Determination of Key Technical Parameters in the Study of New Pressure Sealing Technology for Coal Seam Gas Extraction

Research on the Performance of Cement-Based Composite Borehole Sealing Material Based on Orthogonal Test

In order to achieve better sealing of boreholes, the performance of sealing materials is modified to improve the efficiency of coalbed methane extraction. In this paper, a new type of cement-based hole sealing material was prepared by using silicate cement (PC) and cement sulfoaluminate (SAC) as raw materials, supplemented with various additives, such as fly ash, Na2SO4, Ca(OH)2, and poly(vinyl alcohol) (PVA) fiber. The effects of these additives on the fluidity, setting time, and compressive strength of the PC-SAC compounded cementitious pore sealing material were investigated by orthogonal tests, and the hydration process and hydration products were analyzed by X-ray diffraction (XRD), thermogravimetry-differential thermogravimetry (TG-DTG), and scanning electron microscopy (SEM). The results show that the water-cement ratio has the most significant influence on the various properties of the material; the two additives of Na2SO4 and Ca(OH)2 play a key role in the setting time of the material; the optimal group, i.e., water-cement ratio of 0.5, fly ash of 5%, Na2SO4 of 1%, Ca(OH)2 of 0.75%, and PVA fibers of 0.8%, is obtained by the orthogonal test method, which is the closest to the actual needs of the project. The hydration products of the optimized materials have obvious changes, and the needle-like AFt and C-S-H increase so that the performance of the materials has been significantly improved.

Study of Reasonable Grouting Pressure in the Process of Measuring Coal Seam Gas Pressure and Application

Coal seam gas pressure is an important index to evaluate the risk of coal outbursts. The accuracy of measurement is closely related to the quality of hole sealing, and reasonable grouting pressure is one of the key factors to determine the quality of hole sealing. To obtain a reasonable grouting pressure, a mathematical model for slurry flow was established based on the relationship between the seepage law of the slurry and the properties of the borehole surrounding a rock. According to the conditions of the working face 11111 of the coal seam Ji_15-17 in Pingdingshan No. 13 Coal Mine, the reasonable grouting pressure in the process of hole sealing and pressure measurement were simulated by COMSOL multiphysics numerical simulation software. After comparing the pressure distribution and slurry diffusion characteristics in the borehole under different grouting pressures, it is concluded that the reasonable grouting pressure is 4 MPa. When 4 MPa grouting pressure to seal the hole is used during actual engineering verification, the measured gas pressure is 2.7 MPa, which is more accurate than the result obtained under conditions of sealing with normal pressure grouting.

Discrimination Method of Gas Pressure Measurement in Coal Seams: Physical Experiment and Model Development

Coal seam gas pressure is a key parameter of gas accident control and gas drainage. At present, there are some problems in field pressure measurement, such as long period, and there is often a need to drill more holes to ensure the reliability of pressure measurement. In this research, a physical gas pressure measurement experiment in a coal sample borehole was carried out, and a mathematical model of gas pressure evolution with time was constructed. Based on the OpenFOAM platform and C++ language, a numerical solver was developed, and the mathematical model was verified by the data of gas pressure in coal seam boreholes. The results show that the evolution process of gas pressure in coal seam boreholes can be divided into two stages. In the first stage, the gas pressure increases rapidly, and the pressure change rate decreases continuously. In the second stage, the gas pressure is slow and stable, and the pressure change rate tends to 0. The correlation coefficients between the mathematical model and the field-measured data are more than 0.94, and the calculation and prediction accuracy are high. Therefore, the model can be used to verify the field data during pressure measurement, which has better field significance and application value.