Study on sealing technology and equipment of non-solidified paste materials for large diameter gas extraction boreholes

For large diameter gas extraction boreholes, the solid material sealing method has a good sealing effect at the beginning. However the gas concentration of the extracted gas from the borehole decreases rapidly after the deformation and destruction of the sealing borehole over time. In order to solve these problems, a non-solidified paste material sealing technology for large diameter gas extraction boreholes is proposed. This technology uses expanded cement and non-solidified paste material to form a multi-segment "solid, liquid, solid" structure, uses expanded cement material to form three solid sealing segments, and then injects non-solidified paste material into the solid sealing segments during different extraction time. Hence the technology realizes the effective sealing of the whole process of extraction and the secondary and multiple sealing of the borehole during different extraction time. Based on the theoretical analysis of the fracture radius around the large diameter borehole, the relationship between the optimal grouting pressure and viscosity is numerically simulated. The technical parameters related to the sealing of non-solidified paste material are studied. The optimal grouting pressure is 1.2 MPa and the optimal viscosity is 0.001-0.03 Pa·s. A borehole sealing equipment is developed based on the grouting pressure and viscosity obtained from the research, which utilizes the principle of "solid, liquid, solid" technology to form a multi-segment borehole sealing structure, and realizes the extraction sealing mode of solid sealing liquid and liquid sealing gas. The results of field industrial tests show that the sealing technology of non-solidified paste material in large diameter gas extraction boreholes, combining the characteristics of the paste material changing with the time and space of the extraction, can solve the problems of sealing failure and rapid decay of extraction concentration, which caused by new fracture formed by the solid material sealing due to borehole deformation. Furthermore, the volume fraction of extraction can be increased by about 10% after secondary grouting which can improve the gas extraction rate effectively.