Experimental Study on Methane Desorption with the Coupling Effects of High Pressure Water and Thermal

Moisture and thermal are the key factors for influencing methane desorption during CBM exploitation. Using high pressure water injection technology into coalbed, new fractures and pathways are formed to methane transport. It is existed a phenomenon of water inhibiting gas flow. This study is focused on various water pressures impacted on gas adsorbed coal samples, then the desorption capacity could be revealed under different conditions. And the results are shown that methane desorption capacity was decreased with water pressure increased at room temperature and the downtrend would be steady until water pressure was large enough. Heating could promote gas desorption capacity effectively, with the increasing of water injection pressures, the promotion of thermal on desorption became more obvious. There are the others effects on methane desorption capacity influenced by water injection and thermal.


Introduction
Coalbed methane, CBM, is a significant natural energy resource and plays an important role in the structure of clean and new energy in China.However, currently, China CBM cannot satisfy requirements for effective and efficient recovery due to low permeability, low reservoir pressure and high gas adsorption capacity.(Zhou et al., 2016) With increased mining depth, ground stress, and gas pressure, as well as low gas permeability of coal seams, the gas extraction became difficult before mining.(Fu et al., 2007) The decrease in gas pressure during gas immigration can lead to gas desorption and cause matrix contraction, leading to fracture extension and further improving the permeability of the coal seam.(Li et al., 2016) Furthermore, their permeability in the gas discharge process is complicated and can be affected by the coupled processes of gas migration and deformation of the coalbed.The mechanism of coupled flow and deformation on CBM recovery has drawn immense attention and significant achievements.(Chen et al., 2017) To increase coal seam permeability and reduce the amount of extraction drilling and risk of outburst, researchers studied techniques and methods of hydraulics.These methods include ordinary drill, enhanced drills, coalbed water injection, hydraulic punching, hydraulic cutting and hydraulic fracturing, they are widely used in modern coal mining to improve coalbed permeability and gas output.All these processes are including high pressure water.Thus, if high pressure water has been injected into coalbed, the desorption capacity of methane would be effected due to moisture.(Zhao et al., 2011) Some researchers have engaged in this field in order to improve coalbed permeability and amount of gas output.The two major technologies are hydraulic fracture and thermal injection.Hydraulic fracture will increase the moisture content in coalbed which it is resulting from water injection.According to the mechanism of moisture increased in coalbed, many scholars have performed studies on the influence of moisture on methane desorption (Xie et al. 2011;Zhang and Sang 2009), but they wet the coal samples before the experiment.Actually, after injecting water, the original equilibrium state of absorption and desorption will be broken.It makes very large differences between the experimental process and engineering practice.Therefore, it is very difficult to accurately explain the problems in engineering practice.Some scholars realized this problem (Pakowski et al. 2011;Xiao and Wang 2011;Zhang et al. 2011a;Zhao et al., 2011;Chen et al., 2015), and they perfected the experimental process in which water was injected into the dry coal samples under the equilibrium state of absorption methane.All of these results showed that the maximum desorption quantity decreases gradually with the amount of injected water, but they were not considering different water pressure impacted on same equilibrium adsorption state.
Thermal injection is an new technology in CBM exploitation, and they have been successfully applied to the enhancement of coalbed methane recovery.(Chen et al., 2010;Wang et al., 2015;Song et al., 2015;Lin et al., 2015;Shahtalebi et al., 2016.)Thermal simulation technologies, such as alternative or supplementary methods for the enhancement of gas production from unconventional reservoirs, have gained great attention in recent years.For example, electromagnetic materials, hot water injection, hot gas injection and microwave or radio have been used (Yahya et al., 2012;Salmachi and Haghighi, 2012;Bahrami et al., 2012).Methane desorption rate would be improved by coalbed heating, but the injected water could block the channels of gas flow imposing additional resistance on the transport and production of coalbed methane.Hot gas injection method would induce a binary gas flow and make the forecasting of gas production more difficult.
In this paper a proposal about thermal and water injection technology to improve coalbed methane production has been proposed.Combining high pressure water will block gas desorption and heat will accumulate gas output, a series experiments of methane bearing coal samples with water injection at different water and gas pressure.
And then, heating coal samples after desorption of water injection.After that, desorption capacities of coal samples at different water injection pressure and thermal effects have been illustrated.The experiments on constant and warming temperature of desorption character of gassy coal under different water injection pressures were carried out.These results are expected to provide a clearer understanding of theoretical efficiency of heat water or steam injection into coalbed, they can provide some theoretical and experimental guidance on CBM production and methane control.

