Enhancing the predictability of radon flux in fractured environments, particularly in confined spaces, is a crucial step towards mitigating the profound health risks associated with radon gas exposure. However, previous models on fluid transport through fractured rock have focused on the relationship between radon flux and aperture in fractures and faults. However, there is paucity of understanding on the influence of rock geo-mechanical properties on radon flux. In addition, there are limited methods of characterizing rocks in relation to radon flux. The numerical model presented in this study incorporated rock properties such as Young's modulus and Poisson ratio with rock aperture to develop a dimensionless radon flux for opening-mode fractures, and five dimensionless parameters (e.g., Geofluid number, Decay number, Fracgen number, Geofrac number, and Geopeclet number) were introduced to characterize fractures in terms of radon transport. Furthermore, these newly discovered relationships were used to conduct a series of flow simulations on fracture networks using the discrete fracture network model (DFN). This model establishes a quantitative framework for predicting radon flux through open-mode fractures and the influence of rock geo-mechanical properties.

The numerical modeling of radon concentrations in the fault zone of the underground excavations at Książ Castle was conducted using a stochastic Discrete Fracture Network (DFN) model. Due to the difficulties related with obtaining the exact fractures in a rock mass, the novel approach used in this study incorporates the stochastic model with known site data. The analysis utilized a dataset comprising long-term measurements of 222Rn activity concentration and geodetic measurements for twelve faults in the Książ unit. The parameters considered in the DFN model are: fracture length, Peclet number (Pe = 0.1 and 1.0, respectively), advection velocities (from 10–8 m/s to 10–6 m/s and from range from 10–7 m/s to 10–4 m/s, respectively), radon diffusion (D = 2.1 × 10–61/s), radon decay constant (λ = 1/s), and radon gas generation (q) along the fractures within the range of 1.5 × 10–3 Bq/m3·s to 3.5 × 10–3 Bq/m3·s. The calibration process obtained the best fit when the radon generation rate was uniformly distributed through the rock mass in addition to incorporating a higher value of radon generation rate (q = 3.0 × 10–3 Bq/m3·s) where elevated radon concentrations have been measured. The modeling results also confirmed that the radon generation rate should always be higher where elevated radon activity concentrations were measured regardless of the measurement period. For the indicated “area” the radon generation rate should be higher from 25% to 37.5% between May–October and 18.5% to 40% between November–April. The influence of fracture zones on the recorded radon activity concentrations was noticeable up to a depth of 15 m. Within this range, the highest values of 222Rn activity concentration, ranging from 1,600 Bq/m3 to 2,000 Bq/m3, were consistently observed regardless of the season. However, as the depth increased, the values of 222Rn activity concentration decreased from 800 Bq/m3 to 400 Bq/m3 and became more dispersed. © The Author(s) 2024.

This study simulates the impact of a shale gas well casing breach near a longwall mine. Field studies are conducted to measure mining-induced permeability changes over the abutment pillar of a longwall mine, and a geomechanical model is developed in 3DEC, a three-dimensional numerical modeling code, to predict the aperture of fractures in the overburden at the study site. The predicted aperture values are used to determine mining-induced permeabilities and the results are compared with the field measurements. These aperture values are provided as inputs into fracture flow code (FFC), which generates a stochastic discrete fracture network (DFN) model for the study site and predicts the potential shale gas flow to the mine. Results from 100 DFN realizations are statistically analyzed using the bootstrapping method to compensate for notable variation in fracture geometry. The results show a significant difference between the gas inflow for nearby panels due to increase in the induced permeability during mining of the second panel. The average gas flow to the mine was calculated as 4.72×10−2 m3/s (49 cfm) for a hypothetical breach at the Sewickley horizon during the first panel mining, 8.97×10−3 m3/s (19 cfm) for a hypothetical breach at the Uniontown horizon during the first panel mining, 2.16×10−1 m3/s (458 cfm) for a hypothetical breach at the Sewickley horizon during the second panel mining, and 8.07×10−2 m3/s (171 cfm) for a hypothetical breach at the Uniontown horizon during the second panel mining. Depending on the mine ventilation system, this could result in methane concentrations exceeding regulatory limits. Hence, these findings provide insights into the potential risk of an unconventional gas well casing breach near a longwall mine under shallow cover. © 2023, This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply.

