CARBON SEQUESTRATION
Figure 2. UNGS Facilities and Prospective CDS Saline Aquifers.
rosive — especially under supercritical- fluid conditions and with water vapor present — so that pipelines and wells which carry carbon dioxide can expect issues of mechanical integrity which are more demanding than those affect- ing conventional UNGS infrastructure (NETL, 2012).
UNGS and CDS: Strawman Design Concept
The locations and operational experi- ence of existing UNGS facilities, rather than their associated natural gas stor- age capacities, are the most significant attributes which can inform the design of future CDS operations. Indeed, the total U.S. natural gas storage capac- ity of 9,232 Bcf (Table 1) is equivalent to approximately 0.47 metric gigatons (Gt) of carbon dioxide1 — only about 9 percent of the annual emission of carbon dioxide in the U.S., which is more than 5.4 Gt (EPA, 2020). Therefore, simply re-purposing existing UNGS storage capacity for CDS would not provide a significant mitigation of GHG emissions.
However, many UNGS facilities are located above saline aquifers (Figure 2) which have been independently rec- ognized for their CDS storage potential (Hovorka, 1999). For deep saline aqui- fers in the U.S., the CDS storage capac- ity has been estimated as ranging from 2,379 Gt to 21,633 Gt of carbon dioxide (NETL, 2015). Although those saline aquifers are not involved in current UNGS operations, many of the candi- date saline aquifers could be reached by drilling from surface footprints of UNGS facilities.
Using as an example a UNGS facility built into a depleted oil or gas reservoir (the most common type of UNGS reser- voir), a strawman design concept for a hybrid UNGS-CDS facility (Figure 3) can be leveraged on the following principles:
• Maximize beneficial use of UNGS surface and ROW footprints.
• Maximize beneficial use of exist- ing gas transmission pipelines where appropriate.
• Streamline regulatory oversight of the hybrid UNGS-CDS storage facilities.
The same principles could be used for UNGS facilities built in solution- mined salt caverns although the relative configurations of UNGS and CDS wells would be different.
Beneficial Use of UNGS Surface and ROW Footprints. ROW issues, including litigation over eminent domain used by governments to convert pri- vate property to commercial use, are usually the most contentious issues in any new industrial infrastructure project. Maximum beneficial use of exist- ing UNGS ROWs means substantially reducing, if not eliminating, land and environmental impacts of CDS infra- structure construction. CDS wells could be drilled and completed within the same fields used by the UNGS wells. Similarly, any new CDS pipelines could take advantage of ROWs already owned by natural gas pipelines.
Beneficial Use of Existing Gas Transmission Pipelines. Re-use of
1. The estimated carbon dioxide volume assumes a gas phase where 1 Gt = 19,642 Bcf. 50 TPG •
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