Modeling Methane Liberation in Underground Coal Mines
Projects to capture and utilize methane released from active and abandoned underground coal mines are becoming more attractive in the United States and abroad as the value of the gas increases and as the need to reduce green house gas emissions becomes more apparent. Raven Ridge Resources can provide reliable reserves estimates and production schedules for projects designed to capture this resource.
Raven Ridge Resources, Incorporated has developed the following approach to modeling methane liberation in underground coal mines:
1. First, Raven Ridge Resources develops a generalized geologic model of the area of interest that incorporates the stratigraphy of the mine area, including rock types and thickness’ of various stratigraphic units. We then predict the geometry of roof and floor relaxation and fracturing after the mining of a longwall panel. This is accomplished by using the Roofgas™ and Floorgas™ finite difference rock mechanics modeling software, developed by Lunagas specifically for this purpose.
Inputs to the modeling software include rate of advance, entry configuration, panel size, and roof and floor lithology. The resulting output shows the potential extent of roof and floor strata involvement in gas migration into the mine workings.
Figure 1. Geometry of Roof Relaxation after Longwall MiningSource: L.W. Lunarzewski, Lunagas Pty Ltd
2. Next, Raven Ridge Resources builds a numerical grid for use in reservoir simulation software. The simulation software can model gas migration through coal and overburden into roadways and gob drainage wells. The grid takes into account the vertical geologic section and significant mine attributes such as currently existing or planned development roadways and longwall panels. Raven Ridge assigns each of the grid nodes a material type, such as coal, sandstone, or shale. We then assign “best estimates” of rock properties to the material types, including porosity, permeability and the methane storage function (adsorption isotherm).
Figure 2. Plan View of an Example Longwall Model Calculation Grid
3. The next step is to perform history matching, an essential part of any flow simulation study that will be used to predict future performance. This may include adjustments to coal and sandstone porosity, permeability, or the methane storage function. The simulation software aids in matching the methane emission rate measured during development mining and/or longwall mining to help establish the characteristics of the coal and surrounding strata. This also helps determine the gas contribution from underlying and overlying coals and other gas-charged strata, and establishes the range of permeability enhancement in the relaxed and collapsed areas.
Figure 3. Emissions History Match
4. Raven Ridge Resources identifies material sinks within the model such as actively ventilated roadways, longwall panels, gob wells, and in-mine drainage wells. Roadways are set at atmospheric pressure, which causes the higher pressured gas within the coal and associated strata to flow toward the roads. The model calculates the gas that is removed by a specific sink which equates to the methane liberated by the mine. Figure 4 below shows a plan view of the pressure distribution at different elevations above the coal seam at the same mine depicted in Figure 2.
Figure 4. Pressure Distribution in Mined and Future Longwalls
Raven Ridge Resources then models gob wells and in-mine drainage wells. A well acts as a material sink whose rate of withdrawal is a function of the difference between a specified pressure, Pwell and the pressure of the model cell in which it is completed, Pcell, multiplied by a factor, commonly known as a productivity index, or PI.
Q = PI * (Pwell – Pcell)
The PI is a function of the cell dimensions, permeability, pressure and size of the wellbore. The gob permeability, and hence the PI, should be adjusted to match actual well performance once a gob well is activated. This will provide a more reliable forecast of gob well production and in-mine emissions. The effect of setting a blower to apply a vacuum at the wellhead is simulated by setting the value of Pwell, or bottom-hole pressure, at the value corresponding to the designed suction pressure of the blower adjusted for friction pressure losses.
6. Finally, Raven Ridge Resources predicts gob well in-mine drainage well gas production and in-mine emissions reductions. Figure 5 shows the in-mine emissions with and without gob wells; the gas that is not recovered by gob wells would be liberated into the mine and discharged through the mine’s ventilation system. The difference between the two curves is the net reduction in atmospheric emissions of methane.
Figure 5. The Effect of Gob Wells on Mine Emissions