IMAGE ANALYSIS OF GAS WELL CEMENT EXPOSED TO COAL MINE WATER

Natural gas wells in Northern Appalachia are often drilled in areas where extensive coal mining has occurred. Many of these coal mines are abandoned and a large portion of these abandoned mines (~40%) are flooded with water (Donovan et al., 2004). This coal mine water ranges in pH from acidic to circumneutral and could have an effect on gas well cement, which has a basic pH. Coal mine water has the potential to alter the microstructure of gas well cement and compromise the well’s zonal isolation. This study uses ImageJ, a public domain, Java-based image processing program, to measure and visualize the porosity of cement cores exposed to sampled coal mine water. As a complement to traditional porosity measurements, image processing can produce contextualized measurements that provide visual and localized insight into the interactions taking place between coal mine water and cement.

In this study, lab-generated cement cores were exposed to sampled coal mine water (pH ~6.6) in static reaction vessels for periods of 2, 4, and 6 weeks. Scanning electron microscope (SEM) images were then analyzed for porosity using ImageJ. Porosity measurements were taken from the interior and four quadrants along the rim. Results show a trend of increasing porosity over time in both interior and rim locations. Interior porosity rose from the 2-week (0.39%) to the 6-week exposure (6.33%). There is an even larger rise in rim porosity, as measurements rose from the 2-week (0.92%) to the 6-week exposure (9.75%), suggesting that the coal mine water was reacting more with the cement rim than its interior.

In addition, the relative standard deviations (7.94-44.28%) for rim measurements suggest that alteration of the cement rim is localized. This localization may be due to the heterogeneous nature of cement, in addition to selective mineral dissolution and the precipitation of expansive minerals in micro-fractures. Documenting such specific cement alteration is a special capability of image processing programs, such as ImageJ, that provides significant insight into how coal mine water affects cement on a localized scale.