CSLF-T-2008-04 

21 April 2008 

Comparison between Methodologies Recommended for Estimation of CO2

Storage Capacity in Geological Media 

by 

the CSLF Task Force on CO2 Storage Capacity Estimation 

and 

the USDOE Capacity and Fairways Subgroup of the 

Regional Carbon Sequestration Partnerships Program 

- Phase III Report - 

Prepared for: 

Technical Group (TG) 

Carbon Sequestration Leadership Forum (CSLF) 

by: 

Stefan Bachu (Canada) 

CSLF Task Force on CO2 Storage Capacity Estimation 
i 

The document on “Methodology for Development of Geologic Storage Estimates for Carbon 

Dioxide”, prepared for the U.S. Department of Energy (DOE) National Energy Technology 

Laboratory (NETL) Carbon Sequestration Program by the Capacity and Fairways Subgroup 

of the Geologic Working Group of the DOE Regional Carbon Sequestration Partnerships, 

was prepared in 2007 and, at the time of presenting this Task Force Phase III report to the 

Technical Group at the CSLF meeting in Cape Town, South Africa, April 13-17, 2008, was 

still in the process of being externally reviewed. As such it is not yet a public document and 

it was accessible only to Stefan Bachu and Robert Burruss, who were members of the 

Capacity and Fairways Subgroup. The document was not available to the other members of 

the CSLF Task Force for Review and Development of Standard Methodology for Storage 

Capacity Estimation. The author assumes sole responsibility for the content of this report. 
ii 

Executive Summary 

Implementation of CO2 capture and geological storage technology at the scale needed to 

achieve a significant and meaningful reduction in CO2 emissions requires knowledge of the 

available CO2 storage capacity. The CSLF Task Force for Review and Development of 

Standard Methodology for Storage Capacity Estimation produced, in March 2007, a report in 

which a consistent set of methodologies for estimating CO2 storage capacity in coal beds, oil 

and gas reservoirs and deep saline aquifers was recommended. In parallel, the United States 

Department of Energy (USDOE) Capacity and Fairways Subgroup within the Geologic 

Working Group of the Regional Carbon Sequestration Partnerships Program developed 

standards for CO2 storage capacity estimation for a Carbon Sequestration Atlas of the United 

States and Canada. At the March 2007 CSLF meeting the CSLF Task Force proposed and 

was given approval to conduct a comparison of the methodologies developed in parallel by 

the CSLF Task Force and the USDOE Subgroup. 

The methodologies proposed by the CSLF Task Force and the USDOE Subgroup are 

basically identical, with minor differences in computational formulation. In both cases the 

methods are based on volumetrics and are applicable to country-, regional- and basin-scale 

CO2 storage capacity estimates. Local and site-specific storage capacity estimates should be 

based on numerical modeling that takes into account the dynamic aspect of CO2 injection and 

of CO2 plume evolution. The only difference of significance is that the CSLF Task Force 

proposed to estimate static CO2 storage capacity in deep saline aquifers by considering only 

stratigraphic and structural traps present in these aquifers, while the USDOE Subgroup 

proposes to consider the entire aquifer, not only the traps. In addition, through Monte Carlo 

simulations of CO2 storage in coal beds and in deep saline aquifers for conditions 

characteristic to North America, the USDOE Subgroup obtained a range of values for these 

storage efficiency coefficients for the 15% and 85% confidence intervals, which are between 

0.28 and 0.40 for coal beds, and between 1% and 4% for deep saline aquifers. 

The proposed methodologies are useful in estimating the effective CO2 storage capacity, 

before applying regulatory, land use, economic and other constraining overlays. The only 

regulatory constraint explicitly considered by the USDOE Subgroup is that CO2 storage 

should be at depths greater than the depth of protected groundwater, defined by water salinity 

less than 10,000 mg/l (ppm). This constraint has been recognized also by the CSLF Task 

Force, but no specific value has been recommended, allowing for each jurisdiction to 

establish its own. 

