To solve the problem of unstable sealing in the sealing section of a gassy, soft coal seam, a seal reinforcement material for gas extraction boreholes was developed, which was mainly made of ordinary Portland cement and blended with additives such as aluminium powder, quicklime, and gypsum. Firstly, in order to obtain the necessary expansion and compressive strength of reinforcement material, key factors affecting the material properties were determined. Key factors affecting the expansion properties and compressive strength of reinforcing materials were investigated by a single-factor test. Moreover, according to the central combination (Box–Behnken) experimental principle and response surface analysis (RSA), the interactions of various factors on the expansion and compressive strength were determined, and the optimal experimental conditions were acquired. The experimental results indicated that the optimum ratio of the material was 2% for gypsum and 0.52% for aluminium powder and quicklime at the experimental temperature of 20°C, and the ratio of water to material was 0.6. Finally, in the N1103 working face of No. 3 coal seam of Yuwu coal mine, Luan Group, China, the sealing property of the reinforcement material was validated, and the problem of hole collapse at the borehole orifice was solved (resulting in a gas concentration 2.48 times that measured before borehole reinforcement), and the gas drainage effect was enhanced.
A large amount of gas is often associated with a coal seam and is a valuable source of clean energy, and at the same time, it can threaten the safety of mine operation [
Many researchers have investigated sealing materials [
In addition, some researchers studied the optimal sealing effect of sealing materials by means of mathematical modelling and numerical simulation. Under the condition of different sealing lengths, Guan-Hua et al. [
Most of the published studies on borehole sealing materials have been limited to borehole seals, which belong to the sealing materials with high expansibility and low compressive strength and are not suitable for the sealing of drilled holes in high-gas loose soft coal seams. Soft coal makes a high-gas soft coal seam prone to collapse when the drill pipe exits the borehole, which makes it difficult to reach the necessary sealing depth [
The compressive strength of reinforced materials was determined as follows: the coal seams are divided into extremely hard coal seam 5.0 >
To meet the requirement that the compressive strength of the reinforcement material is 15 MPa to 40 MPa, cement-based material was selected as the base material of the reinforcement material [
The key factors affecting the expansion source of reinforcement materials are as follows: Aluminium powder: the main component of cement is silicate, which is soluble in water and weakly alkaline after hydrolysis. After adding aluminium powder into the cement, the chemical reaction of aluminium powder in the alkaline solution produces hydrogen, which will cause the volumetric expansion of the cement, compensate for the shrinkage of the cement slurry, play the role of expansion filling, and improve the sealing performance of the cement [ CaO: with the hydration of Cao in water, Ca(OH)2 is continuously generated and the lattice volume grows. In addition, a proper amount of CaO can be used as reactant to consume Al(OH)3, promoting the reaction to the right to produce hydrogen and achieve the desired expansion; too much alkaline material not only affects the early deformation performance of cement, but also causes the early deterioration of cement structure due to the segregation and cracking sensitivity induced by alkali, which also has a great impact on the early hydration and microstructure formation of cement [ CaO dissolves in water to form Ca(OH)2, and the reaction is exothermic, as shown in the following equation: Aluminium powder reacts with water to generate hydrogen and Al(OH)3: Al(OH)3 is an amphoteric substance, which reacts with strong bases: Gypsum: the gypsum used in the experiment is gypsum dihydrate (CaSO4·2H2O). Its influence on the expansion performance of sealing materials is mainly concentrated in the reaction with tricalcium aluminate (C3A) in the cement component to produce calcium vanadate (AFt). The expansion is based on water absorption-induced swelling, and the expansion effect is produced by the increase of lattice volume, but the contribution to the expansion is generally small [
Moderate increases in gypsum content can enhance the strength of cement and strengthen and support the surrounding rock after the hole is sealed, but when the gypsum content is excessive, it continues to react with the solid state of hydrated calcium aluminate to generate high sulphur content hydrated calcium aluminate after the cement is hardened, with the volume increased by about 1.5 times, causing cement stone cracking, collapsing the hole during the second drilling operation.
