Geographical distribution and capacity

Summary

Identifying potential sites for CO2 geological storage and estimating their capacity on a regional or local scale should conceptually be a simple task. The differences between the various mechanisms and means of trapping suggest in principle the following methods:

 -  For volumetric trapping, capacity is the product of available volume (pore space or cavity) and CO2 density at in situ pressure and temperature;

capacity is the amount of CO2 that can be dissolved in the formation fluid (oil in oil reservoirs, brackish water or brine in saline formations);

, capacity is the product of coal volume and its capacity for adsorbing CO2;

The major impediments to applying these simple methods for estimating the capacity for CO2 storage in geological media are the lack of data, their uncertainty, the resources needed to process data when available and the fact that frequently more than one trapping mechanism is active. This leads to two situations:

1) Global capacity estimates have been calculated by simplifying assumptions and using very simplistic methods and hence are not reliable;

2) Country- and region- or basin-specific estimates are more detailed and precise, but are still affected by the limitations imposed by availability of data and the methodology used.

Country- or basin-specific capacity estimates are available only for North America, Western Europe, Australia and Japan.

The IPCC special report on CCS estimates the worldwide technical potential for storage in geological formations to be at least 2,000 GtCO2. This is only the lower bound, and the IPCC believes the capacity may be many times higher, but the upper limit estimates are uncertain due to insufficient charting and disagreements on methodology. The capacity for storing CO2 in depleted petroleum reservoirs is known with much greater certainty.

* These estimates may increase by 25 per cent, when undiscovered oil and gas fields are included (IPCC 2005b).

 Content

1. Storage in oil and gas reservoirs

2. Storage in deep saline formations

3. Storage in coal

4. Reports and studies

5. External links

6. References

Storage in oil and gas reservoirs

 This CO2 storage option is restricted to hydrocarbon-producing basins, which represent numerically less than half of the sedimentary provinces in the world. It is generally assumed that oil and gas reservoirs can be used for CO2 storage after their oil or gas reserves are depleted, although storage combined with enhanced oil or gas production can occur sooner.
Short of a detailed, reservoir-by-reservoir analysis, the CO2 storage capacity can and should be calculated from databases of reserves and production (e.g., Winter and Bergman, 1993; Stevens et al., 2001b; Bachu and Shaw, 2003, 2005; Beecy and Kuuskra, 2005).

Many storage-capacity estimates for oil and gas fields do not distinguish capacity relating to oil and gas that has already been produced from capacity relating to remaining reserves yet to be produced and that will become available in future years. In some global assessments, estimates also attribute capacity to undiscovered oil and gas fields that might be discovered in future years. There is uncertainty about when oil and gas fields will be depleted and become available for CO2 storage.
The depletion of oil and gas fields is mostly affected by economic rather than technical considerations, particularly oil and gas prices. It is possible that production from near-depleted fields will be extended if future economic considerations allow more hydrocarbons to be recovered, thus delaying access to such fields for CO2 storage.
Currently few of the world’s large oil and gas fields are depleted. A variety of regional and global estimates of storage capacity in oil and gas fields have been made. Regional and national assessments use a ‘bottom-up’ approach that is based on field reserves data from each area’s existing and discovered oil and gas fields.Currently, this type of assessment is available only for north-western Europe, United States, Canada and Australia.

The storage potential of north-western Europe is estimated at more than 40 GtCO2 for gas reservoirs and 7 GtCO2 for oil fields (Wildenborg et al., 2005b). The European estimates are based on all reserves (no significant fields occur above 800 m). Carbon dioxide density was calculated from the depth, pressure and temperature of fields in most cases; where these were not available, a density of 700 kg m–3 was used. No assumption was made about the amount of oil recovered from the fields before CO2 storage was initiated and tertiary recovery by EOR was not included.

In Western Canada, the practical CO2 storage potential in the Alberta and Williston basins in reservoirs with capacity more than 1 MtCO2 each was estimated to be about 1 GtCO2 in oil reservoirs and about 4 GtCO2 in gas reservoirs. The capacity in all discovered oil and gas reservoirs is approximately 10 GtCO2 (Bachu et al., 2004; Bachu and Shaw, 2005). For Canada, the CO2 density was calculated for each reservoir from the pressure and temperature. The oil and gas recovery was that provided in the reserves databases or was based on actual production. For reservoirs suitable for EOR, an analytical method was developed to estimate how much would be produced and how much CO2 would be stored (Shaw and Bachu, 2002).

