e for radioactive waste
The large-scale gas injection test at the -420m level.
The waste will need to be buried for
between 100,000 and one million years: it’s a long time, and it’s important to get it right. But an experimental approach alone can’t give us all the answers. Most experiments can only be successfully operated for a few years at a time, so FORGE uses mathematical modelling of potential future conditions to get a more complete picture. Modelling is always a difficult process. It
uses mathematical algorithms to replicate natural processes, but to do this we have to simplify complex natural systems down to just a few variables to represent the properties of features such as the host rocks, groundwater flow and gas generation. If we choose the wrong ones, our model won’t replicate the natural environment accurately enough, and it won’t be much use. To get the best possible results,
FORGE’s approach is to fully integrate laboratory and field experiment data with mathematical modelling. Our modellers and experimentalists exchange data and collaborate closely, and as a result our models represent the geological information better than ever before.
Ground breaking One of our full-scale in situ experiments is LASGIT (large-scale gas injection test).
It studies the movement of gas through bentonite clay in a mock deposition hole, 420m down in the Äspö Hard Rock Laboratory in Sweden. LASGIT has two stages, each of which
lasts about a year. In the hydration phase the bentonite is saturated with water and we monitor the suction (swelling) and pressure. In the second phase gas is injected into the clay, and we watch how it flows through the rock. So we get data on the movement of both water and gas through the bentonite buffer and the relationship between gas flow and pressure within the clay. Another crucial part of the field test
involves critical stress theory. Essentially this tells us that fractures or faults at certain orientations to stresses in the surrounding rock will act as barriers to gas flow, while those at other orientations will act as conduits – something we’ve seen in natural gas reservoirs. So this part of the experimental programme looks at the relationship between fractures and stresses in the repository. We’re interested in two distinct areas; close to the repository, where the construction process will have created a complex local stress field with lots of fractures of different orientations; and farther away where the natural fractures in the rock are undisturbed by construction.
Thanks to some new British Geological
Survey equipment we’ve been able to test critical stress theory in repository conditions for the first time. By injecting gas (or fluid) into the plane of the fracture at an angle to the applied stresses we can see how the gas flows and so work out which stresses will help the gas move and which will seal the fault and stop the gas flowing. This work is important because building
a repository changes the stresses in the surrounding rocks. Construction methods that cause the least fluid and gas flow are going to minimise the release of repository gas, so this knowledge helps us determine the best design and orientation for a repository. FORGE is playing a key role in
enhancing and developing European expertise in gas migration, helping to ensure that the partners are global leaders in this fast-developing and important area of science. n
MORE INFORMATION
The British Geological Survey (BGS) is one of two UK partners in FORGE. Emma Ward is based at BGS Keyworth. E-mail:
forge@bgs.ac.uk www.FORGEProject.org
PLANET EARTH Autumn 2011 09
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