Accelerated gravity testing of aquitard core permeability and implications at formation and regional scale

Abstract

Abstract. Evaluating the possibility of leakage through low permeability geological strata is critically important for sustainable water supplies, the extraction of fuels from strata such as coal beds, and the confinement of waste within the earth. The current work demonstrates that relatively rapid and reliable hydraulic conductivity (K) measurement of aquitard cores using accelerated gravity can inform and constrain 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 centrifuge permeameter (CP) unless consolidation or geochemical reactions occur. The CP module was custom designed to fit a standard 2 m diameter geotechnical centrifuge (550 g maximum) with a capacity for sample dimensions of 30 to 100 mm diameter and 30 to 200 mm in length, and a maximum 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. Vertical hydraulic conductivity (Kv) results from CP testing of cores from three sites within the same regional clayey silt formation varied (10−7 to 10−9 m s−1, n = 14). Results at one of these sites (1.1 × 10−10 to 3.5 × 10−9 m s−1, n = 5) that were obtained in < 24 h were similar to in situ Kv values (3 × 10−9 m s−1) from pore pressure responses over several weeks within a 30 m clayey sequence. Core scale and in situ Kv results were compared with vertical connectivity within a regional flow model, and considered in the context of heterogeneity and preferential flow paths at site and formation scale. More reliable assessments of leakage and solute transport though aquitards over multi-decadal timescales can be achieved by accelerated core testing together with advanced geostatistical and numerical methods.

The Influence of Syndepositional Macropores on the Hydraulic Integrity of Thick Alluvial Clay Aquitards

“…Investigations of a 31 m deep clayey silt paleovalley fill from Cattle Lane, Australia (Acworth et al, ), have revealed the potential for hydraulically active flow paths. Geotechnical centrifuge techniques (Crane et al, ; Timms et al, ) found that vertical K varied between and m/s with a geometric mean value of m/s (13 samples, 5–31.4 m BGL), values that were consistent with in situ vertical K values (Timms & Acworth, ). The vertical K values are high for an aquitard (as defined by K of m/s; Neuzil, ) and have prompted more detailed investigation into the depositional properties and groundwater processes at this site.…”

“…Steady state K measurements of core subsamples taken from 5.0, 9.5, and 21.8m depth BGL (below ground level) were reported by Crane et al () and Timms et al (). Details are provided on steps to ensure reliable K measurements of intact samples (Crane et al, ; Timms et al, ), primarily related to core storage, stress application and detection of rapid leakage. Potential errors were evaluated and minimized for example, by using relatively large diameter cores (101.6 mm) and application of stresses less than or equivalent to in situ stresses at the depth of core collection.…”

“…The core barrel was 1.5 m long but as the cores were collected by pushing the central tube down while rotating the outer drill bit, the formation resistance was sometimes too great to completely fill the 1.5 m barrel. A total of 21 cores in 1.5 m lengths (maximum) were recovered directly into the 101.6 mm ID clear PVC core tubes used to line the Treifus barrel (Timms et al, ). Geophysical logging was completed for all the bores.…”

“…Further details on methods to minimize disturbance to the cores during sampling and laboratory testing are detailed by Timms et al (). Minimizing structural and moisture changes to these clayey cores was important for reliable characterization of macropores and hydraulic connectivity.…”

“…This depth corresponded with clay from approximately 74 ka years (Acworth et al, ), with a vertical hydraulic conductivity that was typical for this sequence (Figure ). The vertical hydraulic conductivity of this sample had previously been determined as m/s (Timms et al, ). It is worth noting that drilling and sampling proceeded between 16.5 and 24.0 m (Cores 12 to 16), through massive clay (Figure ), with 100% core recovery.…”