Unconventional natural gas technology

17/6/2010 New Scientist FORGET coal, it’s too dirty. Forget nuclear power, it’s too expensive and controversial. Forget renewables, they’re too unpredictable.To meet our energy needs and cut carbon emissions we need an abundant source of clean, cheap energy, available night and day and in all weathers.

We may be in luck. Natural gas is such a fuel, and it’s sitting right under our noses in abundance. Predominantly methane, it’s the cleanest-burning of all fossil fuels (see chart), so using gas rather than coal to generate electricity could halve greenhouse gas emissions from traditional coal-fired power plants.

But hang on a minute: aren’t natural gas reserves depleting just as quickly as oil? And aren’t most reserves found in countries that might not want to share their riches with the rest of the world? Back in 2006, a political spat in Europe led Russia to temporarily cut off its supply of gas to Ukraine. All of a sudden, gas seemed to have just as many problems as other fossil fuels.

While that may have been the case four years ago, things are changing fast. New technology to extract natural gas from what’s euphemistically called “unconventional” deposits means previously gas-poor countries in the Americas, Asia and western Europe could have enough cheap gas to last for another 100 years at present rates of consumption (see diagram).

Unconventional gas tends to be trapped in impermeable hard rock or sandstone, contained within coal seams, or - most promisingly for gas producers - in shale deposits. For Vello Kuuskraa, president of Advanced Resources International, an energy industry consultancy based in Washington DC, unconventional gas “has the potential for changing the long-term outlook for natural gas in a very dramatic way”.

Extra time
The world consumes around 3 trillion cubic metres of natural gas each year, and the European Union says reserves from proven and conventional sources will run out in 2068. Unconventional reserves could buy us at least an extra 60 years at current rates of consumption. According to research in the late 1990s by Hans-Holger Rogner at the University of Victoria in British Columbia, Canada, there could be 900 trillion cubic metres of unconventional gas worldwide, half of which is shale gas. Of this, the International Energy Agency estimates 180 trillion cubic metres will be recoverable.

3 trillion cubic metres: annual global natural gas consumption
900 trillion cubic metres: unconventional gas resource worldwide
In the US shale gas has been produced on a small scale for decades. The problem with shale gas is that, unlike conventional gas, it is not formed within discrete pockets, making it more difficult to extract. What’s more, shale is not naturally permeable, so fractures have to be created to allow the gas to seep out into wells. The good news is that over the last 10 years, horizontal well drilling and hydraulic fracturing techniques have been developed to get at the gas more easily (see “Fracing deep”). This has opened up basins such as the Barnett shale near Fort Worth in Texas. As a result, US shale gas production is expected to reach 295 million cubic metres per day this year, compared with 31 million cubic metres in 2000.

That is just the start. The US Department of Energy estimates the huge Marcellus shale running along much of the east coast has recoverable gas reserves of up to 7 trillion cubic metres. To put this in perspective, in 2008 US natural gas consumption was around 62 billion cubic metres, so at this rate the reserves would be good for at least a century. Until very recently, the US had been preparing for a decline in its domestic gas supply by building huge terminals to receive imported liquefied natural gas. “We’re now saying we have potentially 100 years of gas,” says Kuuskraa.

If the US experience is replicated throughout the world, it could transform the energy outlook. Around 60 per cent of US gas is produced from unconventional sources. “It is the dominant source of gas production in the US, will become that in Canada in the next 10 years, and has the potential to become that in the rest of the world in 20 to 30 years,” says Kuuskraa.

The big oil and gas firms are now eyeing the rest of the world’s shale. Major hotspots include Australia, China, South Africa and, in particular, Europe. Shell is exploring in Sweden, Exxon Mobil in Germany, while several other companies have Poland in their sights. “In the US all the big companies walked away as they didn’t realise the big improvements in fracturing technology would make such a difference. They don’t want to make the same mistake in Europe,” says Brian Horsfield of the German Research Centre for Geosciences in Potsdam, who leads a consortium of researchers and oil firms studying Europe’s shale basins.

Europe could be sitting on hefty reserves of up to 15 trillion cubic metres of shale gas. All the signs are positive: Horsfield says the Alum shale in Sweden and the Posidonia shale in northern Germany, the Netherlands and southern England look a lot like the US shale sites. The European shales are chiefly black or dark brown in colour, indicating they are rich in organic matter derived from the creatures living in the mud when the shales were formed, most of which has now become gas.

If these basins are as rich in gas as they appear, it could transform the fortunes of many European countries, freeing them from their reliance on imports from Russia, which currently supplies around a quarter of the European Union’s gas needs. “Shales are the single largest deposited rock in the world. There are many areas of the world that are quite poor in conventional gas that are going to have rich shale resources,” says Kuuskraa.

As clean a fuel as shale gas seems to be, there are concerns that the fracturing technologies used to extract the gas may damage the environment. One fear is that groundwater could be contaminated by the chemicals used in the fracturing process, though a 2004 study in the US by the Environmental Protection Agency found no evidence that drinking water was contaminated by these chemicals. The EPA is due to complete a study into the impact of hydraulic fracturing for shale gas by 2012.

