| The methane extraction via in-seam borehole is an important measure for methane disaster management of coal mine.However,due to the air leakage of the surrounding rock of the boreholes,the methane extraction concentration of in-seam boreholes tends to decay rapidly within a short period of time.Therefore,the research of grouting and plugging technology is of great significance to improve the methane extraction effect of in-seam boreholes.In this paper,the methods of combining theoretical analysis,physical tests and field measurements are used to systematically study the failure mechanism and grouting technology of in-seam boreholes.Based on the above research,considering the adverse effects of the surrounding rock instability and deformation on the methane extraction,an integrated grouting and plugging technology for sealing and protection of in-seam borehole that can greatly improve methane extraction performance is proposed.The main research results are as follows:Based on the rock mechanics theory,the relationship between the "stress-strain-permeability" of the coal rock is studied.Under uniaxial stress loading,the coal rock will go through the compaction stage,the elastic deformation stage,the plastic deformation stage and the failure stage in turn.The number of fractures in coal rock is different in different stages,and the permeability is different.In the failure stage,the permeability of the coal rock is the largest.And in the compaction stage,the permeability of the coal rock is the smallest;Taking into account that the air leakage is a cumulative dynamic damage process of the gradual development and creep of the fractures in the surrounding rock of the borehole under the influence of stress and time.This paper studies the dynamic evolution mechanism of borehole surrounding rock fractures under the coupling of time and space,and clarifies the leakage mechanism of borehole “air leakage loop” consisting of mine roadway,air leakage channel in the surrounding rock of borehole and methane drainage pipe.The three main influencing factors that exacerbate the air leakage of the in-seam borehole,including improper sealing depth,inadequate sealing inside the borehole and unstable deformation in the surrounding rock of borehole,are theoretically analyzed.Taking into account the effects of stress disturbance and long-term creeping deformation of coal rock,multiple groups of physical experiments are designed to reveal the evolution characteristics of borehole surrounding rock fractures.The compressive evolution test of borehole surrounding rock fractures is carried out to analyze the relationship between stress loading and the development of fractures.When the loading stress is within 2 MPa,the fractures in the coal sample decrease with increasing loading stress.When the loading stress is higher than 2 MPa,the fractures in the coal samples increase with the increase of loading stress;A constant-pressure creep test for surrounding rock fractures of the in-seam borehole is conducted to analyze the relationship between loading time and fractures development.With the increase of creep time,the fractures in the coal samples increased.And after 8 hours of creep deformation,the fractures in the coal samples is basically stable;The length of the surrounding rock fractures is less than 10 mm.Among them,the proportion of fractures in the surrounding rock of borehole with a length of 5 mm is 58.3%.The ratio of lateral fractures and vertical fractures in the surrounding rock of the borehole is 3:5.The vertical fractures development speed relatively faster,and the smaller the fracture size,the larger the proportion of vertical fractures;In the area of 0-30 mm from the orifice,fractures in the surrounding rock of the borehole are relatively developed.After the application of one-time grouting and staged grouting,the number of fractures in the surrounding rock of the borehole decrease significantly,and then the fractures in the surrounding rock of the borehole continue to develop,and the fracture development and expansion rate are similar.After the use of staged plugging sealing,the fractures in the surrounding rock of the borehole can be significantly reduced again.In order to improve the methane extraction concentration of the in-seam boreholes,based on the above research,an integrated grouting and plugging technology for sealing and protecting borehole is proposed.This technology not only can cut off the air leakage channel of the surrounding rock of borehole in different time and space,but also can protect the instability and deformation of surrounding rock at the same time.Considering that the leakage fractures are mainly distributed in the fracture zone and plastic zone of the surrounding rock of the roadway,the new technology is correspondingly divided into the cavity section and the reserved section to realize staged grouting and plugging;At the same time,providing accurate active support for the easily deformed section of the surrounding rock in the stress concentration area and pressure relief area of the roadway to effectively support the borehole;According to the integrated working principle of the new grouting and plugging technology,a special bag-type borehole sealing device is designed;Among them,the bag can actively support the unstable deformation section after the grouting expansion,and the sealing section between the bad can successfully achieve the staged grouting plugging process;In addition,the new grouting sealing technology process can be divided into the preparation stage,the first grouting stage and the second plugging stage in turn.Field test is carried out in a high gas mine of Shanxi province.The results show that,after the application of integrated grouting and plugging technology for sealing and protecting borehole,the effective extraction time of the in-seam boreholes has increased from about 40 d to about 80 d.The effective extraction periods of the in-seam boreholes using the new method can be extended by more than 2.1 times.And during the entire drainage period,the average methane concentration of the boreholes after using the new grouting and plugging technology should be relatively higher.Moreover,it can simultaneously solve the problem of borehole collapse in the stress concentration zone and fracture zone of the surrounding rock of the roadway. |
