Environmental 'time machine' - the integrity of aquitards overlying coal seams

Posted 16 May 2012

Dr Wendy Timms, David Garces and Gyanendra Regmi looking over the geotechnical centrifuge.

Dr Wendy Timms *

A new geotechnical centrifuge at the University of New South Wales Water is an environmental 'time machine' for coal seam gas (CSG) and mining projects. The new centrifuge facility, funded by the Australian Research Council and the National Water Commission, is one of only two of its kind in the world, providing capability for assessing aquitards. Aquitards, or low permeability sediment or rock that that can store water, are critical to disconnect or partially disconnect flow systems. Also known as confining units, aquitards control recharge and seepage and the hydraulic response of underlying aquifers.

Experimental testing of pressure changes and pore water quality in large drill cores can be completed in less time at accelerated gravity in the centrifuge. This innovative new assessment technique can complement and improve site assessments and numerical modelling of potential cumulative impacts.

Innovative centrifuge testing is part of the National Centre for Groundwater Research and Training (NCGRT) aquitard research program, co-led by Dr Wendy Timms, who was recently appointed to the UNSW School of Mining Engineering. Wendy and Dr David Laurence of the Australian Centre for Sustainable Mining Practices (ACSMP) have commenced discussions on leading practice for CSG and water with industry and government stakeholders. The ACSMP is also playing a key role in promoting the range of UNSW expertise and research that is engaged in CSG and water issues.

The centrifuge facility is located in the University of New South Wales (UNSW) School of Civil and Environmental Engineering, at the Water Research Laboratory. Professor Ian Acworth, leads groundwater research activities there, including new geophysical methods for aquitard characterisation with Dr Anna Greve. Another member of the NCGRT aquitard team, Dr Adam Hartland, is working with Wendy on tracing seepage through aquitards with a suite of inorganic, organic and isotopic signatures.

Aquitard integrity

Aquitards with sufficient integrity could allow time to re-establish pore pressures following dewatering for CSG development. Decades and hundreds of years can be required for hydraulic equilibrium through aquitards, meaning that adjacent aquifers could be targeted for re-injection of treated water before potential impacts spread. However, questions have been raised over long-term connectivity between coal seams, aquifers and surface waters, and the potential for changing water quality. Bore hole and testing of drill core can help in understanding pressure changes that occur with CSG developments and better constrain numerical models that otherwise give non-unique predictions.

Quantifying the integrity of aquitards, with variable stress, pore pressures and chemistry over time requires an inter-disciplinary approach. Matrix permeability and effective porosity testing is an important part of assessment. Geological information and models of 3D stratigraphy and structures are also required to define the continuity of aquitards and preferential flow paths through faults and fracture systems. The geomechanical response of aquitards to depressurisation and desaturation is another aspect of aquitard integrity that warrants investigation.

Aquitard or aquiclude?

In contrast to an aquitard, an aquiclude is a geological material through which zero flux occurs. However, there is typically insufficient data to distinguish aquitards from aquicludes. The reasons for the lack of data and knowledge for aquitards is partly due to the practicalities of measurements in such low permeability materials. Also, hydraulic characterisation has historically focussed on high-yielding aquifers, and relatively permeable reservoirs for oil and gas.

Groundwater studies often assume that hydraulic conductivity (K) is equivalent to the intrinsic permeability of geological material, since there is little variation in fluid density and viscosity in shallow groundwater systems. The terms permeability and hydraulic conductivity are thus often used interchangeably. Low permeability material is commonly defined as K <10-8 m/second (0.001 m/day), although permeability as low as 10-14 m/second (10-9 m/day) has been recorded for shale. A 106 magnitude permeability difference would result in 106 magnitude difference in vertical flow, with similar hydraulic gradient.

Not all aquitards behave like a plastic sheet with zero vertical flow, particularly if pore pressures do not recover sufficiently following extraction. In fact, significant quantities of water can seep through aquitards over large areas, or with depressurization over long time periods. For example, an aquitard with K of 10-8 m/s (0.001 m/day) can allow ten Gigalitres/year of vertical seepage over an area of 500 km2 at unit hydraulic gradient. A gigalitre of water could be a significant component of a catchment water balance in semi-arid areas.

