Background

Deep saline formations (DSF) constitute the largest potential global resource for the geological storage of CO₂ and are therefore crucial to the successful up-scaling of storage from pilot and demonstration projects to commercial operations. However, there are uncertainties relating to the capacity and injectivity of DSF, with particular concerns relating to the management of pressure and potential displacement of formation brines. Extraction of saline waters from storage formations provides a potential solution to pressure management; for example the proposed Gorgon storage project in Australia includes the provision of pressure relief boreholes.

The effect of pressurisation in a storage formation will depend largely on whether the system can be considered as open or closed. In a closed or semi-closed system, the pressure build-up will be determined by the boundary conditions, which include the shale permeability. Recent studies have shown that microdarcy scale shale permeability will allow brine displacement, while very low shale permeabilities on the nanodarcy to subnanodarcy scale will not. Part of the problem comes from the uncertainty in assessing brine displacement due to boundary condition uncertainty. It can be difficult to determine macroscopic scale permeability, even when samples have been obtained, due to problems with up scaling measurements as regional permeability effects also need to be taken into account (IEAGHG, 2010).

Pressure relief wells can compensate for increases in pressure caused by injection, though extraction rates will depend on site-specific factors e.g. geological structure, shale permeability and heterogeneity.

Heterogeneities in the storage formation may cause complexities in predicting flow rate and direction of injected CO₂. If an extraction well is placed along a path of high permeability, then the rate of flow towards the well would be high, resulting in unwanted CO₂ breakthrough. This may necessitate the plugging of the old well and the consequent drilling of a new pressure relief well, thereby increasing the potential cost of the project and possibly affecting the storage security. This possibility highlights the importance of a detailed site characterisation. Brine extraction could also play a part in plume management.

The plume may be managed both laterally and vertically, as the CO₂ will be forced to migrate towards the extraction wells. In the case of forced downward migration, the extraction wells will be towards the base of the storage formation. This will cause a larger vertical proportion of the formation to be used and the lateral extent and contact of the CO₂ plume with the caprock will be reduced. Both of these effects can increase storage security. This also means that CO₂ plumes formed at adjacent or nearby injection wells would be less likely to interact with each other.

For large scale projects, there are likely to be multiple injection and pressure-relief wells. It is important to consider how they will interact with each other, as there will be an overlap of pressure footprints from each well.

The water extracted from the storage formations will need to be used or disposed of in some way, for example, at the proposed Gorgon project in Australia, the planned injection of the extracted brine will be into an overlying saline aquifer. Possibilities for future sites include disposal directly in the sea, which would be dependent on the composition of the brine; alternatively the water could be utilised for other industrial processes, such as the cooling process within power stations or use as geothermal energy or it could be desalinated and used either for irrigation or drinking water. The latter options would depend on the cost and demand of water as a resource.

The Energy & Environmental Research Center, in North Dakota, USA, was commissioned by IEAGHG to provide a thorough review of existing information and published research on the effects of brine extraction from CO_­2 storage sites. The study also aims to highlight the current state of knowledge and / or gaps and recommend further research priorities on these topics.

Recommendations

There is yet to be any large scale demonstration of this topic and most information is currently through modelling studies. It is recommended that IEAGHG continue to follow this topic and any updates, through future storage network meetings, namely the modelling network and by the study programme.

A future review of this topic would be useful as data is generated by future large scale demonstration projects.