What is shale gas?

Shale gas is mainly methane that is trapped within a clay-rich sedimentary rock at depths greater than 1,500 metres. The low permeability of the rock means that gas, either adsorbed or in a free state in the pores of the rock, is unable to flow easily.

Unearthing shale gas [Music plays and text appears: Unearthing shale gas. What is shale gas, how is it extracted and what are some of the challenges involved?] [Image changes to show a computer generated cross-section of land with the shale rock level labelled] Narrator: Shale gas is mainly methane trapped within shale rock layers at depths greater than 1.500 metres. Australia’s shale gas industry is largely in the exploration phase. This involves drilling vertical and horizontal wells and hydraulically fracturing, or fraccing, the shale rock to see if gas can be produced economically. [Image changes to show the drilling lines appearing on the computer generate cross-section of land, with markers at both 1,500 and 3,000 metres deep] When in the production phase, wells are drilled anywhere from 1,500 to 3,000 metres deep, through various layers of rock to access the shale. To protect groundwater from contamination the well is lined with cement and steel casings. [Camera zooms in on the well and shows the cement and steel casings] Horizontal drilling is a technique used to maximise shale gas recovery and minimise surface impacts. [Image changes to show a line appearing underground on the computer generated cross-section, labelled Horizontal Drilling] Before gas production can start hydraulic fracturing needs to occur. [Camera zooms in on the on the section labelled Hydraulic Fracturing] This involves perforating along the horizontal portion of the well to gain access to the shale rock. Water containing chemical additives is pumped under high pressure to open up existing fractures and create new ones within the shale rock. [Image changes to show water running through the fracture. Camera zooms in on a drop of watered labelled 1 % chemical additives and 99 % water proppant] Proppant, such as sand is then added to the water that flows through to the fractures. [Camera zooms in on the sand moving through the water in the fracture] The sand keeps the cracks open allowing the gas to flow to the well and up to the surface. [Image changes to show the process being repeated and new fractures appearing] This process is repeated several times within the horizontal portion of the well, with each fracturing stage separated by a plug. At the end of the hydraulic fracturing process the plugs are removed and production can start. [Camera pans up the computer generated image to reveal the well head where a truck and equipment can be seen extracting the water and gas] Shale gas and any produced water flow to the well and are pumped to the surface and separated at the well head. Extracted gas is processed and transported for domestic and/or international use. [Diagrams of the process appear on screen beginning at the cross-section of land where the shale is extracted, moving to a Gas compressor station (cleaning and compression), with two arrows off that box showing a Domestic use box and Export box] The produced water is treated, then either used in future hydraulic fracturing jobs, or disposed of in accordance to state government regulations. [Diagrams of the process appear on screen beginning at the cross-section of land, moving to a Water treatment box and then to a Re-use or disposal in accordance to state government regulations box] A source of concern is the amount of water used in the hydraulic fracturing process. [Image changes back to show the computer generated cross-section of land with text: 20 Megalitres – eight Olympic pools appear on top of the text] An average of 20 mega litres of water can be used per well, which would fill about eight average Olympic sized swimming pools. Another possible impact is groundwater contamination from accidental surface spills or leaks of produced water and hydraulic fracturing fluids. [Image changes to show the truck reversing and a spill can be seen coming from under the truck] Other potential environmental impacts include the industry’s greenhouse gas footprint, fragmenting of local habitat and changes to rural communities. [Camera pans out on the computer generated cross-section of land] CSIRO is conducting research to better understand the impacts of shale gas development and develop sound technologies and practices to ensure socially and environmentally responsible development. [Text appears: Research to inform decisions. Visit the CSIRO and GISERA websites for more information and latest research. www.csiro.au, www.gisera.org.au] [Music plays and CSIRO logo appears with text: Big ideas start here www.csiro.au]

October 2015.

How is shale gas extracted?

To extract shale gas, wells are drilled anywhere from 1500-4000 meters deep, through various layers of rock to access the shale. The wells are lined with cemented steel casings to protect groundwater from contamination.

To maximise shale gas recovery and minimise surface impacts a technique called horizontal drilling is used. This technique involves the well changing from a vertical to a horizontal direction deep underground.

 

Schematic diagram of shale extraction process

Hydraulic fracturing 

Before any gas can be extracted hydraulic fracturing, also known as fraccing, needs to occur. This involves perforating along the horizontal portion of the well to gain access to the shale rock.

Water containing chemical additives is pumped under high pressure to open up existing fractures and create new ones within the shale rock.

This hydraulic fracturing fluid is typically made up of 99% water and proppant and 1% chemical additives. Proppant, such as sand is then added to the hydraulic fracturing fluid, which carries it into the fractures. The proppant holds open the hydraulic fractures after injection stops, allowing the gas to flow to the well and up to the surface.

This process is repeated several times within the horizontal portion of the well to create multiple hydraulic fractures. Each injection of hydraulic fracturing fluid is called a stage and requires isolation. One common method to separate each fracturing stage is with a plug. At the end of the hydraulic fracturing process the plugs are removed and production can start.

How much water is used in the hydraulic fracturing process for shale gas?

An average of 20 mega litres of water can be used per horizontal well, which would fill about eight average Olympic sized swimming pools.

What happens to the extracted gas and produced water?

The extracted gas is processed and transported for domestic and/or international use.

One of the key differences between shale gas and coal seam gas (CSG) production is that shale gas extraction produces very little water. This is because there is little to no water trapped in shale rocks.

If produced water does flow back to the surface then that is mainly composed of hydraulic fracturing fluid. The produced water is usually treated and then either used in future hydraulic fracturing jobs, or disposed of in accordance to state government regulations.

Potential environmental risks

A source of concern is the amount of water used in the hydraulic fracturing process during shale gas production.

Another concern is that hydraulic fracturing fluids may leave shale rock and enter fresh water aquifers, which are layers of porous permeable rock that allow water to flow through easily.

The depth at which shale gas extraction occurs is far below the location of fresh water aquifers, reducing the risk of aquifer connectivity to a very low level.

Contamination of groundwater during shale gas production is more likely to occur as a result of surface spills, overflows of containment ponds/tanks or leaks of produced water and hydraulic fracturing fluids at the surface.

Other potential environmental impacts include the industry’s greenhouse gas footprint, fragmenting of local habitat and changes to rural communities.

 

 

 

 

Hydraulic fracturing differences in coal seam and shale gas

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