Insights and Commentaries

‘Enhanced Water Recovery’: Brine Extraction with CO₂ Geological Storage as part of CCS

16th July 2016

Topic(s): Carbon capture, use and storage (CCUS)

Deep saline formations (DSF) provide the largest potential storage resource available worldwide to CCS projects; in the United States (US) alone, effective storage resources have been conservatively estimated as in excess of 2,000 Gigatonnes (Gt) of CO₂; to place this resource in context, latest IEA projections in the 2016 Energy Technology Perspectives (ETP) report shows over 80 Gt of storage needing to be achieved by 2050. However, storage resources in DSF are typically subject to a greater level of uncertainty than for depleted oil and gas fields or storage associated with enhanced oil recovery (EOR). Part of this greater uncertainty is due to the often limited availability of information on geological characteristics, and pressure management is a key related technical issue.

The ability of any given storage reservoir to accommodate the pressure increases which result from CO₂ injection is dependent on a number of technical factors, and there have been differing views on how pressure management requirements could impact storage capacity in DSF sites. One theoretical solution to aid pressure management and increase capacity would be to remove some of the highly saline groundwater (or brine) that occupies the pore spaces of DSF, thereby reducing pressures and creating more ‘space’ for injected CO₂. This concept of brine extraction has also been dubbed ‘enhanced water recovery’ (EWR), to draw a parallel with the established EOR industry.

The costs associated with DSF storage operations may rise significantly with the application of EWR, although there may be economic benefits in some cases from the resource value of brines that could offset part or all of these costs. Increased costs could result from the following factors:

• Increased drilling and infrastructure capital/operating costs;
• The potential for CO₂ ‘breakthrough’ in brine extraction wells – in this case CO₂ could be lost through venting or recycled (in similar fashion to EOR) with associated infrastructure and operating costs; or the extraction well might be prematurely shut in or abandoned;
• Brine treatment, where required for utilisation as opposed to disposal.

Economic benefits of brine extraction could be derived from:

1. Utilisation of the extracted brine for industrial purposes, for example to supply part or all of the needs of a CO₂ capture plant. This may bring the added environmental benefit of displacing the use of groundwater or surface water resources, especially in arid regions where potable water resources may be scarce;

2. Extraction of valuable minerals dissolved in the brine, for example lithium.

The relative merits of brine disposal versus utilisation will depend on a number of project-specific factors. One key issue that can determine the viability of brine as a water source is the level of salinity – higher salinity brines are more expensive to treat for utilisation than lower salinity brines. The salinity of brines in DSF varies widely but can be very high in some cases; for example, brine sampled at the Aquistore project in Canada has a total dissolved solids (TDS) content in excess of 300,000 parts per million, almost 10 times the salinity of seawater.

EWR is yet to be applied to any operational CO₂ geological storage project, but large volumes of brine are routinely co-produced and/or re-injected into the subsurface for disposal by extractive industries, most notably in oil and gas operations. The offshore Gorgon project will be the first CCS project with dedicated storage to incorporate brine extraction as part of the operational plan; brine will be disposed of by re-injection into separate rock layers above the storage reservoir.

Despite not being currently employed in operational CCS projects, EWR has been and is the focus of considerable R&D efforts, especially in the US. In September 2015, the US Department of Energy (DOE) sponsored five Brine Extraction Storage Tests (‘BEST’) projects that developed engineering strategies for the management of pressures and fluid flow in CO₂ geological storage reservoirs. Subsequently, the DOE has recently announced that two of the BEST projects will progress to field demonstration, receiving a total of US$31 million in funding to assess reservoir pressure management issues and to test technologies associated with EWR:

1. The Energy and Environmental Research Centre at the University of North Dakota and partners will make use of a commercial brine disposal facility located at Watford City in North Dakota;
2. The Electric Power Research Institute will base their field testing on existing wastewater disposal wells at a site near Panama City in Florida.

As part of the collaboration between the US and China on climate goals announced in September 2015, EWR was included in technologies to be investigated by the two nations. The GreenGen project in China comprises a plan by the Huaneng Corporation to capture CO₂ from coal at an integrated gasification combined cycle plant; DSF storage with EWR is a potential option for the captured CO₂. EWR is also one focus area of the Advanced Coal Technology Consortium, part of the US-China Clean Energy Research Centre.

In conclusion, EWR offers significant potential benefits to CCS projects through increased storage capacity in DSF, improved reservoir pressure management strategies and potential economic benefits from extracted brine. However, at some locations, costs associated with storage may increase with the application of EWR. During the coming 12 months, the storage team within the institute will be compiling a detailed report examining the technical and economic issues surrounding EWR.

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