Coal seam gas is trapped in coal formations, typically 400-1000 metres underground, although it can be sometimes be found at depths of up to 1200 metres.
CSG deposits are usually about 200 metres below the water table used for stock and domestic purposes and are usually separated from these aquifers by impermeable rocks (or “aquitards”) that impede the flow of water.
The transition from exploration to production involves installing production wells drilled into gas-bearing coal seams.
Because CSG wells are not as deep as most other oil andgas wells, the rigs used to drill them and the surface infrastructure are relatively small.
Drilling and well construction are tightly regulated in Australia. The oil and gas industry has a strong understanding of the technologies involved as millions of wells have been drilled around the world in the last 160 years.
Drilling stops at regular intervals so that purpose-built steel pipes – or casing – can be installed.
The gap between the casing and borehole wall is filled with cement. The casing and the cement form a non-porous barrier that prevents cross-contamination between the petroleum-bearing rock formation and any overlying aquifers. The casing and cement are pressure-tested to ensure that they can tolerate higher pressures than those expected over the life of the well.
A wellhead – which contains barriers, valves, seals and a gas/water separator – is placed on the surface to maintain control of the well and the drilling rig is moved from the site.
In onshore drilling, an area is fenced off around the well, which is now ready for production. A CSG well-head and associated infrastructure is smaller than an average water tank and the fenced off area around the well is typically about 23m by 23m.
Once a well has been completed, the drilling site is rehabilitated.
Wells typically produce gas for many years with little surface activity other than regular environmental checks. When a well is no longer producing, it is permanently sealed off by a series of cement plugs designed for durability and withstanding high pressure.