# Study of the Migrating Mine Gas Piston Effect during Reactivation of Tectonic Faults
## 6 Conclusions  
Using computer modeling, the hypothesis of a piston effect during the migration of mine gases caused by the reactivation of faults was investigated to explain the cases of the sudden appearance of mine gases on the surface of the earth in coal mining areas and the main conclusions reached are as follows:  
1. In mine conditions, fault planes can be washed with mine water and have a free volume of void space between the asperities, filled with mine gases. The two opposing types of contacts of fault walls are the \"ridge-ridge\" type (maximum volume of void space along a fault plane) and the \"ridge-sag\" type (minimum volume of void space). The height of the asperities and their sizes are, on average, such that a void volume of up to 0.05 m\({}^{3}\) can be adjacent to a fault plane area of 1 m\({}^{2}\).
2. Mining causes the reactivation of tectonic faults during subsidence processes. The size of a reactivation part of a fault plane can reach several kilometers in a strike and hundreds of meters in a depth. The displacement along the fault plane can be several meters in total and up to 10 cm and more during a single shock, which commensurate with the magnitude of the asperities. In strong rocks of coal-bearing strata, asperities with a gentle angle of inclination are deformed elastically without destruction of the frame. In this case, wall displacement along an uneven fault plane leads to an alternation of typical contacts of the wall surfaces from the \"ridge-ridge\" to the \"ridge-sag\" and a sharp decrease in the volume of the void space surrounding the fault plane, which causes a piston effect of squeezing them towards the earth's surface.
3. According to computer modeling, with a shock-like displacement of the fault walls along an uneven fault plane by 1 cm, and even in the case of filling the void space with crushed material, the gas pressure in the fault plane area can increase by 250-1000 Pa in 1-2 s, which is already sufficient for a gas migration starting in the direction of the earth's surface. With a single-act displacement of the fault walls by 20 cm in an area of 200 \(\times\) 200 m, the volume of gas released to the surface can reach 1500 m\({}^{3}\), which poses a real threat to the environment and the safety of the population.
4. As a recommendation, it can be offered to carry out geomechanical monitoring in potentially hazardous--for gas release--places, regardless of the results of traditional gas monitoring.  
The findings expand our understanding of the sudden gasification of earth surface and living spaces in coal mining areas and contribute to the understanding of gas migration process, thereby helping to monitor hazards.  
Conceptualization, A.B.; methodology, A.B. and K.K.; validation, A.B., A.K. and A.I.; formal analysis and numerical modeling, A.K., A.B. and V.M.; investigation, A.B., E.K. and B.Z.; resources, K.K.; data curation, A.B., A.K. and D.K.; writing--original draft preparation, A.B. and A.K.; writing--review and editing, A.B. and A.I.; visualization, E.E. and D.K.; supervision, A.B.; project administration, E.E.; funding acquisition, A.B. and A.I. All authors have read and agreed to the published version of the manuscript.  
This study was supported by a grant No 22-27-00728 from the Russian Science Foundation (RSF), [[https://rscf.ru/en/project/22-27-00728/](https://rscf.ru/en/project/22-27-00728/)]([https://rscf.ru/en/project/22-27-00728/](https://rscf.ru/en/project/22-27-00728/)) (accessed on 15 October 2023).  
Not applicable.  
Data Availability Statement: The data presented in this study are available on request from the corresponding author.  
The authors are grateful for the help to the staff of the department \"Safety and ecology of mining\" of NUST MISIS.  
The authors declare no conflict of interest.