Repurposing gas fields for energy storage – a proposal for compressed air energy storage in the depleted Rolleston gas field

Introduction

Achieving net-zero emissions is a key objective for Australia. Queensland has abundant space and sunshine for renewable projects in its inland regions, but vast distances to the high-demand east coast population centres pose a challenge. Pumped hydro offers large-scale energy storage; however, the former Pioneer-Burdekin Pumped Hydro was cancelled due to its high cost and the significant environmental impacts.

Compressed air energy storage (CAES) offers a potentially more viable and lower impact alternative. As shown in Fig. 1, CAES uses the excess renewable energy to pressurise the air through compressors and injects it into underground caverns/porous media. During periods of high power demand, the compressed air is withdrawn through wells to drive the turbines and generate electricity. The compression heat is stored in the thermal tank and reused to heat up the air in the expansion process, which increases the round-trip efficiency up to 72%.

Fig. 1.

A schematic illustration of repurposing depleted gas reservoir for CAES.

Reservoir screening

The majority of CAES plants today are constructed on salt caverns. However, salt formation in Australia is scarce and mostly located in remote areas such as the Amadeus Basin in Northern Territory. Depleted gas reservoirs, of which Australia has many, are strong candidates for CAES due to their enormous underground volume and proven stable seals formed over millions of years. The advantages of depleted gas reservoirs for CAES have attracted interests from both industry and researchers (King and Apps 2014; Wang and Bauer 2017; King and Moridis 2022; Medeiros et al. 2022). It is recognised that candidate reservoirs should have properties, including:

After reviewing the geological and production data of Denison’s gas fields in Denison Trough, the Freitag gas reservoir in Rolleston gas field has been selected as a favourable candidate for a CAES plant of 600 MW and 12-hour duration, i.e. 600 MW/7.2 GWh.

Reservoir pressure and permeability

The Freitag gas reservoir in the study area is buried at the depth of ~870–1000 m below the surface (Fig. 2a, surface elevation is ~200 m above sea level) with a thickness of up to 18 m in the centre of the four-way dip trap (Fig. 2b, c) and porosity of ~20%.

Fig. 2.

Burial depth below sea level (a), reservoir thickness (b) and a typical well log (c) of Freitag gas reservoir in Rolleston gas field.

Well testing on the Freitag gas reservoir at Rolleston-1 shows the initial reservoir pressure is 10.6 MPa at a depth of 900 m, indicating an over-pressured reservoir. Drill stem test analysis of Rolleston-16 shows permeability of 430 mD; and the permeability derived from production analysis is over 1000 mD. Troup et al. (2018) reported a porosity of 23–28% and permeability close to 4000 mD. For power generation of 600 MW, approximately 1400 kg/s of air flow, equivalent to approximately 100 million standard cubic metres per day (sm³/day), is required at the inlet of the turbine. The Darcy-level permeability of the Freitag sandstone has the potential to make this achievable.

Reservoir simulation

The commercial simulator tNavigator was employed to evaluate the air productivity of the Freitag gas reservoir for the proposed 600 MW/7.2 GWh CAES plant. The Freitag Formation is represented by 44 × 69 × 10 cells. Porosity and permeability are interpolated to the cells, with a range of 8–25% for the porosity and a range of 400–1500 mD for the permeability. The east-west extended fault (Fig. 2) is incorporated with zero transmissibility. In the central area where the permeable sand developed, 42 injection/withdrawal wells are distributed at distances of 100–200 m.

In the initial stage, it is assumed the residual gas was displaced by nitrogen (N₂) to be below the combustion threshold and air was injected into the reservoir to the original reservoir pressure. The simulation lasts for 30 days, with alternative 12-hour withdrawal and 12-hour injection at a time step of 2 hours. During withdrawal, minimum bottomhole pressure (BHP) of 6 MPa is set for all wells and maximum flow rate ranging from 1.3 to 3.8 million sm³/day is assigned for the wells. During injection, original reservoir pressure is set as the maximum BHP and the same maximum rate is assigned to wells.

On Day 1, the air withdrawal rate is constant at 103.4 million sm³/day in the first 10 hours and slightly drops to 102.4 million sm³/day in the last 2 hours (Fig. 3a); at the end of withdrawal, the reservoir pressure in the well-dense area drops to approximately 6.5 MPa with higher values of 8–9 MPa at the edge of the trap (Fig. 3b). The following air injection rate is maintained at 103.4 million sm³/day in the first 6 hours but drops to 97.3, 63.3 and 40.6 million sm³/day in the next 3 time steps, respectively (Fig. 3a). Less air is injected than withdrawn and therefore the average reservoir pressure is below its initial status, especially at the edge of the trap (Fig. 3c).

Fig. 3.

Air withdrawal/injection rate in 30 days (a), reservoir pressure at the end of withdrawal (b) and at the end of injection on Day 1 (c).

Due to this pressure loss, the air withdrawal rate drops below 100 million sm³/day in the last 2 hours on Day 2 and in the last 4 hours from Day 3 onwards. A pattern is developed from Day 4 onwards. The withdrawal/injection rate maintains at 103.4 million sm³/day in the first 8 hours but drops to ~92 and ~60 million sm³/day in the last 2 time steps. Due to the inadequate air flow, the power generation would therefore be below 600 MW in the last 4 hours. To resolve this inadequacy, probably more wells are needed to drain and recharge the reservoir more effectively.

Generally, it indicates the Freitag gas reservoir has the potential to deliver the required air flow for the 600 MW/7.2 GWh target. According to the current simulation, the deliverability of the 42 wells declines due to the pressure loss as time goes on. This could be improved by more wells with optimised well distribution, or the proposed capacity of the CAES should be adjusted to be more consistent with the reservoir deliverability, with both requiring further analysis.

Concept definition

Repurposing a depleted gas reservoir for CAES incorporates multiple elements including a source of renewable power, gas reservoir, grid connection and environmental impact analysis.

As presented, the Freitag gas reservoir in the Rolleston gas field is selected for the proposed CAES project. Given the project is the first of its kind, a 10 MW/120 MWh pilot has been designed and a regulatory framework is needed, for example, to legislate the maximum injection pressure of the compressed air. Before air injection, N₂ injection would be implemented to displace the residual methane to avoid combustion. A GW-scale solar farm might be constructed as the source of renewable power. Extension of the high-voltage transmission line to the proposed site is required to transmit the clean energy to east coast cities.

These key elements including regulation establishment, extension of transmission line and the pilot have been initiated and progressing with relevant parties.

Conclusion

Repurposing a depleted gas reservoir in the Rolleston gas field into a long-duration CAES could harness the abundant solar resource in central Queensland and help facilitate the Government’s energy transition targets. The Freitag sandstone, with its desirable reservoir properties, is favoured for the proposed 600 MW/7.2 GWh CAES plant. Potential air leakage is derisked by the seal of tight thick shale formed over millions of years. Reservoir simulation indicates that the deployed 42 wells could deliver adequate air for the surface facilities in the first 8 hours, but more wells are needed for a sustainable 12-hour operation.

A broad and close cooperation between the gas reservoir owners, landowners, regulatory departments and the transmission operator is crucial to make this project successful and duplicatable in other gas fields, and ultimately create a cleaner future.