Coal samples and preparation
Coal samples used in experiments were taken from Gucheng and Gaohe Coal Mine Large coal samples were taken from the well and wax sealed on site.After reaching the laboratory, they were processed into Ф100mm×100mm cylindrical specimen.The picture of experimental coal samples is shown in Fig. 1, the results of proximate and reflectance analysis are shown in Table 1.

Fig.1 Experimental apparatus and coal samples used in these experiments
Table 1 Analysis results of experimental coal The samples are lumpy, which contained similar pores and fractures to a coal bed.
Pores are the dominant place for gas storage and fractures for gas transportation.
Methane adsorbed on the coal samples was used for CBM in situ, and enclosed CBM was simulated using water injection after adsorption.

Experimental apparatus
To study the adsorption/desorption characteristics of coal mass samples and to carry out simulated water injection CBM experiments, we constructed a new experimental system.The set-up comprises a stainless steel container that withstands pressures up to 20 MPa and has top and bottom openings with flexible seals.The top part is used to inject and produce gas and the bottom part is used to inject water.A columnar coal sample is placed inside the container, which is surrounded by a temperature-controlled air bath.Water injection and gas production in the apparatus are similar to the one water injection well per gas production well configuration under ideal conditions.The coal mass samples simulate an ideal coal bed after hydro-fracture.Gas usually desorbs rapidly, but once water is injected the process slows down and becomes less efficient.A series of experiments was performed at different water pressures to determine the desorption behavior.The appearance and the schematic diagram of experimental apparatus are shown in Fig. 1 and Fig. 2, respectively.
Fig. 2 Schematic diagram of experimental apparatus used in this study The gas system indicates the gas flow path and the liquid system provides introduces water at different pressure into gas-bearing samples.The system is controlled based on experimental conditions, including pressure and flow.Needle valves (V1, V2, V3) and gauges (G1, G2, G3) are used to regulate and show the pressure of methane (CH4), helium (He) and water, respectively, and flow meters (F1, F2) are used to read the gas and liquid flow rate.In addition a switch valve (S1) is used to switch from methane to helium.Methane, the major CBM component, is supplied from a high-pressure cylinder (C1).Helium from cylinder C2 is used to check for leaks and measure free volume after sample in container.Then the helium was purged using a vacuum pump (M2) until the sample column contained no gas before methane injection.It will last more than 48h.
After testing for leaks and methane injection (the equilibrium pressure is shown on G5), liquid water was injected from the bottom until the container was full of water.
This process simulates an ideal coal bed by injecting water around the surface of broken coal at a certain pressure.An injection device (M1) is used to inject liquid water.The injected water was left in place for approximately 24 h to simulate ideal water-bearing coal after hydro-fracture.
A control device (M3) was used to regulate the temperature via an air circulation system.The system for measuring water outflow consisted of a conical measuring flask to measure the flow volume during desorption.A gas collection system (M4), using displacement of water and a rotor flowmeter, was used to precisely determine the volume of desorbed methane (precision ±2 mL).

Experimental procedures
During the experiments, the coal samples were placed in adsorption-water injection-desorption apparatus at a room temperature of 25℃ which means coal reservoir temperature, and after up to 90℃ which is experimental temperature controlled by a temperature controller using thermal air flow.Before experiments began, the free volume of coal in container is measured.The experimental procedures are divided into three steps.
The first is the adsorption, checking the airtightness of the experimental device by the method of high-pressure helium, and then keep the device vacuum under airtightness more than 48h, finally open the valve for connecting gas container with sample container so that coal sample could be adsorbed gas, the process would last about 3 to 7 days until the gas pressure is constant.Under the rated adsorption pressure and the pressure change during the adsorption process is recorded by the digital pressure gauge.
The second is the high pressure water injection, connecting M1 to M3 after coal sample adsorption reached an equilibrium state.The water pressure is controlled by water pump in M1 and divided into four different pressures.They are water pressure equal gas pressure (1 time), 2.5 times of gas pressure (2.5 times), 6.25 times of gas pressure (6.25 times) and 16 times of gas pressure (16 times).For contrast, natural desorption without water injection after adsorption is necessary.During water injecting process, water has run through the entire coal sample.Then kept the state of water in coal sample container for 24h.This process is simulated to moisture injection in the situ-coal seam.
The third is the desorption, it is separated two different steps.The one is room temperature desorption and the other is heating to 90℃ for desorption.M4 is connected to M3 for desorption under atmospheric pressure.The desorption process of desorption would last 48h in each experiment and the desorption gas is collected.
The amount of gas desorption is measured in real time during the experiment and after less than 10ml/h, it is considered that the desorption equilibrium has been reached.

