AQUIFER COMMINGLING IN ABANDONED OIL AND GAS WELLS IN THE WESTERN CANADIAN SEDIMENTARY BASIN

A single abandoned commingled well, which connects multiple aquifer units together, has the potential to be a point source of pollution. The combined effects of multiple commingled wells spread over a large area can increase the scale of potential water quality impacts from the immediate zone surrounding a single well to that of an entire aquifer system. Such large scale impacts, including large alterations to the water budgets of individual aquifers, have been widely documented for relatively shallow aquifer systems exploited by irrigation and municipal wells, but the potential impacts abandoned commingled oil and gas wells could be having on the deeper subsurface have been sparsely documented. Such impacts may include the upward migration of brines, hydrocarbons, and injected fluids as well as large alterations to the groundwater flow system. This research looks at two study sites in the Western Canadian Sedimentary basin. At these study sites, historical abandonment practices have left behind wells that commingle together aquifers located below potable groundwater aquifers but above reservoirs typically targeted for oil and gas extraction, otherwise known as the intermediate zone. These same commingled aquifers are also the target of extensive salt water disposal operations, which can lead to pressurization of the aquifer being injected into increasing vertical hydraulic gradients. Higher vertical hydraulic gradients would lead to increased rates of inter-borehole flow between any aquifers commingled in nearby abandoned wells. Our findings indicate the potential for widespread unplanned alteration of groundwater flow regimes in the intermediate zone where older abandoned oil and gas wells are present. These findings have implications for regulations concerning injection wells, and how intermediate zone flow systems should be conceptualized in areas of historical oil and gas production.