The match, or identity, of the methodologies for CO2 storage capacity estimation at the 

country, regional and basin scales proposed independently by both the CSLF Task Force and 

the USDOE Subgroup indicates that these methodologies are robust and science-andengineering based, increasing the degree of confidence in their use and in the results obtained 

by using them. Application of these methodologies at various appropriate scales should 

provide decision makers in governments and industry with the information needed to assess 

the potential for CO2 geological storage and in focusing further work for site screening and 

selection.
iii 

Contents Page 

1. Introduction......................................................................................................................1 

2. Concepts and Definitions ................................................................................................3 

 2.1 Assessment Scale .......................................................................................................3 

 2.2 Resource-Reserve Pyramid Concept ..........................................................................6 

3. Estimation of CO2 Storage Capacity ...............................................................................6 

 3.1 Coal Beds ...................................................................................................................9 

 3.2 Oil and Gas Reservoirs ..............................................................................................9 

 3.3 Deep Saline Aquifers ...............................................................................................11 

4 Conclusions ....................................................................................................................14 

5 References ......................................................................................................................17 

Figures 

Figure 1. Techno-Economic Resource-Reserve pyramid for CO2 storage capacity 

in geological media within a jurisdiction or geographic region ..........................5 

Tables 

Table 1: Confidence indicator in CO2 storage capacity estimates, proposed by the USDOE 

Capacity and Fairways Subgroup of the Regional Carbon 

Sequestration Partnership Program ....................................................................16
1 

1. Introduction 

Carbon dioxide capture and sequestration in geological media, also known as Carbon Capture 

and Storage (CCS), is one among several means of reducing atmospheric emissions of 

anthropogenic CO2. This is a technology that: 1) is immediately applicable as a result of the 

experience gained mainly in oil and gas exploration and production, deep waste disposal and 

groundwater protection; 2) has large capacity, although unevenly distributed around the 

world, and 3) has retention times of centuries to millions of years (IPCC, 2005). In regard to 

CCS, the Carbon Sequestration Leadership Forum (CSLF) is an international climate change 

initiative that was established in 2003 and that is focused on the development of improved 

cost-effective technologies for the separation and capture of CO2 for its transport and longterm safe storage. It comprises 22 members, including 21 countries and the European 

Commission, that are significant producers and/or users of fossil fuels and that have a 

commitment to invest resources in research, development and demonstration activities in CO2

capture and storage technologies. The Forum has recognized that geological storage is an 

important element of the CCS chain and that it is critical for CSLF members to have a good 

assessment of the CO2 storage potential and capacity within their national jurisdictions. 

The Technical Group of the CSLF has identified early on that the lack of consistent and 

sound methodologies for estimating CO2 storage capacity represents a barrier to CCS 

deployment and has established at its meeting in Melbourne, Australia, in September 2004 a 

Task Force for Review and Development of Standard Methodology for Storage Capacity 

Estimation. In its Phase I report (CSLF, 2005), delivered in September 2005 at the CSLF 

meeting in Berlin, Germany, the Task Force has identified and critically analyzed the issues 

associated with various methodologies used to date for estimating CO2 storage capacity (see 

also Bradshaw et al., 2007). At the meeting in Paris, France, in March 2007, the Task Force 

submitted a Phase 2 report (CSLF, 2007) that presented definitions, concepts and 

methodologies to be used in estimating CO2 storage capacity (see also Bachu et al., 2007). It 

is hoped that the concepts, definitions and methodologies presented in the Phase II report 

would serve as a basis in CSLF member countries for collecting the necessary data and 

properly estimating the CO2 storage capacity in geological media in their jurisdiction, and 

indeed the proposed methodologies have been and are being applied for estimating CO2

storage capacity in various European countries, in the Indian sub-continent and in Brazil. 

During the same period, starting in 2003, the United States Department of Energy (USDOE) 

established seven Regional Carbon Sequestration Partnerships across the United States, with 

the goal of advancing and demonstrating the CCS technology. During Phase I of the 

Regional Carbon Sequestration Partnership Program (2003-2005), the partnerships 

independently produced quantitative estimates of the CO2 storage capacity within their 

respective areas, which were subsequently compiled into a Carbon Sequestration Atlas of the 

United States and Canada1

 (USDOE, 2007), hereafter labelled Atlas I. The Atlas presents 

information about CO2 storage capacity in the U.S. and parts of Canada both by geological 

medium (unmineable coal seams, oil and gas reservoirs and deep saline aquifers), and by 

individual partnerships. However, some inconsistencies in the evaluation of CO2 storage 

capacity by various Regional Partnerships subsequently became apparent, and in 2006 

USDOE established a Capacity and Fairways Subgroup within the Geologic Working Group 

of the Regional Carbon Sequestration Partnerships for developing standards for CO2 storage 

 

1

 The four western Canadian provinces are also members of two Regional Partnerships, PCOR and WestCarb, 

and as such they are included in the Atlas. 
2 

estimation, with the intent of producing an updated 2008 Carbon Sequestration Atlas of the 

United States and Canada (Atlas II). The primary focus of the update is to “add basins and 

formations to the CO2 storage portfolio, document procedures completely, and provide 

definitions of CO2 resource versus CO2 capacity that reflect the uncertainty of geologic 

storage estimates across the Regional Carbon Sequestration Partnership Regions”. 

