Enhanced Water Resources Risk from Collocation of Disposal Wells and Legacy Oil and Gas Exploration and Production Regions in Texas

An existing probabilistic risk assessment (PRA) for Class I hazardous waste (Class IH) injection well systems is extended for dynamic risk analysis and modified for Class II disposal well systems, which inject large volumes of "exempt" oil and gas (OG) exploration and production (E&P) waste. Disposal system failure is release of waste to the biosphere, including underground sources of drinking water. Comparative PRA analysis of generic Class IH and generic Texas Class II disposal systems suggests Class II systems are 50 times more likely to fail than Class IH systems due to different requirements for pressure monitoring, waste migration monitoring, and number of confining layers. The generic Texas Class II disposal well PRA was extended to portray a generic disposal system in Dimmit County, Texas, which is a legacy OG E&P region, and to account for the increased likelihood of waste migration pathways from unknown wells and plug failure of abandoned wells. Legacy oil-related activities increase the probability of waste migration pathways because well construction, regulatory protections, and information tracking technologies have improved during the last 40 years. The system failure likelihood for Class II disposal well systems, collocated with legacy and active OG activities, is twice that for generic Texas Class II disposal systems, and failure likelihood increases with the assumed proportion of unknown wells to known wells.