August 19, 2014
Unconventional Gas in Western Australia
What is Unconventional Gas?
Natural gas is found in a number of different geological environments. As demand for gas has increased and drilling and extraction technology has improved, so exploration and production is moving into geological environments that were previously too challenging; gas in such locations is called Uncoventional Gas and it is often described based on the type of rock from which it is extracted (Figure 1), such as:
• coal seam gas (CSG) – found in coal seams,
• tight gas – found in compacted sandstone or limestone and
• shale gas – found in dense shale rock.
To release the gas from these different formations, some form of engineering is required to produce commercially viable rates of gas flow. In most cases, hydraulic fracture stimulation (also known as hydraulic fracturing or fracking) is required to improve the flow paths in the gas bearing rocks. For CSG deposits in Australia, 10-40% of wells need to be hydraulically stimulated (Frogtech, 2013), while all shale gas wells will require hydraulic stimulation.
Unconventional Gas in Western Australia
In Western Australia, coal seam gas deposits are very limited, but considerable reserves of shale and tight gas exist, located in the Kimberley, East Pilbara and Midwest regions. WA basins are estimated to contain 280 trillion cubic feet (Tcf) of shale and tight gas (DMP, 2014), as follows:
• 235 Tcf are in the Canning Basin (Kimberley and East Pilbara regions)
• 45 Tcf are in the northern Perth basin (Midwest region)
Since 2005, only 15 exploration wells have been drilled to search for shale and tight gas resources in Western Australia (DMP, 2014). Seven of these were hydraulic fractured to test the gas flow capacity of the reservoir. It is clear then, that the shale gas industry in WA is still in its infancy and considerably more drilling and testing is anticipated in the future.
The primary geological risks associated with shale gas development are induced seismicity, water management and well integrity. From a water management perspective, the concerns that need to be addressed as part of future developments in WA, are:
• the supply water used for fracking and drilling,
• contamination of adjacent potable aquifers due to hydraulic stimulation practises, and
• the treatment of produced water
The volume of water needed to conduct hydraulic stimulation of a well varies from project to project, depending on the stage of the project, size and length of the well and the type of rocks being stimulated. Typically, the volumes required for shale gas wells (DMP, 2014) are:
• Exploration phase – 7ML (equivalent to a water supply bore being pumped continuously at 2.7 L/s for one month)
• Evaluation phase – 7-17 ML(equivalent to a water supply bore being pumped continuously at 2.7 – 6.8 L/s for one month)
• Production phase – 21 ML(equivalent to a water supply bore being pumped continuously at 8.1 L/s for one month)
The volume of water needed to stimulate shale gas strata is typically an order of magnitude higher than for coal seam gas wells, due to greater depths and different geology (Golder Associates, 2010).
In WA, most of this water will have to be supplied from groundwater. In the arid areas of the Canning Basin, a shale gas wellfield has the potential to become a major user of groundwater, relative to other users, with the potential to impact on the sustainable supply potential of local aquifer systems. In the Perth Basin, groundwater abstraction is largely at its sustainable or allocation limit. A key part of well-stimulation operations will be groundwater licensing. Notwithstanding, the re-use of hydraulic stimulation fluids is probably going to be necessary.
Contamination of water supply aquifers
Since shale gas wells are much deeper than CSG wells, the risk of stimulation impacting on water supply aquifers is limited (Frogtech, 2014). Shale gas resources tend to be well below the depth of potable aquifers, while the shales also act as barriers to movement of any contaminants (Figure 2).
Although stimulation is unlikely to impact the shallow aquifers, there are still contamination risks from well failure, and/or poor handling of produced water.
Management of Produced Water
During the stimulation programme, some product water is generated. The water is a usually a highly saline mix of recovered stimulation fluid and connate water from the shale, but is of much lower volumes that in CSG wells. Typically from 30-70% of injected water is recovered. When this water reaches the surface it must be stored, treated and disposed of properly, to avoid damage to the environment, people and water supplies. Considering the arid nature of most of the WA basins and the limited groundwater resources, the treatment and re-use of the product water is likely to be encouraged by the authorities involved (DMP, 2014)
The team at AQ2 have helped resources companies with groundwater assessment, management and licencing in the Canning and North Perth Basins. Our team can help with:
• Identification of aquifers that can be used for water supply
• Installation of water supply bores and design of reticulation systems associated with the borefield
• Assessment of the effects of groundwater withdrawal from these aquifers on the environment and other users, including numerical modelling of the drawdown resulting from pumping
• Delineation of the regional hydrogeology system
• Installation of monitoring bore networks to allow assessment of the impacts of water supply abstraction, stimulation and product water management, on the shallow aquifer system
Department of Mines and Petroleum (DMP), Government of Western Australia), 2014, Natural Gas from Shale and Tight Rocks, An overview of Western Australia’s regulatory framework, February 2014.
Frogtech, 2013. Potential Geological Risks Associated with Shale Gas Production in Australia, Australian Council of Learned Academies, January 2013, Project Code: AAS801.
Golder Associates, 2010. Coal Seam Hydraulic Fracturing fluid environmental risk assessment response to the co-ordinator-general requirements for coal seam gas operations in the Surat and Bowen Basins Queensland. Submitted to Santos Ltd.
Royal Society and The Royal Academy of Engineering, 2012. Shale gas extraction in the UK: a review of hydraulic fracturing: http://royalsociety.org/policy/projects/shale-gas-extraction and raeng.org.uk/shale