Mindful of the similarity between the efforts of the CSLF Task Force for Review and 

Development of Standard Methodology for Storage Capacity Estimation, and of the Capacity 

and Fairways Subgroup of the U.S. Regional Carbon Sequestration Partnership Program, the 

CSLF Task Force has proposed and was given approval at the CSLF meeting March 2007 

that was held in Paris, France, to proceed to a Phase III of the Task Force that would include 

coordination of methodology for CO2 storage capacity estimation with other national and 

international groups working on this subject, including the Capacity and Fairways Subgroup 

of the USDOE Regional Carbon Sequestration Partnership Program. 

In June 2007 the Capacity and Fairways Subgroup of the Geologic Working Group of the 

Regional Carbon Sequestration Partnerships in the U.S. initiated an effort to inventory and 

review the methods used in Atlas I to estimate CO2 storage potential and capacity, to generate 

consistent methods and assumptions across a wide range of data for estimating the geologic 

resource for storing CO2 in the U.S. in unmineable coal seams, oil and gas reservoirs, and 

deep saline aquifers. The author of this Task Force Phase III report has been invited to be a 

member of, and participate in the work of the Capacity and Fairways Subgroup. Through 

various meetings, conference calls and e-mail exchanges, a consensus emerged within the 

Subgroup for an updated approach towards producing Atlas II, summarized in October 2007 

in a draft document titled “Methodology for Development of Geologic Storage estimates for 

Carbon Dioxide” that is currently under review. 

This Phase III Report of the CSLF Task Force for Review and Development of Standard 

Methodology for Storage Capacity Estimation presents a review of the methodologies 

proposed by the CSLF Task Force in its Phase II report, and those proposed by the Capacity 

and Fairways Subgroup of the U.S. Regional Carbon Sequestration Regional Partnership 

Program, with the aim of identifying commonalities and differences in proposed 

methodologies, leading hopefully to improved and harmonized methodologies for estimating 

the potential and capacity for CO2 storage in geological media. 
3 

2. Concepts and Definitions 

The methodologies for estimating CO2 storage potential and capacity are based on widely 

accepted assumptions about geological trapping mechanisms, storage media and operating 

timeframes reviewed previously (e.g., IPCC, 2005; CSLF 2007). Only concepts relevant to 

this comparison between CSLF and USDOE methodologies will be reviewed here. 

2.1 Assessment Scale

Five different assessment scales have been identified in the CSLF Phase II Report (CSLF, 

2007; Bachu et al., 2007): 

1) Country-Scale Assessment, which is a high level of assessment performed for a 

contiguous geographic area defined by national jurisdiction (country) and which 

usually encompasses several sedimentary basins. 

2) Basin-Scale Assessment, which is a more detailed level of assessment focusing on a 

particular sedimentary basin to evaluate and quantify its storage potential and to 

identify the best regions for CO2 storage and the types of storage that might take place 

there, often in relation to the major stationary CO2 sources in the basin or in its 

proximity. 

3) Regional-Scale Assessment, which is performed at an increasing level of detail for a 

large, geographically-contiguous portion of a sedimentary basin, usually defined by 

the presence of large CO2 sources and/or by its known large potential for CO2 storage. 

4) Local-Scale Assessment, which is very detailed, usually performed at a preengineering level when one or several candidate sites for CO2 storage are examined to 

determine site capacity, injectivity and containment prior to site-selection decisions. 

5) Site-Scale Assessment, which is performed for the specific storage unit (hydrocarbon 

reservoir, deep saline aquifer or coal bed), usually to model the behaviour of the 

injected CO2. 

The relationship between basin-scale and regional-scale assessments may reverse in the case 

of large, continental-size countries where a region may contain several sedimentary basins. 

The purpose and envisaged GIS-based structure of the Carbon Sequestration Atlas of United 

States and Canada is to provide quantitative estimates of the geologic resource for CO2

storage in each of the three media under consideration and in aggregate for individual 

sedimentary basins, States (Provinces in Canada), the seven regions covered by the Regional 

Partnerships, and the United States as a whole. As such, the CO2 storage capacity estimates 

correspond to the country-, regional- and basin-scale assessments defined by the CSLF Task 

Force (note that in this case the order of regional and basin scales is reversed). Local- and 

site-scale assessments are explicitly excluded from the Carbon Sequestration Atlas of United 

States and Canada. 