0.25MPa
Table 3 Raw data of the two coal samples at gas adsorption pressure around 0.50MPa The instantaneous desorption rate of all the experiments reaches the final desorption rate of 71.53% in 500 minutes.The instantaneous desorption rate of gas after injection is not obvious with time increase, the data in slowly growth with time.The larger the injection pressure, the more difficult it is to desorb, the lower the final desorption percentage.

Curves
In the same coal sample, with the increase of adsorption pressure, whether in natural desorption or water injection desorption, the final gas desorption percentage of sample has been improved to some extent.When using 2 times of former adsorption pressure in Fig. 4, desorption percentage of Gucheng and Gaohe coal samples increased by 11.44% and 27.82% in Natural desorption.Water injection pressure is higher, the increase of adsorption pressure on gas desorption rate is more obvious in water injection desorption.Under 16 times of water injection pressure, contrast with 0.25 MPa gas pressure, and the desorption percentage of Gucheng and Gaohe coal samples increased by 2.473 times and 2.047 times, respectively.
When the temperature rises to 90℃, the Gucheng coal sample showed a significant increase in the desorption rate of coal methane compared with 25℃ when water injection pressure and adsorption pressure are constant.The final desorption percentage at 90℃ increased by 1.331 times compared with that at 25℃ in natural desorption.With the increase of water injection pressure, the effect of heating on desorption rate increased significantly.At 16 times of water injection pressure, thermal promotes the desorption and the gas desorption rate increased by 10.72 times which is compared to room temperature.This is shown that thermal could enhance the ability of coal methane desorption effectively, and inhibition effect of water injection was weaken.
The time effect of desorption in the process of mining coalbed methane is a very important indicator, which is the direct influence factor of the recovery efficiency.
Therefore, based on the description of the desorption capacity of coalbed-methane with the change of time, the paper (Zhao et al., 2011) uses the following formula to analyze the relationship between desorption rate and time under different conditions: In this formula, t0 and n were the desorption median time parameters and the time effect of divergence; t is the time.The above parameters are related to experimental water pressure and type of coal.
Equation ( 1) is transformed and can be changed to ( 2 According to Equation ( 1) and (2), a regression analysis has been made using the experimental results, the simulation results is shown in Table 4.It can be seen that the time-dependent parameter t0 of desorption gradually increases with the change of water injection pressure, which indicates that high pressure water injection will affect the desorption rate of coalbed methane.Table 4 Simulation results of time effect on desorption

The relationship between final desorption percentage and water injection pressure
Desorption capacity as a function of water pressure for the two coal samples at 0.25 MPa and 0.50 MPa gas adsorption pressure are shown in Fig. 5 and Fig. 6, respectively.From natural desorption to 2.5 times water injection pressure and then (3).
In the expression, ηdmax is the final desorption percentage (%) of coal sample during natural desorption; a and b are related parameters (MPa -1 ).Regression analysis of experimental results and simulation results are shown in Table 5 that b/a is one of the important indexes to measure the effect of water injection on gas desorption and the lower limiting value is 1, indicating that the gas desorption rate is equal to 0 when the water pressure is infinite.The value of b/a is close to 1 in this paper, indicating that the desorption rate of gassy coal will be decrease with the increase of external moisture injection pressure.The desorption rate will be close to zero when the water pressure increases to a certain value.
Table 5 Simulation results between desorption percentage and water injection pressure The influence of moisture on coal desorption based on coal moisture content by using small particle size coal samples had been reveal in following forum (Wang et al., 2010): (5).

In the expression, M is the water content (%).
This formula is similar to this article.The water content of coal is positively correlated with the water injection pressure and can be approximated by comparison, but the value of b/a that obtained is larger, indicating that the desorption rate is still about 4/5 at natural desorption when reaching saturated water content.The smaller the particle size of coal samples are, the less impact of external water injection on the desorption of coal is.

