ENERGY & MINERALS

Project Overview

Greater Aneth oil field, which is the state’s largest, has produced over 450 million barrels of oil since its discovery in the 1950s and continues to produce today. As production has declined over the years, operators have employed enhanced oil recovery (EOR) techniques to recover as much oil as possible from the producing rock layers, or reservoir. Such techniques include injecting water and gases like CO₂ into the reservoir to mobilize and push remaining oil out of the reservoir. The Aneth Unit site, part of Greater Aneth field, was chosen by the SWP for a demonstration project because this technique was already in practice in other parts of the field, and a pipeline system was in place that supplies CO₂ from a nearby, naturally occurring source in southwest Colorado (McElmo Dome). The goals of the demonstration project are to inject CO₂ into an area of the reservoir where EOR has not been undertaken before, monitor where and how the gas moves, determine whether the reservoir at Aneth Unit and others like it are safe sequestration sites, and demonstrate that CO₂ can be sequestered while increasing production in a mature oil field.

Before injection of CO₂ began, the UGS performed several field analyses to help determine whether the project area was geologically suitable for sequestering CO₂. One main objective was to determine if any faults or other fractures occur in strata of the Aneth Unit area, as they can be potential migration pathways for CO₂ to leak from the reservoir to the surface. First we mapped the surface geology to determine if any faults or fracture systems are present within surface formations. We then mapped subsurface formations focusing on groundwater aquifers, the oil reservoir, and the reservoir seal to determine if any faults are present at depth. The reservoir seal is an impermeable layer above the reservoir that keeps reservoir fluids from migrating to the surface. If any surface faults link with subsurface faults cutting the reservoir seal or reservoir, then CO₂ could migrate or leak to the surface. Or, if any faults connect the reservoir to the groundwater aquifers, then this could lead to CO₂ contamination of aquifers that are critically important to the local communities of Montezuma Creek and Aneth.

Our work showed that no major faults occur in the area. We did, however, find several thousand fractures in surface rocks. These fractures, called deformation bands, are small and are confined to the sandstone units of the Morrison Formation. We observe no evidence of these fractures penetrating into the subsurface. Based on these results, migration or leakage of CO₂ from the reservoir to the surface along faults is unlikely and there are no fault or fracture connections between the reservoir and groundwater aquifers, so CO₂ contamination of the aquifers is unlikely.