, external moisture and heat. In order to investigate the effect of these factors, as well as those arising from pressured water and superheated steam on gas desorption, a series of experiments were conducted. The experimental equipment designed by our research group could be used to supply pressured water and superheated steam during the gas ad-/desorption process. Our results were: 1) High-pressure water injection after adsorption resulted in a reduced gas desorption capacity. With an increase in the equilibrium adsorption pressure and water injection pressure, the desorption capacity of coal increased and decreased, respectively. 2) The desorption capacity of the residual adsorbed gas in coal increased with steam action, and it eliminated the influence of low gas adsorption pressure and highpressure water injection on gas desorption. 3) The gas desorption velocity increased with steam action, which was reflected by a decrease in the critical value of time effect. The coal particle size had a certain influence on the desorption of the adsorbed gas. Finally, with an increase in the water injection pressure, the influence of coal particle size became larger. However, the desorption under steam was less affected by the coal particle size. These results may have significant implications for energy exploitation and utilization.
1. Introduction Coalbed methane (CBM) is self-generating and self-storing, if the unconventional, natural gas that is mainly deposited in coal seams is in an adsorbed state. It can be used as a source of energy and raw chemical materials for human use. At the same time, coalbed methane can be used to achieve economic and social benefits in terms of reducing the amount of coal mine gas disasters, improving coal mine safety production, and providing for environmental protection (Zhou et al., 2016). Through the influence of the two-phase gas-water permeability of different coal ranks, and the influence of wettability and overburden pressure on its relative permeability, fissure-based seepage can be obtained (Shi and Durucan, 2005; Durucan et al., 2013). The relative permeability curve exhibits a linear, narrow feature with pore-based percolation, and the curve is nonlinear and has a wide saturation range
(Rahmanian et al., 2010). At the same time, the ratio of the effective coalbed methane relative permeability to absolute permeability increases with coal rank, meaning that in high-rank coals, the gas-flow effect is more pronounced than for low-rank coals (Shen et al., 2011). A new dynamic model can accurately describe the combined effects of coal matrix shrinkage effects, effective stress effects and the Klingenberg effect of coalbed methane reservoir gas. In a two-phase flow experiment, gas breakthrough experiments were carried out by applying different gas pressures to the saturated coal samples, and pressure changes were observed, where the flows of different types of gases through the saturated coal matrix are obtained (Han et al., 2010; Zhao et al., 2014). The gas–water mutual displacement process was observed using NMRI (Nuclear Magnetic Resonance Imaging) technology, and the inconsistency and dominant displacement path of the water-flood front were obtained (Pan et al., 2008). The above study was confirmed by simulation, and that the CBM mining process in the formation is a
∗
Corresponding author. Department of Safety Science and Engineering, College of Safety and Emergency Management Engineering, Taiyuan University of Technology, Taiyuan, PR China. ∗∗ Corresponding author. Key-Laboratory of In-situ Property-improving Mining of Ministry of Education, Taiyuan University of Technology, Taiyuan, PR China. E-mail addresses: [email protected] (D. Zhao), [email protected] (Z. Feng). https://doi.org/10.1016/j.petrol.2019.05.027 Received 12 February 2019; Received in revised form 19 April 2019; Accepted 7 May 2019 Available online 08 May 2019 0920-4105/ © 2019 Elsevier B.V. All rights reserved.
