Helium equilibrium between pore water and quartz: Application to determine caprock permeability.

摘要

The effectiveness of carbon capture and geologic storage depends on many factors, including and especially the permeability of the reservoir's caprock. While caprock integrity is generally assumed if petroleum has been preserved, it is poorly constrained in reservoirs containing only saline waters, and CO2 leakage poses a potential risk to shallow aquifers. Naturally-occurring helium accumulates in pore waters over time with the concentration being strongly dependent on the long-term flux of fluid through the caprock. Furthermore, a small fraction of pore-water helium diffuses into quartz and this may be used as a proxy for helium concentrations in pore water, where dissolved gas samples are difficult to obtain, such as in deep sedimentary basins.;Quartz was purified from core samples from the San Juan Basin, New Mexico and the Great Artesian Basin, South Australia where pore water helium has been previously measured. Quartz separates were heated at 290°C to release helium from the quartz. The quartz from the San Juan Basin and high purity quartz from the Spruce Pine Intrusion, North Carolina was repeatedly impregnated at varying pressures using pure helium, heated and analyzed to build helium sorption isotherms. The isotherms appear linear but vary between samples, possibly due to fluid inclusions within the quartz grains as high purity quartz samples partition only 1.5% of helium that partitions into San Juan Basin samples. Concentrations of helium in the pore water were calculated using the helium-accessible volume of the quartz and the air-water helium solubility. The mean San Juan Basin helium pore water concentration was 2x10-5 cc STP He g-1 water, ∼400 times greater than atmospheric solubility. Great Artesian Basin samples contain a mean helium concentration of 3x10-6 cc STP He g-1 water or 65 times greater than atmospheric solubility. However, pore water helium concentrations in both the San Juan and Great Artesian Basins differ by up to an order of magnitude when compared to samples collected with an alternate method. The reason for the offset is attributable to either partial saturation of the pore volume or a lack of helium equilibrium between quartz and pore water. Coating of clay or other mineral phases on quartz grains, which tends to reduce the effective diffusion coefficient, may cause the latter. Modeling results suggest that helium's high mobility helps constrain formation-scale permeability including the effects of fluid flow through fractures and other seal bypass systems that may not be evident in core samples. This technique of assessing permeability is promising due to the abundance of existing core samples from numerous basins where carbon sequestration may ultimately occur.