EFFECTS OF SAMPLED COAL MINE WATER ON GAS WELL CEMENT

Unconventional natural gas wells drilled in Northern Appalachia often pass through coal formations before reaching the Marcellus or Utica formations. Many of these shallow coal seams have been mined and abandoned. Flooding of these mines is common and it is estimated that ~40% of abandoned Pittsburgh coal mines are flooded (Donovan et al., 2004). The pH of coal mine water varies from acidic to circumneutral and this water is known to flow through pillars into neighboring mines. Interactions between coal mine waters and gas well cements have the potential to alter the cement and compromise its sealing integrity. This study investigates how sampled waters from an abandoned coal mine affect cements in a laboratory setting.

Static reaction experiments were designed to simulate the interactions between gas well cements and coal mine waters in a shallow subsurface environment. In order to recreate these interactions, water samples were taken from an abandoned bituminous coal mine in southwestern Pennsylvania. Class H Portland cement cores were then exposed to this coal mine water. These batch reaction experiments were conducted in static reaction vessels for time periods of 1, 2, 4, and 6 weeks.

Results from this study suggest that the sampled coal mine water altered the permeability and matrix mineralogy of the cement cores. Scanning electron microscope images display an increase in mineral precipitates inside the cement matrix over the course of the experiment. Permeability values for the cement cores decrease over the 6-week experimental period, suggesting that precipitates inhibit connected pore space. Chemistry results from the reaction vessels’ effluent waters display decreases in dissolved calcium, iron, silica, chloride, and sulfate. These results, coupled with the scanning electron microscope images, suggest that mineral formation involving these chemical species occurred inside the cement cores. Effluent waters also exhibit an increase in potassium during the experiment, suggesting that the coal mine water dissolved potassium based minerals from the cement cores.