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complex multiphase fluid flow interaction process. Gas desorption from coal in various conditions is affected by numerous factors including gas content from underground boreholes, coalbed permeability, initial gas desorption properties, and coal strength (Tang et al., 2015; Zhao et al., 2017; Li et al., 2017; Zou et al., 2017). Temperature is an important factor that affects the desorption and transport of adsorbed gases, where many studies have shown that increasing the temperature can effectively promote the desorption velocity and desorption capacity of adsorbed gases in coal (Yang et al., 2008; Zhao et al., 2011, 2012, 2018a, 2018b). Recently, thermal injection has been used as a new technology in gas exploitation which has been successfully applied to enhance coalbed methane recovery. They are used in shale gas, CBM, numerical simulations and so on (Wang et al., 2015; Song et al., 2015; Lin et al., 2015; Shahtalebi et al., 2016). At the same time, water has a large specific heat capacity, and per unit mass it can carry more heat; hence gaseous water in the form of steam is an ideal working medium. The structure and distribution are the most important factors for the adsorption and desorption of gas in coal. Some elegant methods including nuclear magnetic resonance, scanning electron microscopy and X-ray nano computed tomography were used in the study of pore structure. They revealed that the contents of adsorption gas and free gas in the multiscale pore structure increase with the increase of gas pressure (Huang and Zhao, 2017). When using hydraulic or gaseous fracturing technology, the coal mass might change to different particle sizes, where each coal block might have different gas desorption features. In this paper, water vapor was used as a heating medium, and the interaction between water and gas in coal was considered. The adsorbed coal after being subjected to high-pressure water was taken as the research object to study the enhanced desorption characteristics of adsorbed gas under steam. We used different sized coal samples in this study, where the five different coal particle sizes had different adsorption and desorption characteristics under external effects. In real CBM wells, there are different sizes of coal blocks, and they display different desorption features for gas exploitation. Using an experimental model, where we considered different sized coal samples, the gas desorption characteristics for different coal particle sizes, and adsorption pressures under the action of pressured water and superheated steam, were revealed.
injection device, a drying box, a vacuum pump, a weighing device with an accuracy of 0.01 g, and a gas collecting device. The thermostatic control cabinet allowed the experiment to be carried out at a set temperature, which minimized the effects of temperature fluctuations. The experimental system is shown in Fig. 2. The parameters of main parts are as follows: (1) Adsorption tank and reference tank: The upper limit of pressure that the adsorption tank and reference tank could reach 30 MPa, which met the standard of high-pressure test containers. (2) Constant temperature system: The temperature control cabinet had a precision greater than ± 0.5 °C. (3) Gas content and pressure measurement system: A pressure gauge with an accuracy of 0.001 MPa was used. Volumetric method was used to calculate gas ad-/desorption content of coal, the pressure drop during gas injection was adopted to calculate gas content in adsorption tank. The pressure of adsorption tank was used to calculate free gas in it, and the other is adsorption gas. (4) Vacuum system: The vacuum was 8 Pa and was set using a vacuum pump that met those requirements. (5) Gas collection system: It is used to measure the content of gas desorption. The desorbed gas was collected using a drainage method, in real time, by using gas to drive the water into a closed container. (6) Superheated steam-generation system: The steam-generation system was composed of an automatic metering water injection pump and a three-stage heating chamber, which guaranteed an output steam temperature of ∼150 °C (Zhao et al., 2016). We used a constant steam speed of 4 g/min in this study, which was controlled using an automatic metering water injection pump. 2.2. Experimental procedures After vacuuming and loading the sample, gas was introduced into the device to start the adsorption process until the process balanced. The experimental process was then divided into three stages. The first experiment was a conventional adsorption/desorption experiment, where the air tightness of the experimental system was determined. The system was at the constant temperature of 25 °C. Next, the gas pressures were in four different stages, they are 0.5 MPa, 0.75 MPa, 1.0 MPa and 1.25 MPa. Using the gas-storage equipment, the sample coal was adsorbed at a given gas pressure for about 24 h until equilibrium was reached. Changes in pressure that occurred during the process were recorded by a digital pressure gauge, and equilibrium was reached when the rate of change in the pressure was < 0.002 MPa/h. During the desorption process, after reaching equilibrium, the gas collection system was connected, and the desorption process was carried out under atmospheric pressure. The desorption process lasted for approximately 24 h until equilibrium was reached (assumed to be when the desorption velocity was < 9.6 × 10−2 mL/(g × s)). We used the drainage method described in Section 2.1 to measure the desorption capacity in real time. The second experiment used steam-injected adsorption–desorption, where the adsorption process was the same as that in the first stage. After adsorption equilibrium was reached, the steam-injection system was switched on, and desorption of the sample started under atmospheric pressure until the desorption reached equilibrium. The temperature of superheated steam was 150 °C and the pressure of it was in atmospheric pressure. The third stage of the experimental process used adsorption, water injection, conventional desorption, and then steam desorption. In the water-injection experiment, after the sample had adsorbed and reached equilibrium, the water-injection system was turned on, and water was injected from the bottom of the adsorption tank at a set water pressure until the top water outlet was turned on. Then, the water injection valve was closed and the pressure was regulated for 24 h. The pressure was