The integrity of unconventional shale gas well casings positioned in the abutment pillar of a longwall mine could be jeopardized by longwall-induced deformations. Under such scenarios, the surrounding fracture networks could provide pathways for gas flow into the mine creating safety concerns. To provide recommendations for developing guidelines that ensure a safe co-existence of longwall mining and unconventional shale gas production, this study evaluates the impact of parameters that could affect potential shale gas flow into the mine in the event of a casing breach using a discrete fracture network (DFN) model. These parameters are evaluated using a conceptualized DFN realization that is representative of the fractured zone in the overburden, and the range of parameter variations is within values validated with field measurements. The results show that a decrease in fracture aperture (potentially due to longwall-induced stress in the likely vicinity of the breach location) reduces the potential gas flow to the mine by a significantly higher proportion. A 50% decrease in the aperture of the fracture that directly transports the gas from the casing breach location reduces the gas flow to the mine by over 70%. Similarly, changes in the fracture water saturation level significantly affect the gas flow. In all cases, the potential gas flow to the mine is higher if the casing breach occurs at an increased gas well pressure. These findings provide critical information regarding the impact of each of the parameters associated with gas flow in the event of a shale gas casing breach near a longwall mine and could help towards the development of guidelines to ensure a safe coexistence of both industries. © 2022, This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply.

Longwall-induced deformations could jeopardize the mechanical integrity of shale gas well casings positioned in the abutment pillar of a longwall mine. The in situ and induced fracture networks surrounding the gas well could provide pathways for gas flow into the mine creating safety concerns. Hence, this study by the National Institute for Occupational Safety and Health (NIOSH) develops a discrete fracture network (DFN) model to characterize the fractures in the overburden based on geomechanical analyses of mining-induced fracture apertures at a study site in southwestern Pennsylvania. The apertures from the geomechanical model are used to develop a stochastic DFN model of the site in fracture flow code (FFC). Multiple realizations of the stochastic DFN model that replicate potential fracture geometries are simulated, and the fracture permeability is compared with field measurements. A maximum field measurement of 5.03 1012 m2 (5080 mD) and 3.82 1013 m2 (386 mD) was estimated over the abutment pillar at the Sewickley and Uniontown horizon, respectively. The results show that the average permeabilities from the DFN model agree closely with the field measurements. In addition, the comparison of all the field measurements and 100 DFN realizations show the model is representative of field conditions. These findings provide critical information regarding fracture characteristics in the overburden, which will further be used to predict potential shale gas flow to the mine in the event of a casing breach for an unconventional gas well. 2022, This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply.

The stability of shale gas wells drilled through current and future coal reserves can be compromised by ground deformations due to nearby longwall mining. Depending on the longwall-induced rockmass permeability, the high-pressure explosive gas from the damaged well may reach mine workings and overwhelm the mine ventilation systems. This study uses geomechanical models to estimate the rockmass permeability induced by mining. A two-panel longwall model of a deep, 341-m-cover mining site in southwestern Pennsylvania is constructed in 3DEC to explicitly model the rockmass by a discrete fracture network (DFN) technique. Stress-induced fracture apertures and permeabilities are calculated across the model and are validated against permeability measurements. A fracture flow code (FFC) is developed to use these results to predict potential inflow to the mine should a gas well breach occur. One hundred DFN realizations are simulated, and the results show that for a gas pressure of 2.4 MPa, the average of the predicted inflow rates to this deep-cover mine is 0.006 m3/s, significantly lower than the average inflow of 0.22 m3/s for a shallow-cover mine (145-m deep) studied in the previous work (Khademian, et al. 2021). The result can help assess the potential hazards of a shale gas well breach for mine safety and evaluate the ventilation requirements to mitigate the risk. © 2022, This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply.

The environmental risks associated with casing deformation in unconventional (shale) gas wells positioned in abutment pillars of longwall mines is a concern to many in the mining and gas well industry. With the recent interest in shale exploration and the proximity to longwall mining in Southwestern Pennsylvania, the risk to mine workers could be catastrophic as fractures in surrounding strata create pathways for transport of leaked gases. Hence, this research by the National Institute for Occupational Safety and Health (NIOSH) presents an analytical model of the gas transport through fractures in a low permeable stratum. The derived equations are used to conduct parametric studies of specific transport conditions to understand the influence of stratum geology, fracture lengths, and the leaked gas properties on subsurface transport. The results indicated that the prediction that the subsurface gas flux decreases with an increase in fracture length is specifically for a non-gassy stratum. The sub-transport trend could be significantly impacted by the stratum gas generation rate within specific fracture lengths, which emphasized the importance of the stratum geology. These findings provide new insights for improved understanding of subsurface gas transport to ensure mine safety.