Concerns are also growing about the amount of water the fracturing process consumes. “Every gas shale-well drilled takes 4 million gallons of water to crack open the rock,” says Horsfield, “so questions are bound to arise when you are using that much water.”

The industry is trying to reduce the environmental footprint of shale-well drilling and hydraulic fracturing. These include capturing the water flowing out of the well after fracturing for reuse in the next “frac”. Halliburton, based in Houston, Texas, says recycled water makes up 70 per cent of the water used at some of its US shale drilling sites. Many shale sites are close to conventional gas production plants, allowing the waste water they generate to be captured, treated and used to fracture the shale.

Another issue concerns the use of bactericides during fracturing, which some fear could leach into the water table. Chemicals are added to the water to give it a gel-like texture which is better for fracturing, but bacteria present in the water can stop this reaction. Rather than adding bactericides as at present, it might be possible to use high-power ultrasound, filters, or environmentally benign chemicals to kill off these bacteria in the water.

There are some immediate benefits to gas over coal and other fossil fuels. Burning natural gas produces far lower levels of nitrogen oxide and sulphur dioxide compared to coal. What’s more, gas has a carbon footprint half that of coal, so much so that in the UK, where natural gas has grown from just 0.5 per cent of the electricity generation mix in 1990 to 45 per cent in 2008, carbon emissions associated with electricity generation have fallen by 26 per cent, according to the Department of Energy and Climate Change.

These emissions will be cut still further if advocates of natural gas get their way. They are pushing to develop carbon capture and storage (CCS) technology for natural gas power plants. So far, most government funding for demonstration CCS plants has gone to coal-fired power stations, but that may be about to change: in the UK plans are afoot to open up a technology demonstration scheme to a gas-fired plant, says John Davison of the International Energy Agency’s greenhouse gas research programme.

One attractive option for gas-fired power plants is to capture the carbon before the gas is burned, Davison says. The natural gas is first reacted with steam at high temperature and in the presence of a nickel catalyst to produce carbon monoxide and hydrogen. This is then reacted with water vapour at a lower temperature to produce carbon dioxide and more hydrogen. The gas mixture is forced through an alkanolamine solution where the CO2 is absorbed, while the hydrogen is burned in a slightly modified gas-fired power plant to produce electricity. This stage actually cuts the energy required for carbon capture, as the natural gas coming out of the ground is at a higher pressure than the plant’s flue gas and so helps force the intermediate gas mixture through the alkanolamine solution.

Crucially, precombustion capture would allow gas-fired power plants to operate more flexibly, says Davison. The plant could receive and convert a steady stream of natural gas into hydrogen, which could then be stored and only fed into the turbine when electricity demand required it, he says. This would help to smooth out electricity supplies as the mix becomes more diverse.

“When you put relatively inflexible nuclear and variable renewables into the electricity generation mix, the fossil fuel plants have to operate more flexibly to cope,” says Davison. What’s more, hydrogen not needed for electricity generation could be used as fuel for cars and other vehicles, says Paul Hurst, engineering manager for BP Hydrogen Power. His company, together with United Arab Emirates-based alternative energy firm Masdar, are jointly planning to build a hydrogen/CCS plant in Abu Dhabi, to begin operating by 2015. “We see this as a way to advance the move into a hydrogen-based economy,” he says.

If CCS is installed as standard on all gas-fired power stations, and if we can find a way to minimise water usage and the potential for environmental contamination, natural gas will be providing us with clean energy for many decades to come.

Fracing deep
Shale gas is not deposited in neat pockets, like conventional gas, but dispersed across wide basins. That means producers cannot economically extract gas from a shale deposit just by drilling a single vertical well. So back in the 1990s a few small drilling firms began experimenting with horizontal wells, which allowed them to reach deeper into the basins to extract more gas.

This still wasn’t enough to make shale gas profitable, however, as the impermeable nature of shale meant that not enough gas was seeping out into the wells. So the drillers started creating artificial fractures in the rock - a process called “fracing”. “The fracturing technology creates the permeability that conventional gas resources have naturally,” says Kent Perry of the Gas Technology Institute in Des Plaines, Illinois.

Fracing involves mixing millions of litres of water with sand and chemicals, then pumping the fluid into the rock at high pressure, causing it to crack. The fluid enters the cracks but the water seeps back out, leaving the sand behind to prop the crack open and allow the gas to leak out gradually.

Producers in the US are now carrying out multiple fracs in single horizontal wells stretching up to 9 kilometres into the basin. Meanwhile, researchers at oil and gas service companies like Schlumberger and Halliburton, both based in Houston, Texas, are developing techniques to drill multiple horizontal wells off a single vertical well, resembling a tree root system, says Perry. The ideas is that these “tree root” wells will allow producers to reach further into the shale deposit without causing more environmental damage at the surface.

Helen Knight is the clean technology correspondent for New Scientist
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