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What is shale gas?

Drilling site / Pad construction

Roughly one or two hectares of land are cleared for a drilling site. Roads are constructed to connect the site to main public roads. Typically, a well site will hold large equipment, such as the drilling rig and cranes, generators, storage tanks for fluids and trucks.

Several (up to 10 or more) separate wells can be drilled from the same pad.

The details of the site will be set out and approved in the original planning application.

Phases of shale gas production

During the exploration stage any gas discovered is likely to be flared. However, if the well goes into commercial production a wellhead valve will be installed. Tanks will remain on site to collect the gas or it will be connected to the country’s network of gas pipes.

The lifetime of shale gas production site can be up to 20 years.

Both the exploration and production of shale gas require the transportation of people, equipment, materials, water and the gas itself to and from the well site.

When the exploration and production phases of a well site come to an end, the well site will be decommissioned.

Decommissioning involves removal of all equipment and materials, cutting off the drill pipe underground and plugging the hole, removing gravel base and liners and restoring the site to its original state, in accordance with the details set out and approved in the original planning application.

Cement casing

Water is a key element in fracking. Water is mixed with (about 5%) sand and a small amount (about 1%) of chemicals. The precise formula varies according to the particular rock formation being fractured.

The water used in fracking can be purchased from water utilities, sourced from surface water bodies (e.g. rivers, lakes) or from groundwater bodies (e.g. aquifers or public/private water sources). All of which is subject to the approval of the Environment Agency.

About one swimming pool of water is required to fracture a well. The water requirement is not especially large when compared with other industrial users, especially farming. Most UK drinking water comes from surface reservoirs and rivers.

Surface casingCementVertical drilling

Potentially exploitable shale formations are located approximately 1000 - 3000 metres deep. In contrast, aquifers typically lie between only 100 and 200 metres below the surface.

As each section of a well is drilled it must be lined with a steel pipe (casing) that is cemented to the wall rock to make an impermeable seal.

Cement slurry is then injected down the outside of the casing pipe. This casing stops fluids (liquids or gases) passing from the wellbore into the surrounding rocks, and vice-versa. This is particularly important where the wellbore passes through fresh water-bearing formations near the surface, such as an aquifer.

As the hole becomes deeper the drill has to be progressively of smaller diameter so that it will pass through the upper casing section(s), so that by the time it is completed there may be 3, 4 or 5 casings, cemented one inside the other as illustrated on the figure.

A second purpose of the casing is to protect the wellbore against pieces of wall rock falling in, which can jam equipment and tools lowered into the hole. This is particularly important as the hole gets deeper.

Production casing

Horizontal drilling

Shale is the most common sedimentary rock on the planet. It is a sedimentary rock that occurs as horizontal “bed” that are sometimes only a few tens of metres thick but many kilometres wide. It is the source rock for all the oil and gas that we produce, both conventional and unconventional.

Unconventional gas is what is extracted using hydraulic fracturing. This is where the drill is deviated horizontally for one or more kilometres into the shale bed. The rock around the horizontal borehole section is then “fractured” in order to increase the effective surface area of rock connected to the borehole and hence to get a sufficient flow of gas.

Hydraulic fracturing

Hydraulic fracturing, or fracking, is used to increase the flow of gas to a borehole by creating a fracture in the surrounding layer of rock. The fracking fluid acts like a crowbar, wedging the crack open.

A hydraulic fracture is a single, flat crack, growing away from the borehole in two directions. Each half of the crack may grow horizontally for 100 to 200 metres and vertically for about 30 metres.

For shale gas production a lot of hydrofracture treatments are required, e.g. 10 or 20 per well.

The exact procedures and technologies used for hydrofracturing can vary according to local circumstances, but the following is typical:

  1. When the producers have horizontally drilled far enough across the shale layer, they cement the final, production casing into position. The drilling stage is now finished. The drilling equipment is taken away and the fracking equipment is moved in.
  2. Starting at the furthest end of the hole, a section of the hole is packed off, and in the isolated section, a small charge perforates holes in the casing (Fig 01).
    Figure 1
    1. Pipe
    2. Production casing
    3. Cement
    4. Expanding plug
    5. Perforations in casing
    Fig 01
  3. The fracking fluid is then pumped into the packed-off section of the well bore. The water column creates pressure at the bottom of the hole of about 5,000psi (pounds per square inch). Some additional pressure is then applied to crack the shale layer via the perforations previously made into the casing. This creates a hydraulic fracture of about 1 or 2 mm wide that spreads horizontally in to the shale formation for up to 200 metres. The sand in the fracking fluid props open the fracture, which otherwise would tend to close-up under the weight of the overlying rock (Fig 02).
    Figure 2
    1. Pressurised container
    2. Production casing
    3. Cement
    4. Expanding plug
    5. Propogating fracks
    Fig 02
  4. The hydraulic fractures allow gas to flow from the rock layer to the borehole, pushing the fracking fluid back up the hole as ”flowback”.
  5. When each fracking stage is complete, another section of the hole, about 30 m long, is packed off and hydrofractured, until the entire horizontal section had been fracked (Fig 03). This process takes 3-10 days. At the end of this process the plugs are removed and the gas flows. The completion stage is now finished. The fracking equipment is removed.
    Figure 3
    1. Perforations in casing
    2. Plug
    3. Hydro fracks
    4. Cement
    Fig 03
  6. The hydraulic fractures allow gas to flow from the rock layer to the borehole, pushing the fracking fluid back up the hole as ”flowback” (Fig 04).
    Figure 4
    1. Production casing
    2. Cement
    3. Expanding plug
    4. Hydro fracks
    Fig 04
  7. A wellhead is installed to capture the released gas. This step marks the start of the production stage.

Backflow and waste

Between 10 to 50 per cent of the fracking fluid flows back to the surface with the gas when the well is depressurised, this is known as backflow. The rest of the fluid remains underground.

At the surface, tanks will collect the back flow. It is generally re-used in future fracking, or desalinate and disposed of as waste water through the sewage system, or trucked away and disposed of via a wastewater injection well. It is important that flow back water is handled correctly (avoiding accidental spillages on surfaces) and, as appropriate, dispose of correctly.

Flowback water can become contaminated through contact with shale, dissolving salts from it and also picking up traces of natural radioactivity. Shale formations are naturally slightly radioactive. It is important that any radioactive content be taken into account in the procedures for handling the flowback and production fluids at the surface.

During the drilling phase, about 3000 tonnes of drill cuttings of rock (equivalent to a cube of solid rock 10 m x 10 m x 10 m) will have been produced per well. This must be trucked away and disposed of.

Procedures for the disposal of waste fluids and solids are subject to geographical considerations and local and national mining waste disposal regulations.