2. Materials and method 2.1. Sample preparation and experimental system Five different coal samples particle sizes (Φ50 mm × 100 mm (50 mm in diameter and 100 mm in height), 11 mm, 6 mm, 3 mm, and 1 mm, respectively) were prepared for the experiments, which were taken from Sanyuan Zhongneng Coal Mine Co., Ltd in Shanxi Province, China. The coal rank was lean coal. After each piece of raw sample coal was transported from the first-line coal production area, it was immediately sealed with wax and then taken back to the lab. Since the raw material did not meet the requirements of our experiments, each piece was sanded and trimmed so as not to damage its structural features. The mass of the Φ50 mm × 100 mm columnar coal sample and their surfaces were smooth, and for the test 256.42 g was used, while the other four coal samples sizes (11 mm, 6 mm, 3 mm, and 1 mm) were used at 100 g each (the sample size error was less than ± 0.5 mm), as shown in Fig. 1. The adsorbed gas used in this study was CO2, which had a purity of 99.9%. We used CO2 instead of methane, because CO2 has more distinct features than methane in terms of adsorption and desorption. Methane and CO2 both display similar and important gaseous geology in real coal seams. For example, to prevent coal and gas outburst disasters in coal mines, both methane and CO2 are outburst gaseous medium found in inner coalbeds. The test system included an adsorption tank, a high-precision pressure gauge, a constant-temperature control cabinet, a water949
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Fig. 1. Coal samples with different particle sizes.
decrease in the size of the sample. The desorption percentage of the 1 mm sample at 1440 min was ∼3% greater than that of the 50 mm sample, but at 360 min, the former was ∼12% larger than the latter, indicating the effect of coal particle size on the desorption velocity. The smaller the coal sample size was, the shorter the time needed for the desorption to reach equilibrium. However, in general, the particle size had little effect on the final desorption percentage. Fig. 5 shows the final adsorption capacity as a function of gas equilibrium adsorption pressure for different coal particle sizes. It can be seen that the final adsorption capacity gradually increased with an increase in the equilibrium adsorption pressure, and the velocity of change gradually decreased until it became stable.
documented with a recording device. The conventional desorption procedure was the same as described in the first stage, but the only difference was that the desorption process occurred after the injection of high-pressure water. During the subsequent steam-injected desorption process, the desorption equilibrium was determined in the same manner as the first stage. 3. Experimental results 3.1. Gas adsorption-desorption characteristics under natural conditions As shown in Fig. 3, and taking a pressure of 1.0 MPa as an example, the adsorption amount of the different sized coal samples was similar with time (Φ50 mm × 100 mm was marked as 50 mm). The initial adsorption velocity was initially rapid, and then gradually decreased, and once it was close to reaching equilibrium, the quasi-equilibrium state lasted for a long time. The gas adsorption in coal was a simple gasphase flow process, which was related to the size of the adsorbate. This implied that the gas adsorption time to equilibrium decreased with a decrease in the coal sample size, and the gas adsorption capacity gradually increased at the same time. This was because, as the size became smaller, more adsorbate molecules were able to diffuse through the coal. The decrease in the distance used for adsorption was reflected in the adsorption time, where the smaller the size, the shorter the time taken for adsorption. The final desorption percentage was calculated using equation (1):
ηmax = VD/ VA
3.2. Enhanced desorption characteristics of adsorbed gases under steam In the state of natural desorption, gas desorption under atmospheric pressure, the desorption percentage of coal samples with different particle sizes was compared with that of coal samples with different equilibrium adsorption pressures, as shown in Fig. 6. In order to compare the effect of natural desorption and steam desorption more directly, a surface diagram is shown in Fig. 7. As shown in Fig. 6, for coal samples of the same particle size, the desorption percentage increased with an increase in the equilibrium adsorption pressure; while under the same equilibrium adsorption pressure, the desorption percentage did not change with varying coal particle size. When the equilibrium adsorption pressure was 1.25 MPa, the desorption percentage during the natural state reached 70%; however, when the equilibrium adsorption pressure was 0.5 MPa, the desorption percentage during the natural state was only about 35%, about half of that at the 1.25 MPa state. This may be because gases under high equilibrium adsorption pressures have greater internal energies and are more likely to diffuse and desorb. As shown in Fig. 7, the desorption percentage of the coal samples
(1)
where, VA is the adsorption capacity in units of mL/g; VD is the desorption capacity in units of mL/g; and ηmax is the final desorption percentage. As shown in Fig. 4, the desorption velocity [the desorption capacity per unit time, in units of mL/(g × min)] increased slightly with a 950
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Fig. 2. The experimental system.
