Approximately 40% of the mines in the Pittsburgh coal seam are abandoned and filled with water (Donovan et al., 2004). Many abandoned coal mines in Northern Appalachia fill with water when mining operations cease. While the discharges from these mines, also known as acid mine drainage, have been researched (Cravotta, 2008), coal mine water has received less attention. When unconventional natural gas wells are drilled in Northern Appalachia in order to access the Marcellus and the Utica formations, the wells pass through coal mines or coal formations adjacent to mines filled with coal mine water. Coal mine water may compromise the sealing integrity of the wellbore cement used to line or plug a well. This study aims to improve our understanding of how waters in flooded coal mines may affect gas well cements.
Experiments were designed to simulate conditions that a gas well cement might encounter when a well is drilled either through or near a coal mine and is exposed to coal mine water. A series of Class H Portland cement cores were prepared and exposed to a lab-generated solution, whose properties were based on median values for bituminous acid mine drainage discharges from previous research (Cravotta, 2008). Batch reaction experiments occurred in static reaction vessels for time periods of 1, 2, 3, and 4 weeks.
Results suggest that the lab-generated solution impacted the porosity, permeability, and matrix mineralogy of the cement. Scanning electron microscopy analysis shows a steady and incremental growth of void space and calcite crystals as well as the development of reaction rims during the 4-week period. Permeability tended to increase with increased exposure to the lab-generated solution. The lab-generated solution and the effluent from each reaction vessel were monitored for pH throughout the experiment and analyzed for cations and anions at its conclusion. Although the pH of each solution stabilized around 12 after one week, elemental concentrations varied significantly over the 4-week period. The results suggest that Ca, SiO2, K, and Na containing minerals within the cement matrix dissolved steadily throughout the experiment, while Cl and SO4 forming minerals precipitated within the cement cores.
One of the primary functions of well casing and well cement is to preserve zonal isolation throughout the length of the wellbore. Previous research has identified failures between well casing and well cement as a primary pathway for vertical fluid migration in unconventional wells in Northern Appalachia (Jackson et al., 2013; Vengosh et al., 2014). An acidic fluid in contact with well cement may chemically degrade well cement (Kutchko et al., 2007; Kutchko et al., 2008). This degradation of well cement could create a pathway for vertical migration along the well annulus as well as expose the well casing to the same acidic fluid.