Gippsland Basin CO2 Storage Project

Project Overview

As part of the four-year National CO₂ Infrastructure Plan (NCIP), Geoscience Australia conducted a CO₂ storage capacity assessment and seismic data acquisition program in the offshore Gippsland Basin. The project was undertaken to accelerate the identification of suitable long term CO₂ storage sites near the sources of major energy and production emissions in the Gippsland region in Victoria. The study built on previous CO₂ storage studies in collaboration with the Geological Survey of Victoria VicGCS program and CSIRO.

The Gippsland Basin has been actively explored and has produced hydrocarbons since the 1960s. It contains numerous giant oil and gas fields that are located in the central and northern part of the basin, including the Kingfish, Marlin and Barracouta fields. The Latrobe Group is the major reservoir unit in the basin and the overlying Lakes Entrance Formation is the regional top seal unit. The offshore Gippsland Basin has been identified by previous storage prospectivity studies as highly prospective for long-term, high capacity CO₂ storage. Potential CO₂ storage sites that could become available in the next 10-20 years will probably be restricted to sites in the nearshore and southern parts of the offshore Gippsland Basin. This is in order to minimise possible overlaps with ongoing petroleum production in the northern and central parts of the offshore basin and onshore aquifers. Large areas in the southern and nearshore margin were released as areas for Greenhouse Gas Storage Assessment and were gazetted in 2009 and 2014.

The project focused on the CO₂ storage potential of the underexplored southernmost part of the Central Deep, Southern Terrace and Southern Platform areas within the basin. A pre-competitive 2D reflection seismic survey, GA0352, was acquired in the southern part of the basin in 2015 as part of the project to provide new data to support future CO₂ storage activities. The seismic survey was designed to tie the existing extensive 3D seismic coverage in the central part of the basin to the southern part of the basin, so seismic lines were shot perpendicular to the main structural trend of the basin. The survey has improved seismic data density in an area with widely-spaced seismic lines that were acquired in 2010 as part of the VicGCS program. Bathymetric data was also acquired during the survey.

Location of the GDPI10 and GA0352 2D seismic reflection surveys and the extent of 3D seismic coverage in the Gippsland Basin and the location of the geological modelling and reservoir simulation study area (yellow boundary).

Scientific Objectives

The study focused on assessing several factors that are critical to understanding the CO₂ storage potential of the southern Gippsland Basin. These include:

• the effect of coal within reservoir units on potential CO₂ migration and storage;
• the seal quality of the lower Seaspray Group;
• an analysis of potential CO₂ storage sites in the southern part of the basin; and
• CO₂ injection rates and storage capacity in the upper Latrobe Group

Marine biological studies on the potential impacts of seismic noise on fish and other commercial species were also undertaken in the project to monitor the impact of seismic activities on marine life, as it is not yet fully understood.

Scientific Results

As coal measures are common within potential CO₂ reservoirs in the Latrobe Group, and due to the complexity of coal-CO₂ effects, reservoir simulations of CO₂ storage in the Gippsland Basin should consider the nature and distribution of coal in the CO₂ migration and storage process. The migrating CO₂ acts to displace adsorbed methane within the coal. At the same time, the adsorption of CO₂ leads to coal swelling that decreases the permeability. Therefore, even if the coal seams were not seals, they would act to retard and spread the CO₂, and increase potential residual trapping and dissolution of CO₂. The key property that determines if coal seams can act as seals to the vertical migration of CO₂ is the CO₂ capillary entry pressure. In general, coals become increasingly water wet and have higher capillary entry pressure as the coal rank decreases; therefore, the low rank coals of the Gippsland Basin are likely to be water wet and essentially act as intraformational seals. However, if higher rank coals are present, the capillary entry pressure would be low and therefore their potential to act as intra-formational seals would be limited.

A detailed study was undertaken on a major deformed zone located along the southern part of the basin to examine if the seal integrity of the regional seal facies, the Lakes Entrance Formation, has been compromised. The deformed or slump zone is associated with a series of sedimentary mass transport deposits that straddle the southernmost Central Deep, and parts of the Southern Terrace and Southern Platform areas in the basin. The study has concluded that the seal has probably not been compromised across the entire area underlying the slump zone; however, it is recommended future site investigations acquire high resolution 3D seismic data over potential fluid migration pathways to fully access the risk proposed by this feature, and other geological aspects of the seal.

An analysis of potential CO₂ storage sites in the southern part of the basin was undertaken by detailed geological mapping and modelling of a selected area that contained potential reservoir facies in the upper part of the Latrobe Group, an overlying effective regional seal unit, and a lack of overlap with ongoing and potential hydrocarbon production. A series of numerical and analytical fluid flow simulations of the selected area indicated that it is feasible to achieve injection and containment of commercial scale volumes of CO₂ in this basin without adversely impacting existing resources.

Horizontal permeability values (scale in millidarcies; logarithmic scale) in the upper Latrobe Group reservoir simulation model.