Crude oil, condensate and natural gas (collectively known as petroleum) began forming millions of years ago when sand, mud and organic matter (plants and animals) were buried together and heated, forming different types of rock including sandstone (tight sandstone and porous sandstone), carbonate rocks and shale. Over millions of years, the trapped organic matter is converted into petroleum.
Fluids such as oil, gas and water can move through rock formations but become trapped if they reach a more impenetrable layer of rock, known as a seal. This trapped oil and gas may become a resource to be developed into a commercial petroleum field. Only a small number of petroleum traps may be developed, as the characteristics of the rocks in which the resource is trapped will determine whether the resource can be commercially produced.
Conventional resources
“Conventional resources” is a term referring to oil and natural gas trapped in rock that is porous and permeable. The natural pressure of the underground rock formation allows oil and natural gas to flow freely up a petroleum well. For conventional petroleum production, the spaces between the grains of rock (called porosity) need to be connected (called permeability) to allow the flow of fluids through the rock.
Unconventional resources
Oil and natural gas trapped in less permeable rocks is referred to as an unconventional resource because it cannot be explored, developed and produced by conventional processes.
Shale gas is natural gas trapped in shale rock. Shale is an organically rich sedimentary rock formed of very fine-grained, or small, particles, such as clay, that have been compacted to form a layered rock. Shales can be rich sources of oil and natural gas.
Tight gas is natural gas trapped in subsurface rock with a very low permeability of 0.1 millidarcy or less (as defined in the Petroleum and Geothermal Energy Resources Act 1967). Tight gas formations are generally more permeable than shale gas formations.
In Western Australia (WA), shale gas and tight gas resources are typically found between 2,000 and 4,000 metres underground. These depths generally lie significantly below groundwater resources and under multiple thick layers of low permeability rock that act as barriers, or seals, between the gas-bearing rock formation and water resources and the land surface.
What is coal seam gas?
Coal seam gas (CSG), also known as coal bed methane, is natural gas found in association with certain coal deposits that is trapped by water, which must be removed for the gas to flow. CSG typically lies at depths of 300 to 1,000 metres. While the CSG industry is established in Queensland, to date, CSG has not been demonstrated to be prospective in WA. CSG wells are drilled down into coal seams where water is pumped out (a process known as dewatering) and the CSG is released from the coal. If the pressure within the coal seam is high enough, the gas may flow to the surface unaided, otherwise stimulation techniques may be used.
Extraction of unconventional resources
Hydraulic fracturing or fracking/fraccing, is a method of extracting natural gas from underground rocks with low permeability, such as shale and tight sandstone. Fluids are pumped under high pressure into a petroleum well to create fractures in the rock deep underground to enable the flow of the gas resource to the surface.
Hydraulic fracturing has generated controversy, particularly as a result of coal seam gas (CSG) extraction, and has been subject to bans, moratoriums and a number of Parliamentary Inquiries in WA and nationally.
Following careful consideration of the Independent Scientific Panel Inquiry into Hydraulic Fracture Stimulation in Western Australia’s report that found the risk from hydraulic fracturing is low, the State Government has decided to lift the hydraulic fracturing moratorium on all onshore petroleum titles existing as of 26 November 2018. Hydraulic fracturing will be subject to a raft of new, world-class controls to ensure risks associated with hydraulic fracturing are minimised and managed safely.
Table 1: Typical differences between coal seam gas and shale gas. The data shown in this table will vary from case to case. Sources: CSIRO, Northern Territory Government
|
|
Coal seam gas |
Shale and tight gas |
|
Source |
Shallow coal seams |
Deeper shales and tight rocks |
|
Depth |
300m – 1,000m |
1,500m – 4,000m |
|
Drilling direction |
Mainly vertical |
Horizontal and vertical |
|
Proximity to aquifers |
Shallow and therefore closer to potable water resources |
Deeper and therefore further away from potable water sources |
|
Surface footprint |
Single exploration well per drill pad (a drill pad is the area disturbed by the installation of the drilling and extraction equipment) meaning a larger number of wellheads in a given area. |
Multiple wells can be drilled from each well pad, meaning a lower number of completed wellheads in a given area than for coal seam gas. |
|
Hydraulic fracturing |
Hydraulic fracturing in some coal seams with low permeability |
Always requires hydraulic fracturing |
|
Hydraulic fracturing extent (length x height) |
200m – 300m x 5m – 30m |
200m – 6,000m x 30m – 300m |
|
Hydraulic fracturing pressure |
35MPa or 5,000psi |
35 – 70MPa or 5,000 –10,000psi |
|
Water use |
Requires dewatering of coal; and water for drilling and hydraulic fracturing where required |
No dewatering, but water is used for drilling and hydraulic fracturing |
|
Hydraulic fracturing fluid per volume per well |
Approximately 1ML (0.1 – 3ML) |
Approximately 20ML (5 – 40ML) |
|
Number of wells required |
Larger number of wells |
Fewer wells required than for coal seam gas |
|
Productivity (over lifetime of well) |
Lower gas recovery |
Higher gas recovery |
Terms used
carbonate rock: a sedimentary rock composed of minerals such as calcite, aragonite and dolomite (e.g. limestone).
millidarcy: a measure of permeability. A millidarcy (mD) is one thousandth of a darcy, the standard unit of measure.
natural gas: naturally occurring mixture of hydrocarbon gases that is highly compressible and expansible. Methane [CH4] is the chief constituent of most natural gas (constituting as much as 85% of some natural gases), with lesser amounts of ethane [C2H6], propane [C3H8], butane [C4H10] and pentane [C5H12]. Impurities can also be present in large proportions, including carbon dioxide, helium, nitrogen and hydrogen sulphide. Natural gas produced from shale reservoirs is known as shale gas.
permeability: the ability, or measurement, of a rock’s ability to transmit fluids.
petroleum: a complex mixture of naturally occurring hydrocarbon compounds found in rock in liquid, gaseous or solid form (i.e. crude oil, natural gas, or bitumen).
tight sandstone: a sedimentary rock composed mainly of sand-sized minerals or rock grains that has very low permeability and porosity.
seal: a relatively impermeable rock, commonly shale, anhydrite or salt, that forms a barrier or cap above and around reservoir rock in such a way that fluids cannot flow beyond the reservoir.
shale: a sedimentary rock composed of clay-sized particles that is laminated or layered.
tight gas: natural gas trapped in a sedimentary rock composed mainly of sand-sized minerals or rock grains that have very low permeability and porosity.