The Process of Shale Gas Development
Europe is still in the early phases of horizontally drilling and fracturing wells to extract shale gas, however the process of hydraulic fracturing has been used in Europe for decades as an industrial process to stimulate conventional oil and gas wells, water wells and geothermal wells. In Germany, more than 300 conventional wells have been stimulated using this technique. Around 200 onshore oil and gas wells in the UK have been stimulated by hydraulic fracturing.
In the United States, hydraulic fracturing was combined with horizontal drilling during the 1980s and 1990s and enabled natural gas trapped in shale rock to be commercially extracted. This has led the US to become the leading oil and gas producer in the world, where horizontal wells have rapidly outstripped the use of vertical wells.
This section provides further information on the process employed to extract natural gas from shale rock.
The drilling phase of a shale gas well is the same process as for conventional wells.
Hydraulic fracturing is the process most commonly used to release natural gas and oil from deep beneath the surface of the earth. Though it has only recently come to the attention of the general public, hydraulic fracturing is a mature technology, first used in the 1940s in the US to stimulate oil and gas wells and increase production rates. In Europe, hydraulic fracturing has been used since the 1960s, for conventional oil and gas production and for the recovery of geothermal energy. Hydraulic fracturing is particularly important in the context of shale gas development. As shale gas reservoirs are trapped within rock formations of very low permeability, the gas can only be released into the well by stimulating the shale formation through hydraulic fracturing. Combined with horizontal drilling it maximises extraction by allowing multiple fractures along the shale bed which in turn allows the gas to be commercially extracted.
Developing shale gas: a step-by-step guide
- An appropriate site is prepared and a wellbore is drilled to a length thousands of metres below the surface. A horizontal section will then be drilled through the target shale layer.
- Stages of small perforations are created at the very bottom of the wellbore. Water and sand is mixed with a small quantity of additives and pumped at high pressure, several hundreds of metres down the wellbore.
- The fracturing solution is pumped into the shale formation, creating fissures and opening existing fissure in the rock. The sand in the fracturing fluid then remains in the cracks created, keeping them open. This process creates passageways through which the natural gas, previously trapped in the shale rock, can now flow.
- During the fracturing process, companies monitor in real-time the rock response and well pressure, enabling them to ensure the safety of operations.
Hydraulic fracturing operations take place for 3-10 days where water is pumped under high pressure to complete the well.
- Depending on the specific shale formation, it will take between three and ten days to complete the hydraulic fracturing process. Once the water flowing up the wellbore is collected, the well is prepared to produce a steady stream of natural gas for decades to come.
- After the well is complete, the surface level will be restored to its original state.
- Throughout the process, good practice well construction techniques seek to mitigate any risk of chemical additives leaking into surrounding surface water, groundwater, land or air.
While hydraulic fracturing typically takes place one to three kilometres below the surface, drilling for shale gas can (but not always) penetrate the water table. The proper casing of shale gas wells is vital for the safety of operations. Good practice well construction techniques employ multiple barrier layers of steel and cement to isolate the well from groundwater:
- Conductor casing: this is installed prior to the arrival of the drilling rig and is there to prevent the top section of the well from collapsing
- Surface casing: going up to 500m underground, the surface casing helps to stabilise the wellbore as well as to protect groundwater deposits from being contaminated by any potential gas leakage
- Intermediate casing: placed between the surface and production cases, it ensures that there can be no leakage path from the shale reservoir up to the aquifer. The cement layer protects the casing from corrosion and provides zonal isolation
- Production casing: the deepest section of casing in a well, it provides the path from the surface of the well to the gas-producing shale formation
This layered design ensures that even if one of the casings would rupture, nothing would be able to migrate outside the wellbore. In addition, all wells are pressure-tested to guarantee integrity before hydraulic fracturing takes place.
The International Oil and Gas Producers Association (IOGP) states that hydraulic fracturing fluids are composed mostly of water and sand (99.5%). On average, chemical additives make up around only 0.5% of a fracturing solution. A typical fracturing treatment will use around twelve chemical additives at these low concentrations, depending on the characteristics of the water and of the shale formation to be fractured. These chemicals fulfil a specific purpose, from helping to preserve well integrity to reducing onsite emissions. A list of the chemical additives used in the US is available though the FracFocus website or NGS Facts in Europe.
All the chemical additives used in fracturing solutions in Europe fully comply with European and national regulations. Furthermore, the industry supports full disclosure of these chemicals to the appropriate regulatory authorities and is committed to keeping the public updated about the chemical additives used in local fracturing operations. For this purpose IOGP has set up NGSFacts.org. This website mirrors the approach taken by the U.S. Ground Water Protection Council and U.S. Department of Energy in setting up FracFocus. Like FracFocus, NGS Facts allows the general public to search for specific information on the compounds used in the hydraulic fracturing of individual local wells. Whilst shale gas development is still in its infancy in the EU, the NGS Facts database has been designed to expand as operations become increasingly widespread in Europe, as has been seen with Frac Focus in the US. Currently Frac Focus holds details of more than 47,000 individual well sites, with this number growing by several hundred each month.