MIGRATION OF NATURAL CO2 UP A FAULT SYSTEM AT GREEN RIVER, UTAH

Natural CO₂ systems provide invaluable information on the processes which control retention and potential migration pathways for anthropogenic storage of CO₂ in geological formations. Natural CO₂ has been leaking up fault systems for > ~ 100,000 years at Green River, Utah, now enhanced by leakage through many recently abandoned drill holes. Scientific drilling adjacent to the Little Grand Wash fault in 2012 has established the mechanisms of CO₂ transport at shallower stratigraphic levels and the consequent reactions between CO₂-charged brines and formation minerals. Corrosion of caprocks is restricted to a few centimetres and involved dissolution of haematite and dolomite with precipitation of pyrite and Fe-dolomites. CO₂-rich brines were encountered at shallow levels and fluid samples collected at pressure in the underlying Navajo sandstone were CO₂-saturated in the lower half of the formation and close to saturation in the upper half of the formation. Given the marked increase of solubility of CO₂ with pressure at these depths, this requires that both CO₂-saturated brines and free CO₂ must be migrating up the fault zone from a CO₂ source at depth. Numerical modelling illustrates how groundwater flow in the Navajo sandstone dissolves any free CO₂ escaping the fault zone and buffers the flow of denser CO₂-saturated brines away from the fault. It is estimated that > 80% of the CO₂ reaching the level of the Navajo sandstone is removed by dissolution in the groundwater. This implies that CO₂ migrating up fault systems through multiple stacked aquifers may largely be dissolved in formation fluids and not reach the surface.