2.2 Resource-Reserve Pyramid Concept

Carbon dioxide storage capacity constitutes a geological resource (commodity) whose 

availability can be expressed using the concepts of resources and reserves, in the same way as 

other energy and mineral commodities such as oil and gas, coal, uranium, iron, gold, etc., are 
4 

classified2

. Resources are those quantities of a commodity that are estimated at a given time 

to exist within a jurisdiction or a geographic area. Resources are of two types: discovered, or 

in-place (i.e., an existing commodity whose location and characteristics are known, being 

assessed on the basis of scarce data), and undiscovered, or inferred (i.e., not found yet but 

assumed to exist based on inferences from geological knowledge and/or various analyses). 

Reserves are those quantities of a commodity that are known to exist and that are 

commercially recoverable. Their assessment integrates technical, economic, environmental, 

societal and regulatory factors available at the time of the assessment. Reserves are a subset 

of resources, and usually accessibility, technology and economic cutoffs are used to define 

and delineate reserves. 

Using the concept of resources and reserves, the CSLF Task Force proposed a TechnoEconomic Resource-Reserve Pyramid for CO2 Storage Capacity (CSLF, 2007; Bachu et al., 

2007), illustrated in Figure 1. The various capacities, described below in ascending order, are 

nested within the resource-reserves pyramid: 

1) Theoretical Storage Capacity, is the total resource. It encompasses the whole of the 

resource pyramid. It is the physical limit of what the geological system can accept. It 

assumes that the system’s entire capacity to store CO2 in pore space, or dissolved at 

maximum saturation in formation fluids, or adsorbed at 100% saturation in the entire 

coal mass, is accessible and utilized to its full capacity. 

2) Effective Storage Capacity represents a subset of the theoretical capacity and is 

obtained by considering that part of the theoretical storage capacity that can be 

physically accessed and which meets a range of geological and engineering criteria. 

3) Practical Storage Capacity, is that subset of the effective capacity that is obtained by 

considering technical, legal and regulatory, infrastructural and general economic 

barriers to CO2 geological storage. The Practical Storage Capacity corresponds to the 

term ‘reserves’ used in the energy and mining industries. 

4) Matched Storage Capacity is that subset of the practical capacity that is obtained by 

detailed matching of large stationary CO2 sources with geological storage sites that 

are adequate in terms of capacity, injectivity and supply rate to contain CO2 streams 

sent for storage from that source or sources. This capacity is at the top of the resource 

pyramid and corresponds to the term ‘proved marketable reserves’ used by the mining 

industry. 

 

2

http://www.spe.org/specma/binary/files/4675179GuidelinesEvaluationReservesResources05Nov.pdf#search=%

22Classification%20and%20Nomenclature%20Systems%20for%20Petroleum%20and%20Petroleum%20Reser

ves%22

http://www.cim.org/definitions/cimdef1.pdf 

http://www.jorc.org/pdf/coalguidelines2001.pdf#search=%22guidelines%20coal%20resource%20reserve%22
5 

Figure 1. Techno-Economic Resource-Reserve pyramid for CO2 storage capacity in geological media within a 

jurisdiction or geographic region. The pyramid shows the relationship between Theoretical, Effective, 

Practical and Matched capacities. 

The Capacity and Fairways Subgroup of the U.S. Regional Carbon Sequestration Regional 

Partnership Program proposes to provide CO2 resource estimates and, where possible, CO2

capacity estimates. A CO2 Resource Estimate is defined as the volume of porous and 

permeable sedimentary rocks that is most likely accessible to injected CO2 via drilled and 

completed wellbores, and includes all volumetric estimates of geologic storage reflecting 

physical and chemical constraints or limitations, but does not include current or projected 

economic constraints, regulations, or well and/or surface facilities. A CO2 Capacity 

Estimate includes economic and regulatory constraints, such as groundwater protection, land 

use, minimum well spacing, (maximum) injection rate and pressure, number and type of 

wells, operating costs and proximity to a CO2 source. 

From the above definitions it appears that the USDOE CO2 Resource Estimate corresponds to 

the CSLF Effective Capacity, and the USDOE CO2 Capacity Estimate corresponds to the 

CSLF Practical Capacity. The CSLF-defined Theoretical Capacity is not considered in the 

USDOE Carbon Sequestration Atlas of United States and Canada, and commercial-scale 

assessments, which would correspond to the CSLF Matched Capacity, are excluded from the 

USDOE Atlas. 
6 

3. Estimation of CO2 Storage Capacity 

The USDOE Capacity and Fairways Subgroup defines static and dynamic methods for 

estimating subsurface volumes. These are widely used in the oil and gas industry, and in 

underground natural gas storage, groundwater and the underground disposal of fluids. The 

static methods are volumetric and compressibility-based; the dynamic methods are 

decline/incline curve analysis, material balance and reservoir simulations. The dynamic 

methods can be applied only after the start of active injection (although one may argue that 

numerical simulations without history matching can be applied also prior to injection), hence 

only the static methods were and will be used in producing the Carbon Sequestration Atlas of 

United States and Canada (Atlas I and II). The approach taken by the Regional Partnerships 

in the U.S. is basically consistent with the methodology proposed by the CSLF Task Force 

for Review and Development of Standard Methodology for Storage Capacity Estimation, 

which is also based on static methods. 

