Scientific drilling and downhole fluid sampling of a natural CO.sub.2 reservoir, Green River, Utah

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 CO.sub.2 reservoirs. The objective of this research project is to recover core and fluids from natural CO.sub.2 accumulations in order to study and understand the long-term consequences of exposure of supercritical CO.sub.2, CO.sub.2 -gas and CO.sub.2 -charged fluids on geological materials. This will improve our ability to predict the security of future geological CO.sub.2 storage sites and the behaviour of CO.sub.2 during migration through the overburden. The Green River anticline is thought to contain supercritical reservoirs of CO.sub.2 in Permian sandstone and Mississippian-Pennsylvanian carbonate and evaporite formations at depths > 800 m. Migration of CO.sub.2 and CO.sub.2 -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 CO.sub.2 -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. CO.sub.2 -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 CO.sub.2 -charged fluids in open fractures. CO.sub.2 -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 CO.sub.2 -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 CO.sub.2 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 CO.sub.2 -saturated brines through the fault damage zone; that these brines mix with meteoric fluid flowing laterally