To improve coal seam gas drainage performance, we developed a double-expansive (DE) material to seal the borehole. The swelling process of this material includes an initial swelling stage and a secondary swelling stage. We studied the swelling pressure properties of the DE material under four constraint conditions using a self-made swelling test device. Further, the active support effect of the DE material on the borehole was analyzed by simulating borehole stability with COMSOL Multiphysics software. The results exhibit the following: (1) The swelling pressure of the DE material exhibits time-dependent behavior, and the mathematical relationship between the swelling pressure and time can be obtained by nonlinear fitting. (2) The radial swelling potential is principally formed during the secondary swelling stage, providing the main active support on the radial constraint. (3) The active support imposed on the hole wall can prevent the extension of plastic and damage regions around the borehole, for improved stability of the gas drainage borehole. Finally, field tests demonstrate improved gas drainage performance of the borehole sealed by the DE material compared to a conventional sealing material.
Coalbed methane (CBM) is an unconventional natural gas and presents hazards in coal mines [
To reduce the air leakage of gas drainage borehole, efficient sealing materials have been developed. Zhou et al. [
Generally, sealing materials with good swelling properties are required to guarantee the sealing quality and to reduce air leakage of drainage boreholes. The swelling pressure of the material can provide active support that acts on the wall of the borehole, to resist borehole deformation, inhibit the development of fractures around the borehole, and reduce air leakage [
The swelling pressure of a material is a resistance to maintain its initial volume and shape [
The DE material is composed of main raw material and supplementary material, combined at a mass ratio of 17 : 3. The main raw material is composed of OPC and mineral powder (at a mass ratio of 8 : 9). The supplementary material includes a solid expansion agent, bentonite, naphthalene water reducer, and aluminum powder (mass ratio of 26 : 6 : 1 : 1). The chemical composition of the solid expansion agent includes MgO (4.1%), CaO (26.8%), CaSO4 (19.2%), SiO2 (31.1%), and Al2O3 (18.3%), with a loss on ignition of 0.5%. The bentonite that acts as an efficient suspending agent [
According to our prior results of applying the free swelling test to the DE material using Le Chatelier’s rubber bag method [
Free swelling ratio of the DE material during hydration with macrographs of the material at the 16th hour and the 168th hour.
The microstructure properties of the DE material were previously characterized by environmental scanning electron microscopy (FEI Quanta 250 FEG-SEM) at a magnification of 3000 times and using an energy dispersion spectrometer (Bruker Quantax 200 Xflash; resolution is superior to 129 eV). Micrographs of the DE material at the 16th hour and 168th hour after the start of hydration are plotted in Figures
The micrographs of the DE material and the EDS of the CH and Aft crystals: (a) micrograph at the 16th hour; (b) micrograph at the 168th hour; (c) EDS of CH crystal; (d) EDS of AFt crystal.
A swelling test device was designed to measure the swelling pressure of the DE material (Figure
Swelling test device and the connections with the computer and data acquisition devices.
The swelling pressure of the DE material during its hydration can be divided as the radial swelling pressure (
The swelling pressure of the DE material is far less than the yield strength of the cylinder material (355 MPa). Therefore, the entire cylinder remains elastic, and we can obtain
Incorporating (
Then,
Therefore,
Using the swelling test device, the swelling pressure test procedure can be summarized as follows: Test device connection: The swelling test device, data acquisition devices, and computer were connected according to Figure DE paste preparation: According to GB/T 1346-2011, 500 grams of the DE material and 400 grams of water were stirred twice in a slurry agitator, at 25°C. The first stirring is performed at a rate of 140 r/min for 120 s. Then, stirring is stopped for 15 s. Finally, the second stirring is performed at a rate of 285 r/min for 120 s. Monitoring of swelling parameters and data processing: After DE paste sample preparation, the paste sample was immediately transported into the hollow cylinder. When the cylinder was full, the upper plate was installed on the cylinder and fixed by bolts and nuts (Figure
With swelling, the DE material was constrained radially by the hollow cylinder and constrained axially by the bolts. We aimed to investigate the swelling pressures of the DE material in four hollow cylinders of different dimensions and different radial constraint stiffness (
Constraint stiffness in the four tests.
Test no. |
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1 | 2 | 7 | 41.2 | 126.8 |
2 | 3 | 7 | 61.8 | 126.8 |
3 | 4 | 7 | 82.4 | 126.8 |
4 | 5 | 7 | 103 | 126.8 |
The swelling pressure of the DE material exhibited a significant time-dependent characteristic during its hydration, as shown in Figure
Variations of swelling pressure over time: (a) radial and axial swelling pressures-time curves in test no. 1; (b) radial and axial swelling pressures-time curves in test no. 2; (c) radial and axial swelling pressures-time curves in test no. 3; (d) radial and axial swelling pressures-time curves in test no. 4.
The time (
From (
The time-dependent behavior of the swelling pressure is shown in Figure
After nonlinear fitting, the fitting parameters are listed in Table
Fitting parameters of the fitting curves.
