ASSESSING CAPROCK PERMEABILITY USING HELIUM SOLUBILITY IN QUARTZ

The effectiveness of carbon capture and storage depends on the permeability of the reservoir’s caprock. While caprock integrity is generally known if petroleum has been preserved, it is unknown in other basins and CO₂ leakage poses a potential risk to shallow aquifers. 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 can be used as a proxy to 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 then heated at 290°C to release helium from the quartz. The quartz was repeatedly impregnated at varying pressures using pure helium, heated and analyzed to build isotherms. The isotherms appear linear but vary between samples possibly due to fluid inclusions within the quartz grains. Helium pore water concentrations were calculated to be on the order of 10^-5 cc STP g^-1 water, ~200 times greater than atmospheric solubility. Helium’s high mobility helps measure formation-scale permeability including the effects of fluid flow through fractures that may not be present in the core. This technique of assessing permeability is promising due to the abundance of existing core samples from numerous basins where carbon sequestration will ultimately occur.