In the United States, the total storage capacity in discovered oil and gas fields is estimated to be approximately 98 GtCO2 (Winter and Bergman, 1993; Bergman et al., 1997). Data on production to date and known reserves and resources indicate that Australia has up to 15 GtCO2 storage capacity in gas reservoirs and 0.7 GtCO2 in oil reservoirs. The Australian estimates used field data to recalculate the CO2 that could occupy the producible volume at field conditions. The total storage capacity in discovered fields for these regions with bottom-up assessments is 170 GtCO2. Although not yet assessed, it is almost certain that significant storage potential exists in all other oil and gas provinces around the world, such as the Middle East, Russia, Asia, Africa and Latin America.

Global capacity for CO2-EOR opportunities is estimated to have a geological storage capacity of 61–123 GtCO2, although as practised today, CO2-EOR is not engineered to maximize CO2 storage. In fact, it is optimized to maximize revenues from oil production, which in many cases requires minimizing the amount of CO2 retained in the reservoir. In the future, if storing CO2 has an economic value, co-optimizing CO2 storage and EOR may increase capacity estimates. In European capacity studies, it was considered likely that EOR would be attempted at all oil fields where CO2 storage took place, because it would generate additional revenue. The calculation in Wildenborg et al. (2005b) allows for different recovery factors based on API (American Petroleum Institute) gravity of oil. For Canada, all 10,000 oil reservoirs in Western Canada were screened for suitability for EOR on the basis of a set of criteria developed from EOR literature. Those oil reservoirs that passed were considered further in storage calculations (Shaw and Bachu, 2002).

Global estimates of storage capacity in oil reservoirs vary from 126 to 400 GtCO2 (Freund, 2001). These assessments, made on a top-down basis, include potential in undiscovered reservoirs. Comparable global capacity for CO2 storage in gas reservoirs is estimated at 800 GtCO2 (Freund, 2001). The combined estimate of total ultimate storage capacity in discovered oil and gas fields is therefore very likely 675–900 GtCO2. If undiscovered oil and gas fields are included, this figure would increase to 900–1200 GtCO2, but the confidence
level would decrease. In comparison, more detailed regional estimates made for northwestern Europe, United States, Australia and Canada indicate a total of about 170 GtCO2 storage capacity in their existing oil and gas fields, with the discovered oil and gas reserves of these countries accounting for 18.9% of the world total (USGS, 2001a). Global storage estimates that are based on proportionality suggest that discovered worldwide oil and gas reservoirs have a capacity of 900 GtCO2, which is comparable to the global estimates by Freund (2001) of 800 GtCO2 for gas (Stevens et al., 2000) and 123 GtCO2 for oil and is assessed as a reliable value, although water invasion was not always taken into account. 

Storage in deep saline formations

Saline formations occur in sedimentary basins throughout the world, both onshore and on the continental shelves (Chapter 2 and Section 5.3.3) and are not limited to hydrocarbon provinces or coal basins. However, estimating the CO2 storage capacity of deep saline formations is presently a challenge for the following reasons:

• There are multiple mechanisms for storage, including physical trapping beneath low permeability caprock, dissolution and mineralization;
• These mechanisms operate both simultaneously and on different time scales, such that the time frame of CO2 storage affects the capacity estimate; volumetric storage is important initially, but later CO2 dissolves and reacts with minerals;
• Relations and interactions between these various mechanisms are very complex, evolve with time and are highly dependent on local conditions;
• There is no single, consistent, broadly available methodology for estimating CO2 storage capacity (various studies have used different methods that do not allow comparison).
• Only limited seismic and well data are normally available (unlike data on oil and gas reservoirs).