Fitting curves |
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1- |
−0.27 | 1.58 | 41.31 | 21.48 | 0.991 |
1- |
0.03 | 1.37 | 49.43 | 24.39 | 0.990 |
2- |
−0.09 | 1.68 | 91.77 | 31.00 | 0.987 |
2- |
−0.08 | 1.53 | 91.77 | 31.00 | 0.987 |
3- |
−1.42 | 2.15 | 28.67 | 49.57 | 0.986 |
3- |
−0.34 | 1.97 | 66.46 | 45.75 | 0.993 |
4- |
−0.35 | 2.21 | 47.48 | 22.59 | 0.997 |
4- |
−0.33 | 2.09 | 47.48 | 22.59 | 0.997 |
Based on the results of the swelling pressure tests, the radial swelling pressures produced at different swelling stages are plotted in Figure
Radial swelling pressures produced at different swelling stages.
In practice, the swelling potential of the DE material energy is released radially and axially, and it is the radial swelling potential that is a key for the active support acting on the gas drainage borehole wall. For the laboratory tests, the radial swelling pressure, as an internal pressure, does work to the hollow cylinder, and then elastic energy (
The RSPI of the DE material at different swelling stages is illustrated in Figure
RSPI at different swelling stages.
In the initial swelling stage, the swelling of the DE material is mainly caused by the foaming effect of the aluminum powder. Then, a porous structure is formed in the DE material, as shown schematically in Figure
Schematic for the morphology model of hydrates in the DE material: (a) initial swelling stage; (b) secondary swelling stage.
In the secondary swelling stage, the DE material has been solidified. The schematic for the morphology model of hydrates of the DE material in the secondary swelling stage is illustrated in Figure
For
Assuming the coal is an isotropic material, the elastic constitutive relation can be expressed as follows:
Incorporating (
For
Parameters of the coal from Huangling no. 2 coal mine.
Density (g/cm3) |
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1.38 | 1.94 | 3.26 | 117.5 | 10.2 | 0.85 | 17 | 31.5 |
Comparison between experimental data and simulated results.
Physical model and boundary conditions.
Distribution of vertical stress around borehole. (a) No active support, 1d. (b) No active support, 2d. (c) No active support, 5d. (d) Active support, S1-1d. (e) Active support, S1-2d. (f) Active support, S1-5d.
Figure
Distribution of the plastic region around the borehole. (a) No active support, 1d. (b) No active support, 2d. (c) No active support, 5d. (d) Active support, S1-1d. (e) Active support, S1-2d. (f) Active support, S1-5d.
The stability of the borehole weakens over time due to the creep of the coal, which is consistent with the results of Liu and Paraschiv-Munteanu et al. [
To compare the gas drainage borehole sealing effect for the DE material and a conventional material, field tests were carried out at panel 207 of Huangling no. 2 coal mine in China. The buried depth of the panel 207 is 369 m–413 m, and the dip angle ranges from 4° to 7°. The gas pressure of the panel is 0.29 MPa–1.51 MPa, and the gas content ranges from 6.75 m3/t to 14.52 m3/t. The coal seam permeability in this panel is measured as 6.7 × 10−17 m2∼3.6 × 10−16 m2, with a mean value of 2.5 × 10−16 m2. The conventional sealing material used for the field test consisted of OPC, water reducer, and aluminum powder (mass ratio of 1 : 0.006 : 0.002). This material has a free swelling ratio of 15% and a maximum swelling pressure of 50 kPa under a water-solid ratio of 0.6 : 1. In this panel, forty coal seam boreholes were prepared and divided into two groups. The boreholes in Group 1 were sealed by the DE material, and the boreholes in Group 2 were sealed by the conventional sealing material. The sealing length of the gas drainage borehole in the panel 207 is 16 m. After sealing, the boreholes were connected to the drainage net with a negative drainage pressure of −15 kPa.
Figure
Gas drainage concentrations in the field test.
To improve coal seam gas drainage performance, we developed a DE material for borehole sealing. This material is composed of OPC, mineral powder, a solid expansion agent, bentonite, naphthalene water reducer and aluminum powder. The swelling process of the DE material includes an initial swelling stage affected by the foaming effect of the aluminum powder and a secondary swelling stage induced by the growth and formation of the crystals. According to the results, the key conclusions are summarized as follows: The swelling test device was used to test the swelling pressure of the DE material under four constraint conditions. The swelling pressure properties of the DE material revealed that (a) the swelling pressure of the DE material is time-dependent during its hydration, and the relationship described here of the swelling pressure and time can describe this time-dependent behavior accurately and (b) the radial swelling potential is principally formed during the secondary swelling stage. The stability of a gas drainage borehole weakens over time due to the creep of the coal. However, the active support imposed on the hole wall can efficiently inhibit the enlargement of the damage region and the plastic region, for improved stability of the gas drainage borehole. Field tests results show significantly improved gas drainage performance of the borehole sealed by the DE material. The DE-sealed borehole had an average gas concentration in 50 days of 39.1%, more than twice that of the borehole sealed by the conventional sealing material.
With the swelling of the DE material, the radial swelling pressure does work to the hollow cylinder. Part of the radial swelling potential is converted to elastic deformation energy and is saved in the cylinder, and this can be calculated by the integration method. As shown in Figure
Unit division of the hollow cylinder.
Based on the elastic theory, the stresses and strains in the rectangular unit can be expressed as
Incorporating (
Then, combining with (
Finally, incorporating (
No data were used to support this study.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This work was supported by the Henan Province Science and Technology Innovation Talent Program (no. 164200510002).
A dataset is provided as supplementary materials, which includes the experimental results of free swelling ratio, EDS data, and swelling pressure of the DE material and the values of RSPI calculated.