Shale gas

04/20/2011

Trapped inside vast source rocks buried deep underground, shale gas represents an immense resource. Producing it involves adapting techniques conventionally used in the oil industry, the environmental impact of which must be minimized.

What is shale gas?

Shale gas is what is known as an unconventional gas. Why? From a chemical perspective, it is completely identical to ‘conventional’ natural gas. However, in the subsurface it is located in specific rocks that are very different to the usual gas deposits.

Conventional hydrocarbons (gas and oil) have been released from the source rock where they were formed. This is a geological layer made up of sediments rich in organic matter deposited on the sea bed or in lakes that have gradually sunk below ground. As they are buried, the sediments solidify and the organic matter decomposes into hydrocarbons. These are gradually expelled from the source rock after which most of them migrate to the surface. Some hydrocarbons are prevented from rising by an impassable obstacle of rocks. They accumulate under this ‘roof’ and, over time, form a conventional oil and/or gas deposit.

Shale gas remains ‘trapped’ in this clayey source rock, buried deep underground at depths of 2000 to 3000 meters).

This clay, converted by pressure and temperature into shale, a compact flaky rock from which the gas gets its name, is highly compact and impermeable, meaning that the gas cannot escape naturally. This is why specific extraction techniques are required to allow the gas to drain towards the well.

Significant, widespread resources

Source rocks are geological formations that are widespread throughout the world. Therefore, shale gas is found on all continents, particularly in the United States, Canada, Europe, China, Australia and India.

Global resources in unconventional gases (the majority of which is shale gas) are estimated¹ at 380,000 billion cubic meters, the same volume as the remaining conventional gases. In total, this could provide enough gas to supply annual consumption for between 120 and 250 years!

Specific extraction techniques

With conventional gas, all you have to do (in theory!) is drill a vertical well down to the deposit to enable the gas to escape. With shale gas, trapped in the impermeable source rock, a network made up of multiple tiny cracks or ‘micro fractures’ has to be created to allow the gas to drain towards the well.

To do this, a mixture of water, sand and chemical products (known as ‘additives’ and accounting for less than 1% of total volume) is injected at high pressure into the bottom of the well. This technique is known as ‘hydraulic fracturing’ and has been used for several decades. The water opens up cracks in the rock, the sand keeps these cracks open while the chemicals are mainly used to destroy any bacteria present and improve the effectiveness of the process. Finally as these reserves are not concentrated in a conventional deposit but are spread over a wide area of rock, horizontal wells over 1 km long are drilled to recover as much gas as possible.

Environmental issues

Because of the extraction techniques used, producing shale gas has higher environmental impact than conventional hydrocarbons, particularly in terms of water usage and the surrounding landscape.

Hydraulic fracturing requires larger quantities of water than conventional drilling: on average, 10-20,000 cubic meters of water is injected, the equivalent of 4 to 8 Olympic swimming pools. Therefore, the extraction site must have: :

   • significant water resources for fracturing operations, which last several weeks;

   • facilities for treating the water that comes back up the well after injection.

Once treated, the water can be reinjected underground, returned to the circuit of surface water or used in the next well.

Additives are only used in small quantities and at great depths. However, as a precaution, additives sourced from the food industry are being developed to replace these. Finally, extracting shale gas also causes disruption on the surface linked to:

   • Noise: although drilling and fracturing operations only last a few weeks per well, they are very noisy. In urban areas, sound barriers may need to be built. 

   • Road traffic: many trucks are used to deliver the drilling material, sand and sometimes water. As with the noise-related issues, the traffic problem is short-lived, lasting only as long as it takes to build the wells.

   • The number of wells: as each well produces less gas than with conventional production, more have to be built. To limit the amount of space taken up by these (their ‘footprint’), the wells are grouped together in groups of 10 or 15 in the same drilling platform, called a ‘cluster’. It should be noted that only the wellheads remain during the production phase as the derrick is removed at the end of each drilling operation.

A resource for the future?

With considerable global reserves spread over all continents, shale gas could provide an effective response to the planet’s growing energy demand as well as reduce dependence on a few oil-producing countries. By replacing coal in power plants, it could help meet soaring electricity demand and reduce greenhouse gas emissions for a given electricity output.

However, to ensure that this unconventional gas keeps all its promises, its environmental impact must be assessed before operations begin in order to be limited. In particular, it is clear that shale gas operations can only be envisaged within well-defined regulatory frameworks and issued authorizations, and with full respect for protected areas after consulting the local population. This is the focus of research into water and additives currently being carried out by the industry.
 

[1]Source: International Energy Agency – World Energy Outlook 2009