3.1 Coal Beds

Carbon dioxide storage in coal beds occurs when CO2 is preferentially adsorbed onto coal. 

Coal has higher affinity for gaseous CO2 than for methane, which naturally occurs in coals, 

the volumetric ratio between the two ranging from as low as 1 for mature coals such as 

anthracite, to as high as 10 for younger, less altered coals. Thus, CO2 storage in coal beds is 

based on the concept that the injected CO2 will replace the methane in coal and stay adsorbed 

onto the coal surface as long as the coal is undisturbed (i.e., the pressure doesn’t drop). 

CSLF-Proposed Methodology. The methodology proposed by the CSLF Task Force is based 

on two steps, consistent with the resource-reserves pyramid (Figure 1). The theoretical 

storage capacity MCO2t for a given coal bed is calculated first according to: 

(1 ) ~ 2 2 a m CO t CO s C C M = ρ × A× h × n ×G × − f − f (1) 

where ρCO2s = 1.873 kg/m3

 is CO2 density at standard (surface) conditions, A and h are the 

area and effective thickness of the coal zone, respectively, C n

~ is the bulk coal density 

(generally nC

~ ≈1.4 t/m3

), GCS (in units of volume of gas per unit of coal mass) is the coal gas 

content (dry, ash free) at saturation assuming that the coal will be 100% saturated with CO2, 

and fa and fm are the ash and moisture weight fraction of the coal, respectively. The coal gas 

content at saturation, GCS, is generally assumed to follow a pressure-dependent Langmuir 

isotherm of the form: 

L

CS L P P

P G V

+ = × (2) 

although other isotherm functions can be used. In relation (2), VL and PL are Langmuir 

volume and pressure, respectively. The Langmuir volume, VL, represents the maximum gas 

adsorption capacity of a particular coal at the given temperature, and is usually given in cc/g, 

which is equivalent to m3

/t. The Langmuir isotherm expressed by eq. (2) displays an increase 

in adsorption capacity with increasing pressure as the gas content GCS tends asymptotically 

towards VL with increasing pressure P. 

The effective storage capacity, MCO2e, is obtained according to: 
7 

MCO2e = Rf ×C × MCO2t (3) 

where Rf is the recovery factor and C is the completion factor, and together they express the 

reservoir gas deliverability. The completion factor C represents an estimate of that part of the 

net cumulative coal thickness within the drilled coal zone that will contribute to gas 

production or storage, it strongly depends on the individual thickness of the separate coal 

seams and on the distance between them, and is lower for thin coal seams than for thick ones. 

The recovery factor Rf represents the fraction of gas that can be produced from, or stored in, 

the coal seams. 

USDOE-Proposed Methodology. The methodology proposed by the Capacity and Fairways 

Subgroup of the Regional Carbon Sequestration Partnerships is consistent with the definition 

of CO2 resource estimate, which is equivalent with the effective storage capacity defined by 

the CSLF Task Force. The proposed relationship for calculating CO2 storage capacity in coal 

beds is: 

MCO2 = ρ CO2s × A× h ×C × E (4) 

where C is CO2 concentration (standard volume) per unit of coal volume (Langmuir or 

alternative), and E is CO2 storage efficiency factor reflecting the fraction of the total coal 

bulk volume that is contacted by CO2. The concentration C assumes 100% coal saturation 

with CO2. If C is for dry, ash-free (daf) conditions, then A and h have to be corrected 

accordingly. 

Monte Carlo simulations produced a range for E between 28 and 40% for a 15 to 85% 

confidence range, with an average of 33% for 50% confidence. In the Monte Carlo 

simulations that produced the recommended range for E, various calculation components 

have been varied as follows: 

- Fraction of the coal bed that is suitable for CO2 storage: 0.6 to 0.8 

- Fraction of coal seam thickness that has adsorption capability: 0.75 to 0.90 

- Areal displacement efficiency: 0.7 to 0.95 

- Vertical displacement efficiency: 0.8 to 0.95 

- