Abstract
The potential for gas loss in inclined coal seams has economic implications in terms of lost resource; potential environmental implications, in terms of gas migration away from the well; and socio-economic implications in terms of the potential to impact groundwater users in gas-bearing basins. This paper investigated the effect of the dip angle on the flux of the methane up-dip, away from a coal seam gas (CSG) production well in a simple, planar, homogenous synthetic models. This paper reports the findings regarding gas movement during the production (pumping) period only, and gas movement after the cessation of pumping is not considered here.
Production well pumping scenarios were simulated using a box model, with a geometry of 1km×1km×5m that assigned representative fluid and rock properties via a dual-continua modelling approach. A model was tilted by 1°, 2°, 3°, 4°, 5°, and 10° to simulate formation bed dip angles. Ten vertical surfaces, distanced 100m, 200m, 300m, 400m, and 500m from the production well were defined in both the up-dip and down-dip directions, and the flux of gas crossing these surfaces were modelled for 80 years.
Results confirm a dependency of the gas flux away from the well upon the dip angle. The simulated flux is not symmetrical across the model due to the different interactions between viscous and buoyancy forces. Dip angle also significantly influences the propagation of the pressure-decline within the model as the compressibility of the aqueous phase becomes transient and non-uniform. At dip angles > 3°, the production well failed to capture desorbed gas at a distance of 500m up-dip from the well at some point in time over the 80 year pumping simulation, around four times higher than a typical production time period for CSG wells. Dip angles > 5° greatly increase the amount of gas flux away from the well (non-captured gas), with the radial well-capture zone steeply decreasing from 5° - 10°. Generally, most desorbed gas is initially captured by the well but gas fluxes away from the well increase over time due to the higher gas generation in down-dip zones. This suggests that some gas produced down-dip of the production well might not get captured by the production well.
This research expands the current understanding of the factors influencing the gas loss during production from dipped coal seams. Although the general effect of dip on buoyancy-induced flow in porous media is intuitive, the effects of formation dip angle on potential gas loss during production, are not documented for CSG applications. While this is a simplified, homogenous model, combined with the well pumping regime and well spacing information, the results of this work can start to inform successful management strategies to reduce gas loss.
