Going underground, again
14 December 2010 by David Evans
With domestic supplies dwindling, gas storage has become a major issue for the UK's energy security. British Geological Survey expertise has been helping develop our underground gas storage capability. David Evans gives us the low-down.
Natural gas is the UK's energy source of choice. As the world's third-largest consumer, after the USA and Russia, gas accounts for more than half the country's energy needs.
Since the late 1960s and 1970s, the UK has been able to rely on its huge offshore oil and gas reserves. But production has declined so much that domestic supply can no longer meet periods of peak demand. It's estimated that by 2020 the UK will be importing more than 80 per cent of its gas.
Managing gas supplies is no simple matter because demand varies as much as the weather: on the coldest winter days we use three or four times as much as on an average summer day. Seasonal variation in demand is reflected in daily movements in the prices charged by gas exporters, which are highest in winter. Aside from the economic implications, this puts increasing pressure on the national transmission system, which has to move gas around the country according to seasonal and daily changes in demand. So relying on imports makes us vulnerable.
Countries without domestic oil and gas reserves have been using underground gas storage (UGS) for many years. The first storage sites opened in the USA in 1915, but the UK's oldest purpose-built facility, at Hornsea in East Yorkshire, opened in 1979 and our capacity is currently only around 4 per cent of our consumption.
In the UK there are three main places where natural gas can be stored underground:
Depleted oil and gas reservoirs - which once held gas or oil in tiny connected pore spaces between sand grains. Gas is injected back into these pores.
Large caverns - effectively large, gas-tight underground pressure vessels created in thick beds of halite (rock salt) by dissolving the salt away.
Aquifers - porous sandstone rocks where the water held naturally in the pore spaces is pushed out and replaced by the injected gas.
Market demand for UGS is growing, and the government recognises that it offers the potential for the safe and reliable extra storage that will buffer the UK against demand and supply fluctuations.
How does it work?
UGS works by injecting gas deep underground, where it's held under pressure in the rocks. The higher the pressure, the more gas can be stored. Storage pressures are limited by rock strength, which normally increases with depth so, in general, deeper formations can store more gas. Deeper storage has the added advantage that the overlying strata will be tightly compacted by the weight of the rocks above, so it's much harder for gas to leak out and make its way to the surface. But this isn't always the case and careful studies are needed for each site.
Different gas storage types together create a hierarchy of supply flexibility. The above-ground tanks we're all familiar with provide small volumes for daily supplies; they empty quickly but can be refilled quickly too. Salt caverns can respond on a daily or weekly basis. Depleted reservoirs and aquifers take longer to fill and empty, and this is generally done during times of lower demand (the summer), to provide monthly or seasonal supply.
Developing UK capacity
Because of their very specific requirements, UGS locations are limited to places with suitable geological features, reservoirs and/or gas-tight rocks. Britain currently has several operational facilities but there is significant potential for more, both on- and offshore. There are currently no plans to develop any aquifer sites.
Planning and commissioning UGS facilities takes a long time in the UK. Safety is paramount in their design, construction and operation, and potential sites have to be investigated in detail to ensure the geological structure is sound and that the rocks are up to the job.
These concerns can be addressed through detailed investigation of potential sites, which means coring and sampling in the field, with in-situ tests in boreholes, followed by further tests, analysis and modelling in the lab.
BGS and UGS
We've been working on UGS since 2003, using oil-industry techniques to describe the geology of potential storage sites. Experts in rock mechanics or cavern design then use our data to assess the suitability and operational limits of the site in question.
Storage sites in the UK
In the field we use seismic reflection surveys to image the subsurface strata, recording sound waves as they 'bounce' back off the different rock layers deep below ground. This effectively produces a cross-section through the rock layers. Then, to take a more direct look, we drill boreholes and lower a range of geophysical probes and sensors that give us information about the properties of the rocks. Core samples (cylinders of rock) are retrieved during drilling, which are described and logged in detail. Selected samples are then sent to the lab where the rock is tested for strength and gas tightness.
By combining all this information we have produced 3D models of rock-salt beds for developers, which make it easy to relate the geology to surface features like towns or major infrastructure. All of this helps the relevant experts to identify any potential problems which can be investigated further, so that if construction does go ahead it will be safe at the outset and won't encounter problems down the line.
One such project saw myself and colleagues Ed Hough and Marcus Dobbs working on a drilling rig in north-west England in winter - calling for hard hats, steel-toe-capped boots and lots of warm drinks! We had to oversee the drilling and take cutting samples from the borehole every 15 minutes or so. The cuttings are crushed bits of rock produced by the drilling bit, which are flushed to the surface by the drilling fluid in the borehole and caught in a large shaker (sieve). By carefully monitoring these we know when the drill has reached the right place for larger core samples to be taken.
Drilling rigs are dangerous places at the best of times, with lots of large machinery to negotiate and 9m-long steel drilling rods and core barrels being swung around. One slip could lead to the loss of fingers, or worse (it is rare to find a driller with all his teeth and fingers intact!). Drilling also needs lots of water, which quickly soaks everything, and handling freezing steel drill-pipes, casings and cutting samples on a bitterly cold February morning is no fun.
BGS's growing experience of UGS means we sometimes have to swap our fluorescent jackets for suits. I have advised county council planning officers on the geological aspects of planning applications, ensuring that the developer has demonstrated a good understanding of the site geology, and our expertise has also been called on in public inquiries.
We have hosted an international conference to share knowledge of, and developments in, UGS across the UK and Europe. In 2008 we produced a report for the Health and Safety Executive (HSE) to develop a risk assessment methodology for land-use planning. And we will continue to provide regulators and potential developers with the tools and means with which to understand the geology of proposed storage sites, to help ensure safety and develop mitigation strategies in the event of any problems.
So, whether it means stamping our feet against the cold or tapping our fingers on a keyboard, BGS will continue to provide and develop geological expertise in this exciting and important field.
Dr David Evans is a geologist / geophysicist at the British Geological Survey in Keyworth, Nottingham.