3.3. Desorption characteristics during the natural state after being subjected to high-pressure water
increased significantly after steam action, steam is water vapor in superheated state, where the desorption percentages increased by more than 80%. For coal samples with an equilibrium adsorption pressure of 0.5 MPa, the desorption percentages increased to twice their initial values. At 1.25 MPa, the desorption percentages increased by about 15%. These results showed that steam significantly increased the desorption energy of the adsorbed gases, and promoted the desorption of residually adsorbed gases under low-gas pressures. The results of the high gas pressure experiment showed that the enhanced desorption effect under steam was limited because of the large initial internal energy of the gas. Therefore, steam significantly improved the desorption capacity of residually adsorbed gases in coal. For coal reservoirs with low adsorption pressures, the strengthening effect was more obvious.
Based on the gas equilibrium adsorption pressure, we selected three different water injection pressures: (1) isobaric water injection, where the water injection pressure was equal to the gas equilibrium adsorption pressure; (2) triple water injection, where the water injection pressure was three times larger than the gas equilibrium adsorption pressure; (3) nine-fold water injection, where the water injection pressure was nine times larger than the gas equilibrium adsorption pressure. The purpose of this experiment was to study the effects of different coal particle sizes, different equilibrium adsorption pressures, and adsorption gases with different desorption characteristics after being subjected to a stage of sustained water pressure. Fig. 8A–C shows bar graphs of the gas desorption characteristics of isobaric water injection, triple water injection, and nine-fold water injection, respectively, as a function of different coal particle sizes and
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Fig. 3. Adsorption capacity as a function of time for different coal particle sizes at 1.0 MPa gas equilibrium adsorption pressure.
compared with the natural state, according to different particle sizes and adsorption pressures. The pressure was maintained at equilibrium, and surface diagrams are shown in Fig. 9A–C. As shown in Fig. 9, after steam injection, the residual adsorptive gas after high pressure water was obviously desorbed, and the desorption percentage increased by about 80%. At an equilibrium adsorption pressure of 0.5 MPa, the desorption percentages of isobaric water injection, triple water injection, and nine-fold water injection increased from 18%–25%, 14%–22%, and 11%–15% to 79%–80%, 78%–79%, and 77%–78%, respectively. At an equilibrium adsorption pressure of 1.25 MPa, their respective desorption percentages increased from 54%–62%, 45%–54%, and 27%–33% to 83%−84%, 81%–83%, and 80%–81%, respectively. The results of our study confirmed that the desorption percentage after steam strengthening was less affected by reservoir pressure, and the desorption percentage at 1.25 MPa reservoir pressure after steam was only 1.04 times that at 0.5 MPa. It could be concluded that for a low-pressure reservoir coal seam, the coalbed gas mining efficiency of conventional mining method was relatively low and that the mining efficiency caused by the combined two-phase gas–water flow was extremely low, while the use of steam significantly improved coalbed gas mining efficiency.
