Influence of hydraulic fracturing on overlying aquifers in the presence of abandoned and converted oil and gas wells : numerical, spatial, uncertainty, and geochemical investigations.

The association between hydrocarbon-rich reservoirs and organic-rich source rocks means unconventional oil and gas plays usually occur in mature sedimentary basins – where large-scale conventional development has already taken place. Abandoned wells in proximity to hydraulic fracturing could be affected by increased fluid pressures and corresponding newly generated fractures that directly connect (frac hit) to an abandoned well or to existing fractures intersecting an abandoned well. If contaminants migrate to a pathway hydraulically connected to an abandoned well, upward leakage may occur. This study evaluated potential risks associated with hydraulic fracturing near abandoned oil and gas wells, with attention to abandoned oil and gas wells converted into water wells. Potential risks of hydraulic fracturing on abandoned oil and gas wells were evaluated with four investigations: (1) development of a numerical model to assess magnitude of upward leakage along a leaky abandoned well in proximity of hydraulic fracturing, (2) characterization of abandoned and converted oil and gas wells and probability of their intersection with stimulated areas surrounding horizontal wells, (3) interrogation of model uncertainty and evaluation of data crucial for future investigations, and (4) preliminary assessment of an organic geochemical fingerprint to identify potential groundwater quality impacts. Collectively, investigations suggest there is potential for abandoned oil and gas wells near hydraulic fracturing to represent a risk to groundwater quality if certain spatial and hydraulic conditions exist. Overall, the results of this study underscore a critical need to evaluate historical oil and gas activities in areas with modern unconventional oil and gas activities, and improve historical and future data contained in public records.