Horizontal Hydraulic Conductivity Estimates for Intact Coal Barriers Between Closed Underground Mines

Abstract

Unmined blocks of coal, called barriers, separate and restrict horizontal leakage between adjacent bituminous coal mines. Understanding the leakage rate across such barriers is important in planning mine closure and strongly affects recharge calculations for postmining flooding. This study presents upper-limit estimates for hydraulic conductivity (K) of intact barriers in two closed mines at moderate depth (75-300 m, or 250-980 ft) in the Pittsburgh coal basin. The estimates are based on pumping rates from these mines for the years ranging from 1992 to 2000. The two mines do not approach the outcrop and are sufficiently deep that vertical infiltration is thought to be negligible. Similarly, there are no saturated zones on the pumped mines' side of shared barriers with other mines, and therefore pumping is the only outflow. Virtually all of the pumping is attributed to leakage across or over the top of barriers shared with upgradient flooded mines. The length of shared barriers totals 24 km (15 mi) for the two mines, and the barriers range in thickness from 15 to 50 m (50-160 ft). K values calculated independently for each of the 9 years of the pumping record ranged from 0.037 m/d to 0.18 m/d (0.12 ft/d to 0.59 ft/d) using an isotropic model of barrier flow. Using an anisotropic model for differential K in the face cleat (K f ) and butt cleat (K b ) directions, results range from 0.074 to 0.34 m/d (0.24 to 1.1 ft/d) for K f and from 0.022 to 0.099 m/ d (0.072 to 0.32 ft/d) for K b .

A simple, low-cost approach to predicting the hydrogeological consequences of coalfield closure as a basis for best practice in long-term management

“…Younger et al 1995;Sherwood and Younger 1997;Burke and Younger 2000;Adams and Younger 2001;Whitworth 2002;Boyaud and Therrien 2004;Winters and Capo 2004;McCoy et al 2006;Gandy and Younger 2007;Hamm et al 2008;Kortas and Younger 2007;Light and Donovan 2015). Post-audits of these applications have shown that they produce useful and realistic results, and that engineering design decisions based on their application have stood the test of time (Younger, 2004;Adams 2014).…”

Bulking Based Porosity Method to Predict Floodable Volume of Longwall Gob

“…Vertical inflow of groundwater is derived from overlying aquifers and surface water sources at shallow overburden conditions [1][2][3] but could also be associated with drainage from overmined seams in multiple seam mining scenarios. Inflow has been shown to occur as horizontal seepage across common barriers between contiguous mines [4,5] and spillage through discrete connections [6] but is typically undifferentiated. Following mine closure or sealing, dewatering pumps are typically deactivated, which leads to resaturation and filling of the abandoned mine voids.…”

Numerical Modeling and Field Monitoring of Overburden Response during Geologic Sequestration

“…Permeability values and gas content values for different coal groups of northern Appalachian coals can be found in published literature (Bruner, 1995). Moreover, anisotropic ratio (face cleat to butt cleat ratio) of 3.4 within the Upper Freeport coal formations has been found (McCoy et al, 2006). Adsorption studies on coal samples from nearby core holes of both the Pittsburgh coal seam and Upper Freeport coal seam were performed by Consol Energy (Winschel, 2009).…”

“…Moreover, the reservoir models also incorporate permeability anisotropy, which is common to coalbed reservoirs. A permeability anisotropy ratio of 3 has been assumed in these models based on the anisotropic values reported in the literature (McCoy et al, 2006). Cleat spacing was varied from 0.025 feet to 0.25 feet based on the published information for coal seams (Law, 1993;Steidl, 1977;Levine, 1996).…”