equilibrium adsorption pressures. It can be seen that the influence of water injection on the desorption capacity of the coal samples with different particle sizes was different, where the desorption percentage decreased with an increase in the coal particle size. For different equilibrium adsorption pressures, water injection had a different influence on the desorption properties of the coal samples. The gas desorption capacity arising after a lower adsorption pressure was much more affected by water injection than that arising after higher adsorption pressure. When the adsorption equilibrium pressure was 0.5 MPa, the desorption percentages of isobaric water injection, triple water injection, and nine-fold water injection were between 18%–25%, 14%–22%, and 11%–15%, respectively. When the adsorption equilibrium pressure was 1.25 MPa, their respective desorption percentages were between 54%–62%, 45%–54%, and 27%–33%, respectively. With an increase of water injection pressure, the gas desorption percentage at the same particle size and equilibrium adsorption pressure gradually decreased. As shown in the bar graphs from Fig. 8A–C, the larger the water injection pressure was, the more obvious its effect on the desorption. 3.4. The desorption characteristics of steam enhanced by high pressure water As described in Section 3.3, the desorption percentage of gas after being treated with high-pressure water was greatly reduced. In order to explain the effect of steam-enhanced desorption, steam was injected into the adsorbed sample under the action of high-pressure water, and
Fig. 4. Desorption percentage as a function of time for different coal particle sizes at 1.0 MPa gas equilibrium adsorption pressure. 952
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Fig. 5. Final adsorption capacity as a function of gas equilibrium adsorption pressure for different coal particle sizes.
Fig. 6. Final desorption percentage as a function of gas equilibrium adsorption pressure for different coal particle sizes at the natural desorption stage.
Fig. 7. Surface diagrams of desorption percentage as a function of gas equilibrium adsorption pressure for different coal particle sizes in comparison with the natural desorption (lower panel) and steam enhanced stages (upper panel).
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Fig. 8. Desorption percentage as a function of gas equilibrium adsorption pressure for different coal particle sizes after water injection at the natural desorption stage.
4. Discussion
the adsorption of coal after applied to coal and gas desorption gas internal migration, based on the thermal extraction is a kind of effective method in low permeability CBM exploration. However, when using superheated steam or water as medium of heat injection, desorption and migration of coalbed methane are influenced by the steam or water. After water is injected into a coal seam, the water-saturation degree
4.1. Mechanism of inhibited water injection desorption and steam-enhanced desorption Steam forms by a certain quantity of heat, and can be increased, and 954
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Fig. 9. Surface diagrams of the desorption percentage as a function of gas equilibrium adsorption pressure for different coal particle sizes after water injection, in comparison with natural desorption (lower panel) and steam-enhanced desorption (upper panel).
gas molecules. There are a large number of pores in coal, which are connected via fissures, some of them are affected by extra fluid flow. As coal physically adsorbs gas and water molecules, after the coal is injected with water, water molecules are absorbed into the pores. Because water molecules block the gas-seepage channels, there is only a certain
