Abstract
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.
Highlights
Understanding the geochemical behaviour of gaseous and supercritical carbon dioxide stored in geological reservoirs, over a range of timescales, is crucial for quantifying leakage risk and the geochemical evolution of the stored CO2 through the life of an individual storage site (e.g. Bickle, 2009)
Whilst it is expected that a well-sited CO2 storage facility will not leak, some natural CO2 reservoirs have conductive features where CO2 and CO2-charged fluids are able to escape from depth to surface
Narrow zones of mineralogical alteration are observed in the cap rock units in contact with the CO2-charged reservoir sandstones
Summary
Understanding the geochemical behaviour of gaseous and supercritical carbon dioxide stored in geological reservoirs, over a range of timescales, is crucial for quantifying leakage risk and the geochemical evolution of the stored CO2 through the life of an individual storage site (e.g. Bickle, 2009). Whilst it is expected that a well-sited CO2 storage facility will not leak, some natural CO2 reservoirs (such as the Green River site) have conductive features where CO2 and CO2-charged fluids are able to escape from depth to surface These can provide sampling opportunities and insights into processes that may inhibit CO2 migration through the overburden, such as dissolution into shallow reservoirs, capillary trapping of CO2 gas or precipitation of carbonate minerals. The geochemical reactions occurring in CO2 reservoirs, including dissolution into brine, and the subsequent reaction of the CO2charged brine with reservoir minerals are all sensitive to temperature Thermodynamic parameters such as the degree of CO2 and mineral saturation in the fluid, or physical parameters related to flow of the CO2, or surface properties of the minerals are as important in controlling the rates of these geochemical processes. The behaviour of cap rocks is reviewed in Song and Zhang (2012) and geochemical processes occurring during CO2 leakage are reviewed in Harvey et al (2012)
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