Placement of a saltwater disposal well (SDW) within the footprint of a mature oil and gas (O&G) exploration and production region is a bad idea. A long history of O&G exploration means many deep wells (> 5,000 ft below ground surface, bgs) have been installed, and many of these wells were installed prior to modern construction standards, permitting requirements, and data tracking capabilities. Consequently, there are likely to be many poorly constructed and unplugged deep wells whose locations have been forgotten. The purpose of deep well disposal is to segregate the disposal fluids, i.e., harmful waste, from the environment which includes underground sources of drinking water (USDW). Sequestration and containment of harmful wastes is eliminated when there are unknown deep and improperly abandoned wells that pierce containment. This issue of lack of adequate confinement for deep waste disposal is common in Texas because of the prevalence of legacy O&G fields and relatively relaxed permitting requirements for SDWs. This paper demonstrates that locating Texas Class II disposal wells (SDWs) and O&G activities within the same area increases waste containment failure likelihood by 2 times relative to generic SDWs in other states and 100 times relative to Class I hazardous waste (Class IH) injection well systems.

Abstract: 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.