increases. From a microscopic point of view, the coal absorbs water because the surface of the adsorbent attracts water molecules, which leads to multi-layered adsorption. The way in which the adsorbent adsorbs the coalbed methane is also a physical adsorption process, which is the result of the pores of the adsorbent and the pores attracting 955
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4.2. Analysis of the time effect of steam-strengthening desorption
Table 1 Time effect on the natural desorption stage after water injection for the coal sample particle size of Φ50 mm × 100 mm. Sample particle size
Desorption condition
Φ50 mm × 100 mm
Natural Isobaric water injection Triple water injection Nine-fold water injection
The time effect of desorption is crucial to the efficiency of coalbed methane production, and can directly reflect the level of mining efficiency and operating time. According to Airey's desorption characteristics of broken coal with time, the correlation between the desorption percentage and time is summarized by equation (2) (Airey, 1968):
R2
Simulation results
t 0.9433 ⎫ ⎤ 569 ⎦⎬
0.9928
t 0.8831 ⎫ ⎤ 822 ⎦⎬
0.9898
t 0.892 ⎫ ⎤ 880 ⎦⎬
0.9914
t 0.902 ⎫ ⎤ 949 ⎦⎬
0.9917
η = ηmax ⎧1 − exp ⎡− ⎨ ⎣ ⎩
( )
η = ηmax ⎧1 − exp ⎡− ⎨ ⎣ ⎩
( )
η = ηmax ⎧1 − exp ⎡− ⎨ ⎣ ⎩
( )
η = ηmax ⎧1 − exp ⎡− ⎨ ⎣ ⎩
( )
⎭ n
t η = ηmax ⎧1 − exp ⎡−⎛ ⎞ ⎤ ⎫ ⎢ ⎝ t0 ⎠ ⎥ ⎬ ⎨ ⎦⎭ ⎣ ⎩
⎭
⎜
⎭
Desorption condition
⎭
Φ50 mm × 100 mm
Natural Isobaric water injection Triple water injection Nine-fold water injection
R2
Simulation results
t 0.9758 ⎫ ⎤ 320 ⎦⎬
0.9997
t 1.1123 ⎫ ⎤ 345 ⎦⎬
0.9939
t 1.1141 ⎫ ⎤ 364 ⎦⎬
0.9875
t 1.1147 ⎫ ⎤ 368 ⎦⎬
0.9862
η = ηmax ⎧1 − exp ⎡− ⎨ ⎣ ⎩
( )
η = ηmax ⎧1 − exp ⎡− ⎨ ⎣ ⎩
( )
η = ηmax ⎧1 − exp ⎡− ⎨ ⎣ ⎩
( )
η = ηmax ⎧1 − exp ⎡− ⎨ ⎣ ⎩
( )
⎭
⎭
⎭
⎭
Table 3 Theoretical desorption percentage as a function of coal particle size for different adsorption pressures from the four desorption conditions. Desorption condition
Natural
Isobaric water injection
Triple water injection
Nine-fold water injection
Sample particle size (mm)
Gas equilibrium adsorption pressure (MPa) 0.5
0.75
1.0
1.25
1 3 6 11 Φ50 × 100 1 3 6 11 Φ50 × 100 1 3 6 11 Φ50 × 100 1 3 6 11 Φ50 × 100
100% 98.95% 98.07% 97.64% 97.14% 100% 98.49% 88.27% 81.43% 72.88% 100% 97.31% 75.11% 68.74% 62.24% 100% 90.02% 81.69% 75.02% 70.85%
100% 97.90% 95.45% 94.44% 93.55% 100% 98.61% 94.22% 88.04% 82.85% 100% 96.92% 82.00% 72.97% 66.98% 100% 90.83% 86.59% 74.73% 69.64%
100% 99.02% 98.80% 97.03% 96.97% 100% 97.77% 94.51% 90.77% 87.14% 100% 97.67% 89.30% 85.59% 82.87% 100% 91.47% 89.00% 83.03% 80.71%
100% 99.52% 98.81% 96.93% 96.04% 100% 99.06% 95.29% 90.79% 86.66% 100% 99.08% 90.37% 85.70% 82.42% 100% 95.79% 92.59% 86.04% 82.93%
(2)
where η is the real desorption percentage, n and t0 are the critical values of time effect divergence and time effect respectively, which are related to the water injection pressure and the coal scale, and t is the time. The unit of t and t0 are both minute. Taking the sample of Φ50 mm × 100 mm as an example, the fitting results of natural desorption are shown in Table 1, and the fitting results of steam-enhanced desorption are shown in Table 2. According to the simulation results presented in Table 1, t0 gradually increased with an increase of water injection pressure, the natural desorption percentage was the smallest, the maximum was nine-fold water injection, and the value of n was quite small (0.90). This was because desorption became difficult with extra water pressure increasing. According to the fitting results shown in Table 2, under the action of steam, t0 was significantly lower (about ½) than during natural desorption, and the time effect of steam-enhanced desorption was not obvious with any increase in the water-injection pressure. The maximum value was approximately 1.15 times that of the minimum value. Although the value of n had changed, the magnitude of the change is still limited. It is influenced by extra high-pressure water injection, but no matter water pressure.