Centrifuge permeameter facility

Centrifuge systems engineer, Mark Whelan is responsible for new instrumentation that extends core testing capabilities. The permeability of drill core from clayey sediment and shale is tested under accelerated gravity in a Broadbent G-18 geotechnical centrifuge (2 m diameter, 2 x 4.8 kg load at 500 g-max). Two modules, a centrifuge permeameter and strong boxes for physical modelling were successfully commissioned in early 2011. Drill cores of 65-100 mm diameter and a length of 20-200 mm are set within clear acrylic liners in the permeameter chambers for testing.

Over 2400 hours testing of drill cores has been completed for NCGRT aquitard research and two contract projects for industry since the facility was commissioned. Clayey sediments and shale rock cores have been tested using an American Standard Test Method (ASTM) for centrifuge permeameter and other types of experiments are soon to commence.

There are several types of tests that can be completed in the geotechnical centrifuge, with flow or no flow through the permeameter module. Under no flow test conditions, porewater can be extracted from some types of core and with additional instrumentation it will be possible to measure displacement and compression. Steady state flow conditions are required for the ASTM permeability method and, if these tests are extended over time, effective porosity and migration of solutes can be studied. Also, by using pumps to vary the influent flow rate, transient permeability tests and water retention curves and recharge or seepage rates can be defined.

In the centrifuge permeameter module, researchers are able to preview the long-term effects on groundwater aquifers of coal seam gas and longwall mining in the geotechnical centrifuge under conditions similar to void ratios and stresses from which the core was drilled. Total stresses up to a depth of approximately 100 m can be applied, with flow being driven by accelerated gravity, rather than a hydraulic pressure gradient.

The strong box module of the centrifuge is for physical models of geotechnical designs, such as the long term performance of natural and engineered seepage barriers. The current tests and experiments are not directly testing the potential effects of fracking. The NCGRT centrifuge facility also includes an Allegra X-15R centrifuge (6 x 100 mL load at 11 400 g-max) for fluid-solid separations.

Flow and transport processes at accelerated gravity

Centrifuge tests of drill core can be designed to simulate groundwater conditions in low permeability units. Flow through tests in a centrifuge for transport of dissolved solutes in aquitards is more realistic than bench top static tests, if flow is slow enough that chemical equilibrium is maintained.

The retardation, migration and fate of a wide range of solutes and contaminants can be tested. For example, a smaller centrifuge at the University of Saskatchewan, Canada was used by Dr Timms and Professor Jim Hendry to measure the sorption of trace metals and rare earth lanthanides over tens of thousands of years of flow in a clayey drill core.

Depending on depth and consolidation state of the sample, the standard centrifuge steady-state flow method is suitable for core permeability in the range of 10-6 to 10-12 m/s. Measuring ultra-low permeability of 10-14 m/s could be achievable in the future in this facility, potentially testing the limits of Darcy's law. New instrumentation and data acquisition systems are a challenge to design for high gravity conditions.

Industry linkages

Use of this new facility for CSG related assessments is encouraged via research linkage programs and contract projects. This innovative centrifuge is only the second of its type in the world and only two cores are tested simultaneously, so testing is not cheap. However, the costs of centrifuge testing of drill core are small compared with the cost of drilling core samples, adding considerable value in terms of data collection from investment in drilling. The facility was established primarily as a research facility, with some time dedicated for contract projects. As such, the facility is in high demand with several months backlog of tests waiting at the current time. Potential partners for research linkage programs are welcomed to help develop new leading practices for aquitard testing and site assessment.

Contact Dr Wendy Timms for information on the NCGRT centrifuge permeameter, aquitard assessments and PhD scholarship opportunities.

* Dr Timms is from the University of New South Wales School of Mining Engineering and National Centre for Groundwater Research and Training.

References:

Timms, W, Hendry, J, Muise J, and Kerrich, R (2009). Coupling Centrifuge Modeling and Laser Ablation ICP-MS to determine contaminant retardation in clays. Environmental Science and Technology. 2009, 43, 1153-1159.

Timms, W A, Hendry, M J (2008). Long term reactive solute transport in an aquitard using a centrifuge model. Ground Water. 46(4): 616-628.

Timms, W and Acworth, I, (2005). Propagation of porewater pressure change through thick clay sequences: an example from the Yarramanbah site, Liverpool Plains, New South Wales. Hydrogeology Journal, 13:858-870.

Republished with permission from The AusIMM Bulletin

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