Investigation of Extraction of Formation Water From CO2 Storage: Beneficial Use Options and Requirements for Extracted Water

Deep saline formations (DSFs) constitute the largest potential global resource for the geologic storage of carbon dioxide (CO₂). Their use is, in turn, crucial to the successful scale-up of storage from pilot and demonstration projects to commercial operations. A proposed method for managing DSFs for CO₂ storage is the use of formation water extraction. Extraction of saline waters from CO₂ storage formations may improve reservoir storage volumes, aid in management of CO₂ plume migration, reduce cap rock exposure to CO₂, manage storage reservoir pressure, and/or generate a new source of water for a variety of beneficial surface uses. It is expected, in most cases, that any extracted water would be managed through direct injection into an appropriate overlying saline formation. However, indirect benefits derived from the treatment and sale of the extracted water may also provide additional economic incentives or cost offsets for formation water extraction.

The quality of extracted waters will vary from low-salinity waters from former oil and gas reservoirs (where hydrocarbons may be the main component of concern) to very high salinity waters from saline formations (where beneficial use of the water is unlikely). Generally, potential CO₂storage formations are required to have formation water with a total dissolved solids (TDS) concentration of 10,000 mg/L or greater, as salinities any lower are considered potential sources for use as drinking water. Ideally, prior to formation water extraction, operators would obtain information concerning TDS, specific ions present, organics, radionuclides, pH, alkalinity, hardness, temperature, and other “common” water quality measures. Any potential for beneficial use for that water could be identified at that time as well.

Salinity and chemical makeup of the dissolved species are highly dependent on the characteristics of the formation and cannot be simply inferred from geographic location or depth. The potential for beneficial use is largely dependent on the quality of the water present in the target formation, and the majority of storage targets are unlikely to contain economically treatable formation water. However, in regions of the globe where available water resources are highly limited and storage formation water is of reasonable quality (perhaps below 50,000 ppm total dissolved solids [TDS]), treatment and use may be economically viable.

The quality of water required for beneficial use of the extracted water also varies based on the intended use of that water, which will further influence the required level of treatment. These uses include geothermal heat recovery, various agricultural applications, industrial applications such as thermoelectric power facilities, and as a drinking water source. The quality of water required for each of these uses varies and, in some instances, has specific requirements.

It may be feasible to use extracted water as a source of drinking water. Water intended for this purpose would need to be economically treated to meet local drinking water standards for water quality. Some of these standards are mandatory requirements, i.e., primary standards; some are recommended, or nonmandatory, values, i.e., secondary standards. The primary standards are set to protect the public against consumption of drinking water contaminants that present a risk to human health. The secondary standards are established as guidelines to assist public water systems in managing their drinking water for aesthetic considerations. These standards vary on both local and national levels and may mean the same extracted water could be rejected as a water source in one region but accepted in another.

Several agricultural uses for extracted water exist. Assuming organic content is not of concern for water being considered for use in irrigation, the most critical salt-related water quality requirements, with respect to use for irrigation, are salinity and sodium adsorption ratio (SAR). Alkalinity, pH, nitrate, and other water quality parameters are also important, but salinity and SAR are of primary importance. Salinity is a problem because of its effect on plants; the higher the salinity, the lower the water activity and the more difficult it becomes for the plant to extract water from the environment for use. If the salinity and SAR characteristics are within acceptable limits, it is likely that the water will be acceptable for use at relatively low cost, even if other parameters need to be adjusted to more favorable values.

The quality of water acceptable to livestock is similar, in general, to the quality of water acceptable to humans. It should not contain unacceptable concentrations of potential toxicants, it should be microbially acceptable so as not to risk the spread of disease, and it should have an acceptable taste and odor. Like humans, livestock will consume pleasant-tasting but unsafe water while avoiding consumption of unpleasant-tasting water of acceptable quality if both are available.

Extracted water may also be put to beneficial use at power plants. Generally, it is desirable to have relatively high quality water for use as the supply of makeup water for recirculated cooling water operations, but it is possible to use water with relatively high TDS, provided that scaling and corrosion problems are controlled. The issues then become the economics of using water, which will allow for only a small number of cycles of concentration, and how to handle concentrated waste brine. Potential synergies between water users and storage operations also exist (e.g., extracted water used for cooling at the facility that provides CO₂to the storage project).

Treatment of saline extracted waters up to concentrations as high as that in seawater (TDS of ~35,000 mg/L) is likely to be economically feasible, especially in areas of significant water demand and low freshwater availability. Globally, the market for desalination has risen dramatically over the past 20 years as the cost of desalination has come down. In 2011, the global capacity for desalination was projected to grow to 77.4 million m³/day (71.9 million m³/day of this was online, and the remaining 5.5 million m³/day of this capacity was under construction). During this time, the cost of traditional water treatment has risen, as has the cost of transporting freshwater long distances.

The three major high-volume water desalination technologies are reverse osmosis (RO), multistage flash distillation (MSFD), and multieffect distillation (MED). While several plants exist that use MED alone, without employing another integrated desalination technology, effectively all recent MED facilities are hybrid plants that most commonly integrate the use of thermal vapor compression (TVC) with MED. Part of the reason these three systems, RO, MSFD, and MED–TVC, are the most popular for high-volume applications is that they are all applicable to the use of seawater and higher-concentration brackish water as the feedwater stock. The only other desalination system that has been commonly applied for high-volume applications at reasonable frequency is electrodialysis reversal (EDR), which is generally limited to desalination of low-concentration brackish waters.

Extensive industry experience in underground injection for enhanced oil recovery (EOR), gas storage, and deep well waste injection demonstrates that injection into geologic environments is feasible using existing technology. However, only a limited number of saline formations are generally used for these purposes, and detailed documentation of the properties of DSFs are generally not compiled in an easy-to-access format. Realistic and quantitative information about the relevant characteristics of the subsurface is needed to assess feasibility, costs, and risks associated with various options for water extraction in conjunction with CO₂storage.

Formation water extraction from CO₂ storage reservoirs is applicable for increasing storage capacity, managing reservoir pressure, and controlling plume movement. Analysis of the resulting water quality and quantity, available treatment technologies, and potential transportation costs reveals there is likely to be limited potential for the beneficial use of extracted water from CCS facilities. This potentially limited applicability for beneficial reuse derives in large part from the small range of water qualities that appear to fall between the poorest-quality water likely to be economically treatable (TDS ~50,000 mg/L) and the highest-quality water present in the resource that is generally considered acceptable for use as a CO₂ storage resource (TDS >10,000 mg/L). Although variable on a site-specific basis, the difference between the highest quality of water appropriate for CO₂ storage and the lowest quality of water economically viable for treatment and beneficial use is relatively small, ruling out a large majority of potential storage targets whose water quality exceeds those treatment limits. It is thus suggested that in most cases any extracted water would be managed through direct injection into an appropriate overlying saline formation. However, ideal circumstances of relatively high quality reservoir water and highly stressed or limited regional water resources could coexist, allowing beneficial use of extracted water to be considered. Additional work classifying the water quality of potential DSF storage targets is necessary before these conclusions can be revisited.