Sequential evaluation of the potential geologic repository site at Yucca Mountain, Nevada, USA

This paper discusses the changes that are planned for the characterization program at Yucca Mountain due to budget changes. Yucca Mountain is the only site being studied in the US for a geologic repository. Funding for the site characterization program at Yucca Mountain program was cut by roughly one half from the 1994 projected budget to complete three major milestones. These project milestones included: (1) a time-phased determination of site suitability, and if a positive finding, (2) completion of an Environmental Impact Statement, and (3) preparation of a License Application to the US NRC to authorize repository construction. In reaction, Yucca Mountain Site Characterization Project has shifted from parallel development of these milestones to a sequenced approach with the site suitability evaluation being replaced with a management assessment. Changes to the regulatory structure for the disposal program are under consideration by DOE and the NRC. The possibility for NRC and Doe to develop a site-specific regulatory structure follows from the National Energy Policy Act of 1992 that authorized the US EPA to develop a site specific environmental standard for Yucca Mountain

Transport and Fate of Natural Gas and Brine Escaping from a Hydrocarbon Reservoir Through a Failed Deepwater Well in the Oceanic Subsurface of the Gulf of Mexico

The possibility of broaching, or the release of fluids at the seafloor due to a damaged or faulty well, is a hazard that must be assessed in the well permitting process. This paper describes a numerical simulation study of a real-life scenario where a complex, permeable sandy formation, connected to the seafloor via known chimneys/seeps, is intersected by a damaged production well that drains another deeper, gas-bearing formation. The objective of the study is to determine the transport and fate of hydrocarbon reservoir fluids (gas and brines) escaping into the sandy formation through the casing shoe of the failed well, and to determine the time it takes for these contaminants to reach the ocean floor. We conducted a detailed simulation study to represent the conditions, properties, and behavior of the system under such failure conditions, and we investigated the migration of gas and brine for a range of reservoir and chimney properties. A key conclusion is that, for such complex systems, modeling the three-dimensional geometry of the system in detail is the key to describing transport and assessing the time and magnitude of potential releases. For the system studied here, transport times range from under 2 years (highest permeabilities) to many decades, ensuring significant time to respond to potential broaching hazards. Under the conditions investigated in this study, we also determine that gas-dominated releases associated with low rates of water flow into the sandy formation are likely to cause hydrate formation that can reduce permeabilities in the colder, upper regions of the chimneys and possibly mitigate releases

Evaluation of hydrocarbon broaching after subsurface containment failure, Gulf of Mexico

Broaching refers to the release of hydrocarbons at the seafloor after a loss of subsurface well containment. An underground blowout may allow migration of fluids upward through adjacent formations or the annulus to the seafloor. After the BP 001 Macondo well was capped, an oil slick was observed near the well. Available data were interpreted to determine if the slick was reservoir fluid broaching the seafloor or if it was a natural seep in the Gulf of Mexico. The experience revealed that no basis existed for estimating subsurface broach rates or how a potential to broach should be considered during well permitting decisions. Lawrence Berkeley National Laboratory (LBNL) modeled two broaching failure scenarios selected by the Bureau of Ocean Energy Management (BOEM) and representative of Gulf geosystems. Geoscientists from BOEM provided reservoir parameters and interpreted geologic surfaces from depth-migrated three-dimensional seismic volumes. The LBNL adapted existing supercomputing capabilities to model the traveltime for formation fluids to reach the seafloor after subsurface containment failure. The most sensitive parameters affecting traveltime are the mixture of formation fluids and the permeability of the geologic media through which it passes. Assuming nondepletion of reservoir pressure, gas-prone multiphase fluids may broach within a matter of decades. Wet oil and oil-prone multiphase fluids are slower by one to three orders of magnitude. The likelihood for the modeled systems to broach was mainly dependent on casing failure depth. The BOEM concluded that casing failures greater than 7500 ft (>2280 m) below mudline (seafloor) will not broach within a 6-month time frame of regulatory concern