A dual-continuum model (TOUGH2) for characterizing flow and discharge in a mechanically disrupted sandstone overburden

Abstract

Underground hard coal mining usually disrupts the mechanical equilibrium of rock sequences, creating fractures within minor permeable rocks. The present study employs a dual-continuum model to assess how both fractured and porous sandstone media influence the percolation process in postmining setups. To test the approach, the software TOUGH2 was employed to simulate laminar fluid flow in the unsaturated zone of the Ibbenbüren Westfield mining area. Compared to other coal mining districts in Germany, this area is delineated by the topography and local geology, leading to a well-defined hydrogeological framework. Results reveal good agreement between the calculated and measured mine water discharge for the years 2008 and 2017. The constructed model was capable of reproducing the bimodal flow behavior of the adit by coupling a permeable fractured continuum with a low-conductivity rock matrix. While flow from the fractured continuum results in intense discharge events during winter months, the rock matrix determines a smooth discharge limb in summer. The study also evaluates the influence of individual and combined model parameters affecting the simulated curve. A detailed sensitivity analysis displayed the absolute and relative permeability function parameters of both continua among the most susceptible variables. However, a strong a priori knowledge of the value ranges for the matrix continuum helps to reduce the model ambiguity. This allowed for calibration of some of the fractured medium parameters for which sparse or variable data were available. However, the inclusion of the transport component and acquisition of more site-specific data is recommended to reduce their uncertainty.

Embedded Discrete Fracture Model for Investigating the Effect of Fractures on Gas Hydrate Production

“…25 A two-dimensional model is built to assess how both fractured and porous sandstone media influence the percolation process in post-mining setups at the Ibbenburen Westfeld mining area. 26…”

“…These models view matrix and fractures as interacting media at the same point, with the interaction between these two media defined through an exchange term . As shown in Table , the dual-permeability model has been commonly employed to simulate fractured reservoirs in Tough2 simulator. , Generally, the dual-permeability model is suitable for dense fracture networks that cannot be well defined. On the other hand, there also exist other ways of explicitly expressing fractures (e.g., discrete fracture model), in which the geometry of fractures (e.g., fracture width and direction) can be considered .…”

Ein multiples interaktives Kontinuumsmodell (MINC) zur Simulation des reaktiven Stofftransports in einem Bergbaufolgegebiet: Eine Fallstudie über das Ibbenbürener Westfeld

“…We used the multiple interacting continua approach (MINC Pruess 1983;Pruess and Narasimhan 1985) integrated into the TOUGHREACT software to set up a reactive transport model for the unsaturated overburden of the Ibbenbüren Westfield (For a detailed explanation of the mathematical principles underlaying the MINC modelling employed in this paper, please refer to the Supplementary Information). The model was constructed using the dual continuum fluid flow model developed by Bedoya-Gonzalez et al (2022) as a basis. Specifically, all the calibrated hydraulic parameters defined in the original dual model were maintained, as well as the model geometry.…”

“…Setting θ r to zero would result in the relative permeability nullifying the exchange with the matrix, leading to numerical deficiencies. Note that the parameters used in both the matrix and fracture continua correspond to calibrated values for the best volumetric discharge agreement with respect to the measured data for the Dickenberg adit (Bedoya-Gonzalez et al 2022). Therefore, disturbances of the flow component derived from porosity and permeability changes by dissolution and precipitation of minerals were not considered in this reactive model.…”

“…The importance of considering both fractures and rock matrix has been corroborated by Bedoya-Gonzalez et al (2022) who used a dual continuum model (DCM) to fit the transient mine adit discharge of the Ibbenbüren Westfield. Their results indicate that the fractured continuum is primarily responsible for the intense and short discharge events throughout the months of heavy rainfall.…”