To understand the difficulties in assessing CO2 storage capacity in deep saline formations, we need to understand the interplay a CO2 plume (Section 5.2 and Figure 5.18). In addition, the storage capacity of deep saline formations can be determined only on a case-by-case basis. To date, most of the estimates of CO2 storage capacity in deep saline formations focus on physical trapping and/or dissolution. These estimates make the simplifying assumption that no geochemical reactions take place concurrent with CO2 injection, flow and dissolution. Some recent work suggests that it can take several thousand years for geochemical reactions to have a significant impact (Xu et al., 2003). The CO2 storage capacity from mineral trapping can be comparable to the capacity in solution per unit volume of sedimentary rock when formation porosity is taken into account (Bachu and Adams, 2003; Perkins et al., 2005), although the rates and time frames of these two processes are different. More than 14 global assessments of capacity have been made by using these types of approaches (IEA-GHG, 2004). The range of estimates from these studies is large (200–56,000 GtCO2), reflecting both the different assumptions used to make these estimates and the uncertainty in the parameters. Most of the estimates are in the range of several hundred Gtonnes of CO2. Volumetric capacity estimates that are based on local, reservoir-scale numerical simulations of CO2 injection suggest occupancy of the pore space by CO2 on the order of a few percent as a result of gravity segregation and viscous fingering (van der Meer, 1992, 1995; Krom et al., 1993; Ispen and Jacobsen, 1996). Koide et al. (1992) used the areal method of projecting natural resources reserves and assumed that 1% of the total area of the world’s sedimentary basins can be used for CO2 storage. Other studies considered that 2–6% of formation area can be used for CO2 storage. However, Bradshaw and Dance (2005) have shown there is no correlation between geographic area of a sedimentary basin and its capacity for either hydrocarbons (oil and gas reserves) or CO2 storage.

The storage capacity of Europe has been estimated as 30– 577 GtCO2 (Holloway, 1996; Bøe et al., 2002; Wildenborg et al., 2005b). The main uncertainties for Europe are estimates of the amount trapped (estimated to be 3%) and storage efficiency, estimated as 2–6% (2% for closed aquifer with permeability barriers; 6% for open aquifer with almost infinite extent), 4% if open/closed status is not known. The volume in traps is assumed to be proportional to the total pore volume, which may not necessarily be correct. Early estimates of the total US storage capacity in deep saline formations suggested a total of up to 500 GtCO2 (Bergman and Winter, 1995). A more recent estimate of the capacity of a single deep formation in the United States, the Mount Simon Sandstone, is 160–800 GtCO2 (Gupta et al., 1999), suggesting that the total US storage capacity may be higher than earlier estimates. Assuming that CO2 will dissolve to saturation in all deep formations, Bachu and Adams (2003) estimated the storage capacity of the Alberta basin in Western Canada to be approximately 4000 GtCO2, which is a theoretical maximum assuming that all the pore water in the Alberta Basin could become saturated with CO2, which is not likely.

An Australian storage capacity estimate of 740 GtCO2 was determined by a cumulative risked-capacity approach for 65 potentially viable sites from 48 basins (Bradshaw et al., 2003). The total capacity in Japan has been estimated as 1.5–80 GtCO2, mostly in offshore formations (Tanaka et al., 1995). Within these wide ranges, the lower figure is generally the estimated storage capacity of volumetric traps within the deep saline formations, where free-phase CO2 would accumulate. The larger figure is based on additional storage mechanisms, mainly dissolution but also mineral trapping. The various methods and data used in these capacity estimates demonstrate a high degree of uncertainty in estimating regional or global storage capacity in deep saline formations. In the examples from Europe and Japan, the maximum estimate is 15 to 50 times larger than the low estimate. Similarly, global estimates of storage capacity show a wide range, 100–200,000 GtCO2, reflecting different methodologies, levels of uncertainties and considerations of effective trapping mechanisms.

The assessment of the IPCC special report on Capture and Storage (2005) is that it is very likely that global storage capacity in deep saline formations is at least 1000 GtCO2. Confidence in this assessment comes from the fact that oil and gas fields ‘discovered’ have a global storage capacity of approximately 675–900 GtCO2 and that they occupy only a small fraction of the pore volume in sedimentary basins, the rest being occupied by brackish water and brine. Moreover, oil and gas reservoirs occur only in about half of the world’s sedimentary basins. Additionally, regional estimates suggest that significant storage capacity is available. Significantly more storage capacity is likely to be available in deep saline formations. The literature is not adequate to support a robust estimate of the maximum geological storage capacity. Some studies suggest that it might be little more than 1000 GtCO2, while others indicate that the upper figure could be an order of magnitude higher. More detailed regional and local capacity assessments are required to resolve this issue.