Table 2 Time effect on steam enhanced desorption after water injection for the coal sample particle size of Φ50 mm × 100 mm. Sample particle size
⎟
4.3. Analysis of the effect of particle size scale on desorption Desorption of the adsorbed gas is affected by both gas diffusion and seepage, which are both related to the adsorbate size. By studying the desorption characteristics of five different sized samples, it was shown that the desorption capacity gradually decreased with an increase of the size of the sample, and the natural desorption and the injection effects showed different phenomena. However, the desorption capacity under the action of steam injection was minimally affected by the size, which was not discussed further in our study. In order to better understand why the desorption capacity was affected by the coal particle size, because the desorption percentage of 1 mm sample was the largest under the same conditions, the desorption percentage of the 1 mm sample was marked as 100%, and the desorption percentage of the other samples were all less than 1. The calculated results are shown in Table 3. As shown in Table 3, in the natural desorption state, the desorption was less affected by the size. The 50 mm coal sample had 93.6% of the desorption percentage as that of the 1 mm coal sample at 0.75 MPa. However, under the conditions of water pressure injection, the desorption was affected by the size. The desorption percentage of the 50 mm column coal was 72.9%–87.1% under different gas pressures, while under the triple water injection conditions, the effect of size on the desorption was further increased. The desorption percentage of the 50 mm coal sample was 62.2%–82.9% under different gas pressures, while under the condition of nine-fold water injection, the effect of size on desorption tended to be stable. The desorption percentage of the 50 mm coal sample was 69.6%–82.9% under different gas pressures. Therefore, when water exists, the coal size has a great influence on its desorption capacity. The desorption capacity may decrease with an increase of water injection pressure, but it tends to be stable when the pressure reaches a certain value.
scale and pore diameter that can let water molecules through. When water molecules enter and occupy in pore or fissure, they can reduce the connectivity with the originally interlinked pore structure of the adsorbate. This isolation effect makes the adsorbate gas closed in connected pores due to moisture absorption, and the high-pressure water results in pressure instead of connected pores of coal. The gas in a free state has changed into an adsorbed state due to the effect of water injection, gas desorption is prevented.
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5. Conclusions
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In our desorption experiments, coal samples with different particle sizes and different equilibrium adsorption pressures were used. We introduced steam to strengthen the coal desorption ability. In summary, our results show. (1) High-pressure water injection after coal sample adsorption equilibrium is reached will significantly reduce the desorption capacity of gas. With an increase in the water injection pressure, its desorption capacity gradually decreases, and with an increase in the equilibrium adsorption pressure, its desorption capacity will gradually increase under the same conditions. (2) Steam action can significantly improve the desorption capacity of the residual adsorbed gas in the coal and can eliminate the influence of low gas adsorption pressure and high-pressure water injection to its desorption capacity. Therefore, the steam enhanced desorption effect is more obvious for coal reservoirs with lower adsorption pressures. (3) The action of steam can accelerate the desorption velocity and percentage of gas, which is reflected in a decrease in the critical value of time effect. The coal particle size has an influence on the desorption of adsorbed gas. With an increase of water injection pressure, the influence of size becomes greater. However, desorption under steam is less affected by the size. Conflicts of interest The authors declare that there is no conflict of interest regarding the publication of this paper. Acknowledgement We sincerely thank the anonymous reviewers for improving the quality of this manuscript. This study was supported by the National Natural Science Foundation of China (Grant No. 51304142 and No. 21373146); the Program for Outstanding Innovation Teams of Higher Learning Institutions (Grant Year 2014), the Project of Applied Foundation Research (Grant No. 201801D121283 and 201801D221366), and the Education Reform Project about Graduate Students of Shanxi Province in China (Grant Year 2018). We thank International Science Editing for editing and checking the language of this manuscript, and sincerely thank Sanyuan Zhongneng Coal Mine Co., Ltd in Shanxi Province of China for providing raw coal samples. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.petrol.2019.05.027. References Airey, E.M., 1968. Gas emission from broken coal: an experimental and theoretical
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