# Study of the Migrating Mine Gas Piston Effect during Reactivation of Tectonic Faults
## 3 Computer-Generated Model and Its Parameters
#### 3.1.2 Description of the Computer-Generated Model  
The graphic showing mine gas migration associated with fault reactivation is provided in Figure 2.  
Gas flow model for migrating gas by piston effect is solved by CFD ANSYS 16-CFX software. The simulation is carried out in a transient setting. Air ideal gas and methane CH4 are used as the fluids. Turbulence model is the k-Epsilon. The gravitational force \(g\) is directed along the y axis. \"Rock\" and \"Excessive fissuring zone\" are presented in the form of porous domains. \"Cavity\" and \"Air\" are represented as fluid domains. Some parameters of the simulation are shown in Table 1.  
The process of gas release into a cavity (void space) located in a rock mass was simulated (Figure 3). The cavity represents a domain filled with mine gas. This cavity has inner and outer surfaces. The inner surface is used as a zone whereupon the pressure surge is set. The outer surfaces border on rock mass and on the zone of excessive fissuring. The simulated process is proposed as an analogue of sharp rock displacement process and increasing fluctuating pressure. There is a surface of excessive fissuring zone that is adjacent to an object on the surface, represented by the \"air\" domain. The simulation parameters (creation of a computational mesh, selection of initial and boundary conditions, turbulence models, etc.) were selected, being based on a previously performed study [53].  
In computer simulation, pressure build-up as a result of rock displacement was set as a pressure surge on the inner surface of the cavity (Figure 3b). The rock properties were set as a porous medium with \(1\times 10^{-14}\) m\({}^{2}\) permeability (Figure 4a). The permeability of the fault zone (Figure 4b) was assumed to be \(1\times 10^{-5}\) m\({}^{2}\) and \(1\times 10^{-7}\) m\({}^{2}\). The pressure surge on the inner surfaces of the void is assumed to be 250 Pa and 1000 Pa. Three stages of the process were simulated, each lasting 1 s. In the period between 0 and 1 s (stage 1), the pressure in the fault zone did not change; in the period between 1 and 2 s (stage 2), the pressure increased stepwise to 250 Pa or 1000 Pa; in the period between 2 and 3 s (stage 3), the pressure decreased (gas outflow from area of high pressure).  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **Value** & **Notes** \\ \hline Analysis type & Transient & \\ \hline Total time & 3 (s) & \\ \hline Timesteps & 0.01 (s) & \\ Fluid & Air ideal gas, CH\({}_{4}\) & Material Library \\ \hline Gravity Y Dirn. & \(-\)g & \\ \hline Turbulence model & k-Epsilon & \\ \hline Turbulence & Intensity 5\% & \\ \hline Multiphase & Homogeneous model & \\ \hline Fluid buoyancy model & Density difference & \\ \hline “Excessive fissuring zone” & \(1\times 10^{-5}\), \(1\times 10^{-4}\) (m\({}^{2}\)) & \\ Permeability & \(1\times 10^{-14}\) (m\({}^{2}\)) & \\ \hline “Rock” Permeability & \(1\times 10^{-14}\) (m\({}^{2}\)) & \\ \hline “Air” Volume fraction & 100\% & Air ideal gas \\ \hline “Cavity” Volume fraction & 100\% & CH\({}_{4}\) \\ \hline Pressure increased & 250, 1000 (Pa) & Cavity inner surface \\ \hline \hline \end{tabular}
\end{table}
Table 1: Parameters of the simulation.  
Figure 3: Computer-generated model of the subject of research: (**a**)—general view, (**b**)—close-up view of area with cavity.