Scientific drilling and downhole fluid sampling of a natural CO₂ reservoir, Green River, Utah

Abstract. A scientific borehole, CO2W55, was drilled into an onshore anticline, near the town of Green River, Utah for the purposes of studying a series of natural CO₂ reservoirs. The objective of this research project is to recover core and fluids from natural CO₂ accumulations in order to study and understand the long-term consequences of exposure of supercritical CO₂, CO₂-gas and CO₂-charged fluids on geological materials. This will improve our ability to predict the security of future geological CO₂ storage sites and the behaviour of CO₂ during migration through the overburden. The Green River anticline is thought to contain supercritical reservoirs of CO₂ in Permian sandstone and Mississippian-Pennsylvanian carbonate and evaporite formations at depths > 800 m. Migration of CO₂ and CO₂-charged brine from these deep formations, through the damage zone of two major normal faults in the overburden, feeds a stacked series of shallow reservoirs in Jurassic sandstones from 500 m depth to near surface. The drill-hole was spudded into the footwall of the Little Grand Wash normal fault at the apex of the Green River anticline, near the site of Crystal Geyser, a CO₂-driven cold water geyser. The hole was drilled using a CS4002 Truck Mounted Core Drill to a total depth of 322 m and DOSECC’s hybrid coring system was used to continuously recover core. CO₂-charged fluids were first encountered at ~ 35 m depth, in the basal sandstones of the Entrada Sandstone, which is open to surface, the fluids being effectively sealed by thin siltstone layers within the sandstone unit. The well penetrated a ~ 17 m thick fault zone within the Carmel Formation, the footwall damage zone of which hosted CO₂-charged fluids in open fractures. CO₂-rich fluids were encountered throughout the thickness of the Navajo Sandstone. The originally red sandstone and siltstone units, where they are in contact with the CO₂-charged fluids, have been bleached by dissolution of hematite grain coatings. Fluid samples were collected from the Navajo Sandstone at formation pressures using a positive displacement wireline sampler, and fluid CO₂ content and pH were measured at surface using high pressure apparatus. The results from the fluid sampling show that the Navajo Sandstone is being fed by active inflow of CO₂-saturated brines through the fault damage zone; that these brines mix with meteoric fluid flowing laterally into the fault zone; and that the downhole fluid sampling whilst drilling successfully captures this dynamic process.