Geochemical baseline monitoring: quartz-helium trial

[摘要] The estimation of permeability is essential for groundwater modellers and ultimately policy makers, because the amount of leakage, in or out of an aquifer or reservoir, is strongly dependent on this parameter. Furthermore, the permeability of geological materials can vary by many orders of magnitude.In the Surat Basin, there is a concern about inter-aquifer leakage brought on by the dewatering of coal seams for the extraction of natural gas. In this study, we attempted to determine formation-scale vertical permeabilities in low permeability formations within the Surat Basin. To achieve this, we used a relatively new method where quartz is separated from core samples and the helium contained within the quartz grains is quantified to determine concentrations of helium dissolved within the pore water. These concentrations were fit to solute transport models to ultimately determine the fluid velocity and formation-scale permeability. The advantage of this approach is that permeability is estimated from archived cores (which are relatively easy to obtain) rather than from aquitard pore water (which is very difficult to collect).Key findings of this report include:-Diffusion rate measurements suggest that helium results are representative of the past 2-20,000 years-Monte Carlo simulations of solute transport (constrained by quartz-helium concentrations) within the Evergreen Formation at the Condabri site suggest a fluid velocity of 0.31±0.06mm/year-Quartz-helium results generally agree with direct dissolved helium measurements (where available), suggesting that the system has been in (quasi) steady state, long enough that helium has equilibrated between the quartz and pore water-The method appears to be effective in deeper (hotter) sites, but may be unreliable at more shallow (cooler) sites because of the additional time required for helium to reach equilibrium between pore water and quartzShortcomings of the method include uncertainties and scatter in the data which in some cases makes interpretation difficult. The uncertainty in fluid velocity may be underestimated because scenarios with transient boundary conditions were not considered - this omission could greatly affect the fluid velocities and vertical hydraulic conductivities presented here. This method is constrained to one-dimensional estimates and without more spatial data, two- and three-dimensional parameter estimates cannot be determined. However, the quartz-helium method can provide estimates formation-scale of vertical permeability in aquitards, where little data currently exist.

[发布日期] 2015-03-22 [发布机构] CSIRO

[效力级别] Hydrogeology [学科分类] 地球科学(综合)