Disposal of Produced Water into Depleted Oil Reservoirs: Economic Use and Risk of USDW Pollution

Produced water from unconventional hydrocarbon reservoirs is characterized by high concentrations of total dissolved solids (TDS) and variable concentrations of heavy metals whose concentrations in most cases largely exceed EPA maximum contaminant levels (MCLs). However, produced water is also characterized by containing indigenous microbial communities which are well adapted to extreme deep subsurface (high temperate, pressure and salinity) conditions. Disposal of produced water into depleted oil reservoirs and deep saline aquifers are two common practices to prevent the pollution of underground source of drinking water (USDW) by contaminants contained in produced water. We are developing a new method to stimulate microbial crude oil biodegradation via methanogenesis in depleted oil reservoirs injected with produced water. This method has the potential to enhance the recovery of oil from depleted oil reservoirs. Its potentiality will be discussed based on our experimental results showing that the addition of stimulating nutrients to produced water of certain microbial composition can trigger crude oil biodegradation via methanogenesis if such produced water is injected into depleted oil reservoirs of different microbial composition. The risk of USDW pollution by heavy metals due to the possible upward migration of disposed produced water into deep saline aquifers will be discussed based on our experimental results showing that the mobility of heavy metals is much higher in brine than in freshwater and that salinity promotes the desorption of heavy metals in deep carbonate saline aquifers.