The single-factor experiment was used to investigate the effects of A (Al), B (gypsum), and C (the ratio of water to material) on the expansion properties and compressive strength of the reinforcement materials in the experiment (assuming there is no interaction between the three factors). The expansion ratio and compressive strength are the main objects of analysis, and the time of condensation was used as the secondary analysis object. Finally, the selection range of each other amount of admixture was determined. It is worth noting that, in the single-factor experiment stage, since aluminium powder and CaO are two reactants inducing gas expansion, they are not separable and were equally divided and repeatedly prepared as a swelling agent for these experiments. The multifactor experimental level design was determined according to the conclusions obtained from the single-factor experiment [
To avoid the limitation of single variable experiment and consider the influence of interaction among multiple factors, a multifactor experiment was conducted to study the significance. According to the single-factor experiment in the early stage, three main influencing factors were selected: A (Al), B (gypsum), and C (ratio of water to material). A total of 17 groups of experimental schemes were designed with three factors and three levels (Tables
Factors and their levels used in the Box–Behnken design.
Factor | Unit | Level | ||
---|---|---|---|---|
Aluminium powder content | % | 0.43 | 0.52 | 0.68 |
Gypsum content | % | 1 | 2 | 3 |
Ratio of water to material | 0.5 | 0.6 | 1 |
Experimental plans and results.
Class number | A | B | C | E | S |
---|---|---|---|---|---|
1 | 0.52 | 1.00 | 0.50 | 10.9 | 11.1 |
2 | 0.52 | 2.00 | 0.60 | 10.5 | 10.6 |
3 | 0.52 | 3.00 | 1.00 | 8.7 | 7.8 |
4 | 0.52 | 1.00 | 1.00 | 7.4 | 5.7 |
5 | 0.52 | 2.00 | 0.60 | 10.5 | 10.6 |
6 | 0.68 | 2.00 | 1.00 | 9.6 | 5.6 |
7 | 0.43 | 1.00 | 0.60 | 7.5 | 11.8 |
8 | 0.68 | 1.00 | 0.60 | 12.1 | 8.2 |
9 | 0.43 | 2.00 | 1.00 | 6.9 | 8.9 |
10 | 0.68 | 2.00 | 0.60 | 12.4 | 8.7 |
11 | 0.52 | 3.00 | 0.50 | 11.9 | 12.2 |
12 | 0.43 | 2.00 | 0.50 | 9.5 | 15.6 |
13 | 0.68 | 3.00 | 0.60 | 12.9 | 9.4 |
14 | 0.68 | 2.00 | 0.50 | 13.8 | 10.6 |
15 | 0.43 | 2.00 | 0.50 | 9.3 | 14.3 |
16 | 0.52 | 1.00 | 0.60 | 9.8 | 10.1 |
17 | 0.43 | 3.00 | 0.60 | 8.2 | 13.2 |
E denotes the expansion ratio and S is the compressive strength.
Expansibility experiment of reinforcement materials was assessed by first, weighing a set amount of cement, and then the additive was weighed according to the experimental design table and added to the cement for uniform stirring [
Compression strength testing of reinforcement materials was carried out [
So as not to affect the smooth operation of downhole drilling, considering the possibility of field operation and the time taken, it is necessary to strengthen the material to meet the required compressive strength within 48 h (above 15 MPa). Therefore, the compressive strength of the reinforcement material at 48 h was measured.
The experiment determined that the aluminium powder and CaO doses were 0%, 0.43%, 0.52%, and 0.68%; the experimental temperature was 20°C; and the ratio of water to material was 0.6.
As shown in Figure
Effects of aluminium and quicklime content on material properties. (a) Effects of aluminium and quicklime content on expansion. (b) Effects of aluminium and quicklime content on solidification time and compressive strength.
In Figure
Effect of gypsum content on material properties. (a) Effect of gypsum content on expansion. (b) Effect of gypsum content on solidification time and compressive strength.
Effect of the ratio of water to material on material properties. (a) Effect of the ratio of water to material on expansion. (b) Effect of the ratio of water to material on solidification time and compressive strength.
The experiment determined that the amount of gypsum added was 0%, 1%, 2%, and 3%; the experimental temperature was 20°C, and the ratio of water to material was 0.6.
Figure
Figure
The experiment determined that the amount of the ratio of water to material was 1, 0.6, and 0.5; the aluminium powder and calcium oxide contents were both 0.52%, the gypsum content was 2%, and the experimental temperature was 20°C.
As seen from Figure
Figure
To elucidate the effect of the interaction of three factors on the reinforcing material, a 17-group orthogonal experiment (Table
As can be seen from Table
Source | Sequential | Adjusted | Predicted | |
---|---|---|---|---|
Linear | <0.0001 | 0.9239 | 0.8845 | |
2FI | 0.4083 | 0.9250 | 0.8414 | |
Quadratic | 0.0007 | 0.9893 | 0.9500 | Suggested |
Cubic | 0.1535 | 0.9976 |
Figure
Normal probability distribution diagram of the studentised residuals.
According to the quadratic model multiple regression equation, the optimal contour lines between the respective variables and the response surfaces between the respective variables are drawn (Figures
Optimised contours between the respective variables. (a) Optimised contour between A and B. (b) Optimised contour between A and C. (c) Optimised contour between B and C.
Response surface between the respective variables. (a) The response surfaces of A and B. (b) The response surfaces of A and C. (c) The response surfaces of B and C.
The shape and trend of contour lines can reflect the intensity and significance of the interactions between the two factors. As illustrated in Figure
Figure
As can be seen from Table
Source | Sequential | Adjusted | Predicted | |
---|---|---|---|---|
Linear | <0.0001 | 0.4507 | 0.9150 | |
2FI | 0.7690 | 0.3865 | 0.9008 | |
Quadratic | 0.0100 | 0.8493 | 0.9693 | Suggested |
Cubic | 0.8493 | 0.9430 |
In Table
Response surface quadratic model and analysis of variance results.
Source | Sum of squares | Degree of freedom | Mean square | |||
---|---|---|---|---|---|---|
Model | 116.93 | 9 | 12.99 | 57.15 | <0.0001 | Significant |
A | 26.87 | 1 | 26.87 | 118.21 | <0.0001 | |
B | 3.96 | 1 | 3.96 | 17.40 | 0.0042 | |
C | 56.02 | 1 | 56.02 | 246.41 | <0.0001 | |
AB | 4.579 | 1 | 4.579 | 0.020 | 0.8911 | |
AC | 0.25 | 1 | 0.25 | 1.12 | 0.3259 | |
BC | 0.35 | 1 | 0.35 | 1.55 | 0.2528 | |
A2 | 2.97 | 1 | 2.97 | 13.05 | 0.0086 | |
B2 | 0.37 | 1 | 0.37 | 1.62 | 0.2439 | |
C2 | 1.13 | 1 | 1.13 | 4.97 | 0.0610 | |
Residual | 1.59 | 7 | 0.23 | |||
Lack of fit | 0.75 | 5 | 0.15 | |||
Pure error | 0.84 | 2 | 0.42 | |||
Cor. total | 118.52 | 16 |
According to the quadratic model multiple regression equation, the optimal contours between the respective variables and the response surface between the respective variables are drawn (Figures
Optimised contours between the respective variables. (a) Optimised contour between A and B. (b) Optimised contour between A and C. (c) Optimised contour between B and C.
Response surface between the respective variables. (a) The response surfaces of A and B. (b) The response surfaces of A and C. (c) The response surfaces of B and C.
It can be seen from Figures
Figure
In order to further analyse and verify the experimental results, the optimised experimental scheme was obtained by Design-Expert, and then five sets of recommended experimental schemes were selected for verification.
Table
Optimised ratio and result verification.
No. | Al (%) | Gypsum (%) | Ratio of water to material | Predictive expansion ratio (%) | Experimental expansion ratio (%) | Expansion ratio error | Predictive compressive strength (MPa) | Experimental compressive strength (MPa) | Compressive strength error (MPa) |
---|---|---|---|---|---|---|---|---|---|
1 | 0.51 | 1.37 | 0.6 | 10.7 | 10.3 | 0.4 | 15.1 | 14.8 | 0.2 |
2 | 0.52 | 1.86 | 0.55 | 10.5 | 10.3 | 0.2 | 15.2 | 15.5 | −0.3 |
3 | 0.52 | 2 | 0.6 | 10.4 | 10.5 | −0.1 | 15.4 | 15.3 | 0.1 |
4 | 0.52 | 1.88 | 0.53 | 10.9 | 10.2 | 0.7 | 15.2 | 15.7 | −0.5 |
5 | 0.53 | 1.05 | 0.6 | 10.4 | 10.7 | −0.3 | 15.6 | 15.2 | 0.4 |
In summary, in order to ensure the practicability and simplicity of the fieldwork, under the premise of meeting the experimental requirements, the compressive strength is about 15.6 MPa, and the expansion ratio is about 10%. The optimised experimental conditions were selected as follows: aluminium powder and quicklime content 0.52%, gypsum content 2%, and the ratio of water to material 0.6.
The N1103 working face of No. 3 coal seam in Yuwu coal mine of Lu’an Group in Shanxi Province was selected for field testing. The gas content of the coal is high (the gas content of the raw coal is 3.06–23.69 m3/t), and the coal quality is relatively soft (0.6 <
To investigate the practical application effect of drilling reinforcement seal, in the field test, the method of drilling combination cross was used. A total of 24 experimental boreholes were divided into four groups: A, B, C, and D: each group had three optimised boreholes and three ordinary boreholes. After the completion of hole sealing, the gas concentration of 24 holes was monitored for 2 months, and the changes in gas content in the holes are demonstrated in Figure
Changes in gas concentration in boreholes with time. (a) Gas content in group A. (b) Gas content in group B. (c) Gas content in group C. (d) Gas content in group D.
It can be seen from Figure
After 60 days, the average gas concentration of the reinforced hole is 44.62%, the average gas concentration of the unreinforced hole is 17.92%, and the gas concentration of the reinforced hole is 2.48 times that of the unreinforced hole. This is because the collapse of the opening section of the ordinary borehole without reinforcement occurred, and the insufficient feeding position of the gas drainage pipe led to the short length of the sealing section of the borehole and the failure to block the gas flow between the borehole and the roadway, resulting in the low concentration of gas drainage. The results confirm the effectiveness of drilling reinforcement and the practicability of the proposed reinforcement materials.
To solve the problem of hole sealing (whereby it is easy to lose stability in the sealing section of high gas content, soft coal seams), a new type of drilling reinforcement sealing material was developed. At the same time, the better to combine with the field test in Yuwu coal mine, the following conclusions can be drawn: Through the division of the hardness of soft coal seams and coal cutter, the range of compressive strength of reinforcement material was determined. Based on cement-based materials and the mechanism of chemical reaction, the influences of each component of reinforced cement material and the ratio of water to material on its expansivity were studied. A new reinforcement material supplemented with additives such as aluminium powder, quicklime, and gypsum for drilling holes was developed. The influences of aluminium powder, gypsum, and the ratio of water to material on the expansivity and compressive strength of the solidified material were studied by single-factor experiment, and the influence of the interaction of three factors on the reinforced material was analysed by a response surface method. A reinforced material with an expansion ratio of about 10% and compressive strength of about 15 MPa was obtained. Verified by experiments, the optimal ratio of key components was obtained as follows: aluminium powder (quicklime), 0.52%; gypsum, 2%; water-to-material ratio, 0.6. The better to verify the effectiveness and reliability of the reinforcement material parameters obtained from the physical simulation experiment, Yuwu coal mine was selected as a test bed upon which to validate the effect of the reinforcement material. The gas drainage concentration of the reinforced hole decreased from 80.815% to 44.62%, which was 2.48 times higher than that of the unreinforced hole, achieving the purpose of hole reinforcement and improving the gas drainage effect.
The experimental data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This work was financially supported by the National Natural Science Foundation of China (Grant nos. 51974241, 51674189, 51874233, and 51327007).