Discussion
The essence of gas drainage is the conversion process of gas from adsorption to free state , and this process must involve two physical quantities which are gassy potential and chemical potential produced by coalbed-methane adsorption.The matter always tends to transform from a state with high chemical potential to a low one.Both reducing the pressure and increasing the temperature reduce the chemical potential of the free gas, which is favorable for the gas to be converted from the adsorbed state to the free state.Gas adsorption potential is to reflect the coal surface adsorption capacity of the methane.The larger the adsorption potential energy is, the less it is easily desorbed.And the higher the temperature is, the lower the adsorption potential energy is.In view of this, adsorption pressure and desorption temperature are inevitably important factors that affecting the gas from adsorption to free state transformation.Desorption capacity as a function of water pressure at 90℃ for coal sample 1# is shown in Fig. 7. Temperature has a promote effect on gas desorption, it is different from various water injection pressure.The larger adsorption pressure it has, the greater desorption pressure it will reduce and it will be more favorable to the release of chemical potential energy and will get higher the desorption rate.In this paper, 0.2MPa adsorption pressure desorption rate was significantly higher than 0.1MPa adsorption pressure.The heating process of Gucheng coal samples is very significant that the process of heating is promoted the desorption.In this paper, it can be seen from the research of different moisture injection pressure conditions that the desorption rate is getting lower and lower with the increase of moisture injection pressure because the moisture molecules obviously block the pores and cracks in the coal through water injection and the amount of desorption energy of the molecule greatly increases so the greater the pressure of water injection, the more difficult it is for the gas in the pore fissure to desorb.Throughout all the experiments, it can be confirmed that moisture injection can effectively suppress gas desorption process.
Heating can effectively promote gas desorption, and with the increase of water injection multiples, the promotion of heating on the desorption more obvious.For the desorption rate in heating, equal pressure water injection,2.5,6.25 and 16 times water injection increased about 6 times, 6 times, 7 times and 21 times respectively.
There is a certain time-median effect on the desorption of coal gas along with time after water injection, and it will vary with the water injection pressure.Under different water injection pressures, the final desorption rate of coalbed-methane under different moisture injection pressures and natural desorption under the same conditions have a certain functional relationship.(C1, C2: methane(CH4) and helium(He) cylinder storage; F1, F2: flow of gas; G1, G2 or G3: gauge for displaying the pressure of methane, helium and water; G4: negative gauge for air pressure used for vacuum; G5: gauge for sample gas pressure; V1, V2, V3, V4 or V5: needle valves; S1, S2 or S3: three-port valves; M1: water injection pump; M2: vacuum pump; M3: temperature-controlling device; M4: measurement system.) Fig.3 Desorption capacity versus time in different water injection pressure at

Fig. 5
Fig.5 Desorption capacity as a function of water pressure for the two coal

Fig. 7
Fig.7 Desorption capacity as a function of water pressure at 90 ℃ for coal

Fig. 8
Fig.8 Mass of coal sample as a function of water pressure after desorption for

Fig. 2
Fig.2 Schematic diagram of experimental apparatus used in this study.

Fig. 5 Fig. 6 Fig. 8
Fig.5 Desorption capacity as a function of water pressure for the two coal samples at 0.25MPa gas adsorption pressure

Table 2
and Table3showed the raw data of adsorption, water injection and desorption of the coal sample, in which 1#,2# and 12# coal samples represent desorption of Gucheng coal sample at 25℃, desorption of Gaohe coal sample at 25℃ and desorption of Gucheng coal sample at 90℃, respectively.The definitions of parameters in the table are shown in Table2.The data of gas adsorption pressure around 0.25 MPa and 0.5 MPa are shown in Table2 and Table 3, respectively.

Table 2
Raw data of the two coal samples at gas adsorption pressure around Preprints (www.preprints.org)| NOT PEER-REVIEWED | Posted:

Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 28 November 2017 doi:10.20944/preprints201711.0179.v1
Note: ΔP means D-value between initial and final gas pressure of gas storage tank, which the content is 3.8L; P 1 means gas adsorption pressure at final stage; V 1 means desorption volume of coal sample at each stage; P 2 means water injection pressure, it is depends on final gas adsorption pressure; V 2 means free content after coal into sample container, it is measured by Helium; m 1 means the mass of dry coal sample; m 2 means the mass of wet coal sample after water injection and desorption; ω is the moisture content of the coal sample appending on dry coal after water injection and desorption; V 3 and V 4 are free gas volume and adsorption gas volume, respectively; dV 5 means adsorption gas volume per unit mass of coal; and η max is accumulative desorption rate after 48h desorption.All data is converted to the value at standard status (273.15K, 1 atm).