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

[摘要] 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 researchproject is to recover core and fluids from natural CO₂ accumulations in order to study andunderstand the long-term consequences of exposure of supercritical CO₂, CO₂-gas and CO₂-chargedfluids on geological materials. This will improve our ability to predict the security of future geologicalCO₂ 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 Permiansandstone 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 oftwo major normal faults in the overburden, feeds a stacked series of shallow reservoirs in Jurassicsandstones from 500 m depth to near surface.The drill-hole was spudded into the footwall of the Little Grand Wash normal fault at theapex of the Green River anticline, near the site of Crystal Geyser, a CO₂-driven cold water geyser. Thehole was drilled using a CS4002 Truck Mounted Core Drill to a total depth of 322 m and DOSECC’shybrid coring system was used to continuously recover core. CO₂-charged fluids were firstencountered at ~ 35 m depth, in the basal sandstones of the Entrada Sandstone, which is open tosurface, the fluids being effectively sealed by thin siltstone layers within the sandstone unit. The wellpenetrated a ~ 17 m thick fault zone within the Carmel Formation, the footwall damage zone ofwhich hosted CO₂-charged fluids in open fractures. CO₂-rich fluids were encountered throughout thethickness of the Navajo Sandstone. The originally red sandstone and siltstone units, where they arein 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 positivedisplacement wireline sampler, and fluid CO₂ content and pH were measured at surface using highpressure apparatus. The results from the fluid sampling show that the Navajo Sandstone is being fedby active inflow of CO₂-saturated brines through the fault damage zone; that these brines mix withmeteoric fluid flowing laterally into the fault zone; and that the downhole fluid sampling whilstdrilling successfully captures this dynamic process.

[学科分类] 地质学