Accelerated gravity testing of aquitard core permeability and implicationsat formation and regional scale

摘要

Evaluating the possibility of leakage through low-permeability geologicalstrata is critically important for sustainable water supplies, the extractionof fuels from coal and other strata, and the confinement of waste within theearth. The current work demonstrates that relatively rapid and realisticvertical hydraulic conductivity (K_v) measurements of aquitard cores using accelerated gravity can constrain and compliment larger-scale assessments of hydraulic connectivity. Steady-state fluid velocity through a low-K porous sample is linearly related to accelerated gravity (g level) in a centrifugepermeameter (CP) unless consolidation or geochemical reactions occur. A CPmodule was custom designed to fit a standard 2 m diameter geotechnicalcentrifuge (550?g maximum) with a capacity for sample dimensions up to 100 mm diameter and 200 mm length, and a total stress of ?～??2?MPa at the base of the core. Formation fluids were used as influent to limit any shrink–swell phenomena, which may alter the permeability. K_v results fromCP testing of minimally disturbed cores from three sites within a clayey-siltformation varied from 10−10 to 10−7 ?m?s−1 (number of samples, n =?18). Additional tests were focussed on the Cattle Lane (CL) site, where K_v within the 99 % confidence interval (n =?9) was 1.1?×?10?9 to 2.0?×?10?9?m?s?1. These K_v results were very similar to an independent in situ K_v method based on pore pressure propagation though the sequence. However, there was less certainty at twoother core sites due to limited and variable K_v data. Blind standard 1?g column tests underestimated K_v compared to CP and in situ K_vdata, possibly due to deionised water interactions with clay, and were moretime-consuming than CP tests. Our K_v results were compared with the set-up of a flow model for the region, and considered in the context ofheterogeneity and preferential flow paths at site and formation scale.Reasonable assessments of leakage and solute transport through aquitards overmulti-decadal timescales can be achieved by accelerated core testing togetherwith complimentary hydrogeological monitoring, analysis, and modelling.