CARBON SEQUESTRATION
ask what role UNGS might play in a future low-carbon economy — especially whether UNGS infrastructure could be eventually re-purposed for CDS or at least used as a model for how CDS infra- structure might work.
UNGS and CDS: Comparison and Contrast
Underground Storage Concepts. UNGS and CDS have overlapping but non-identical technical requirements (Table 2). The main difference is that UNGS relies upon fully reversible injec- tion and withdrawal of gas to fulfill the commercial purpose of the associated facilities whereas CDS presumes a one- way injection into permanent isolation. Otherwise, UNGS and CDS both depend upon suitable geology and the availabil- ity of pipelines to transport the gaseous commodity from points of origin to points of underground injection.
UNGS is a mature and profitable industry while CDS has been demon- strated mostly as pilot projects with limited commercial significance (NETL, 2015). The most well-known and situ- ational application of CDS has been for enhanced oil recovery (EOR) (NETL, 2010) where economic benefits were clear. At least in the U.S., non-EOR applications of CDS have been mostly proof-of-concept experiments, especially for capture of GHG emissions from coal- fired electric-power plants.
Geology. For UNGS, sedimentary geology is dominant as many of the storage reservoirs are the depleted rem- nants of formerly productive oil or gas plays, especially involving sandstone or limestone host rocks. However, CDS wells have been constructed in a wider variety of rock types, including coal and basalt (NETL, 2012). For CDS reser- voirs, porosity and permeability are key and any chemical reactivity (including mineralization reactions), which might further immobilize the carbon dioxide, is considered a desirable attribute.
Although some UNGS storage res- ervoirs are sufficiently saturated with water to be classified as aquifers, drier reservoirs usually are more effective for natural gas storage. In contrast with UNGS, saline aquifers are frequent- ly discussed as advantageous to CDS, based on the widespread occurrence and typically deep isolation of saline aquifers (Hovorka, 1999). In addition, the density and chemistry of saline water tends to retard carbon dioxide mobility.
Wells and Pipelines. UNGS and CDS wells span similar ranges of depth and
www.aipg.org Figure 1. Pressure-Depth Relationships in Gas Storage Wells by Reservoir Type.
Oct.Nov.Dec 2021 • TPG 49 Table 2. UNGS and CDS Characteristics Storage Characteristic
Undergound Natural Gas Storage (UNGS)
Carbon Dioxide Sequestration (CDS)
High Permeability Reservoir
Very Low Gas Migration
Reversible Injection/ Withdrawal
Sedimentary Geology A Proximity to Transmission Pipeline B
Criticality: Required Optional Notes:
A. Saline aquifers belong almost entirely to sedimentary geology. Finding suitable reservoirs in non-sedimentary geology is highly situational.
B. Proximity to transmission pipeline is optional only if the CDS location is immedi- atelyy adjacent to the source of the carbon dioxide. Otherwise, transportation of the carbon dioxide (possibly over long distances) becomes necessary.
operating pressure (Figure 1) although other differences involve the phase behavior of carbon dioxide compared with natural gas. To achieve increased transportation efficiency, where eco- nomic return is a significant consider- ation, CDS for EOR compresses carbon dioxide to a supercritical fluid (NETL, 2010) which places special requirements on pipeline infrastructure. However, if requirements for economic return are relaxed, compressed gaseous car-
bon dioxide can be transported through pipelines which more closely resemble those used for natural gas (Peletiri et al., 2018). Indeed, the existing network of natural gas transmission pipelines in the U.S. has been used as one possible model for a future network of carbon dioxide pipelines (Dooley et al., 2009).
One notable difference facing pipe- lines and wells used for CDS rather than UNGS is that carbon dioxide is cor-
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