Storage in coal

No commercial CO2-ECBM operations exist and a comprehensive realistic assessment of the potential for CO2 storage in coal formations has not yet been made. Normally, commercial CBM reservoirs are shallower than 1500 m, whereas coal mining in Europe and elsewhere has reached depths of 1000 m. Because CO2 should not be stored in coals that could be potentially mined, there is a relatively narrow depth window for CO2 storage.
Assuming that bituminous coals can adsorb twice as much CO2 as methane, a preliminary analysis of the theoretical CO2 storage potential for ECBM recovery projects suggests that approximately 60–200 GtCO2 could be stored worldwide in bituminous coal seams (IEA-GHG, 1998). More recent estimates for North America range from 60 to 90 GtCO2 (Reeves, 2003b; Dooley et al., 2005), by including sub-bituminous coals and lignites. Technical and economic considerations suggest a practical storage potential of approximately 7 GtCO2 for bituminous coals (Gale and Freund, 2001; Gale, 2004). Assuming that CO2 would not be stored in coal seams without recovering the CBM, a storage capacity of 3–15 GtCO2 is calculated, for a US annual production of CBM in 2003 of approximately 0.04 trillion m3 and projected global production levels of 0.20 trillion m3 in the future. This calculation assumes that 0.1 GtCO2 can be stored for every Tcf of produced CBM (3.53 GtCO2 for every trillion m3) and compares well to Gale (2004).

Reports and studies

 The main source of this article is the IPCC (2005) Special Report on Carbon Dioxide Capture and Storage, available from Cambridge University Press.

Other sources referred to in article:
- Akimoto, K., H. Kotsubo, T. Asami, X. Li, M. Uno, T. Tomoda and T. Ohsumi, 2003: Evaluation of carbon sequestrations in Japan with a mathematical model. Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies (GHGT-6), J. Gale and Y. Kaya (eds.), 1-4 October 2002, Kyoto, Japan, v.I, 913–918.
- Bachu, S. and J.C. Shaw, 2003: Evaluation of the CO2 sequestration capacity in Alberta’s oil and gas reservoirs at depletion and the effect of underlying aquifers. Journal of Canadian Petroleum Technology, 42(9), 51–61.
- Bradshaw, J.B., E. Bradshaw, G. Allinson, A.J. Rigg, V. Nguyen and A. Spencer, 2002: The potential for geological sequestration of CO2 in Australia: preliminary findings and implications to new gas field development. Australian Petroleum Production and Exploration Association Journal, 42(1), 24–46.
- Dooley, J.J., R.T. Dahowski, C.L. Davidson, S. Bachu, N. Gupta and J. Gale, 2005: A CO2 storage supply curve for North America and its implications for the deployment of carbon dioxide capture and storage systems. Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies (GHGT-7), September 5–9, 2004, Vancouver, Canada, v.I, 593-602.
- Garg, A., D. Menon-Choudhary, M. Kapshe and P.R. Shukla, 2005: Carbon dioxide capture and storage potential in India. Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies (GHGT-7), September 5–9, 2004, Vancouver, Canada.
- Holloway, S. (ed.), 1996: The underground disposal of carbon dioxide. Final report of Joule 2 Project No. CT92-0031. British Geological Survey, Keyworth, Nottingham, UK, 355 pp.
- Larsen, M., N.P. Christensen, B. Reidulv, D. Bonijoly, M. Dusar, G. Hatziyannis, C. Hendriks, S. Holloway, F. May and A. Wildenborg, 2005: Assessing European potential for geological storage of CO2 - the GESTCO project. Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies (GHGT-7), September 5–9, 2004, Vancouver, Canada.
- Rigg, A., G. Allinson, J. Bradshaw, J. Ennis-King, C.M. Gibson-Poole, R.R. Hillis, S.C. Lang and J.E. Streit, 2001: The search for sites for geological sequestration of CO2 in Australia: A progress report on GEODISC. APPEA Journal, 41, 711–725.
Amadeo, N., H. Bajano, J. Comas, J.P. Daverio, M.A. Laborde, J.A. Poggi and D.R. Gómez, 2005: Assessment of CO2 capture and storage from thermal power plants in Argentina. Proceedings of the 7th International Conference on Greenhouse Gas Technologies (GHGT-7), September 5–9, 2004, Vancouver, Canada, v.I, 243-252.