Radionuclide Migration at the Rio Blanco Site, A Nuclear-stimulated Low-permeability Natural Gas Reservoir

Abstract

EXECUTIVE SUMMARYThe U.S. Department of Energy and its predecessor agencies conducted a program in the 1960s and 1970s that evaluated technology for the nuclear stimulation of lowpermeability gas reservoirs. The third and final project in the program, Project Rio Blanco, was conducted in Rio Blanco County, in northwestern Colorado. In this experiment, three 33-kiloton nuclear explosives were simultaneously detonated in a single emplacement well in the Mesaverde Group and Fort Union Formation, at depths of 1,780, 1,899, and 2,039 m below land surface on May 17, 1973. The objective of this work is to estimate lateral distances that tritium released from the detonations may have traveled in the subsurface and evaluate the possible effect of postulated natural-gas development on radionuclide migration. Other radionuclides were considered in the analysis, but the majority occur in relatively immobile forms (such as nuclear melt glass). Of the radionuclides present in the gas phase, tritium dominates in terms of quantity of radioactivity in the long term and contribution to possible whole body exposure. One simulation is performed for 85 Kr, the second most abundant gaseous radionuclide produced after tritium.A geologic model was developed that includes the Mesaverde Group and overlying Fort Union Formation. The upper device was detonated in the lower part of the Fort Union Formation, while the middle and lower devices were detonated in the Mesaverde Group. Both formations are low-permeability shales (permeability ~10 -17 m 2 ), with lenses of slightly higher permeability sandstone spread throughout. The length of some of these lenses is as great as 1,000 m. The formations are hydrostatically pressured; horizontally, the pressure gradient(s) are not well known, as production tests were not run to completion due to the length of time required to reach quasi-steady state. In the vicinity of the emplacement hole, the formations dip approximately 0.19 m m -1 . Formation fluids (gas and water), however, are not confined to the dipping strata such that fluid interfaces, where they exist, are not dependent upon the direction or location of bedding planes.The geologic model was incorporated into a conceptual flow and transport model that includes transport of radionuclides (tritiated water and krypton gas) in a two-phase (gas and liquid) system. The conceptual flow and transport model was developed into a numerical model. The model was implemented into the TOUGH2 computer program, which is a nonisothermal, multicomponent flow and transport code capable of modeling flow in three dimensions. Radionuclides released from the cavity were transported in both liquid and gas phases, and were allowed to partition between phases in accordance with Henry's law. Two types of simulations were conducted: one that investigated flow away from the three nuclear cavities in a regional pressure field, and the other that investigated flow from only the middle cavity toward a producing gas well. In addition, a few simulations were conducted that ...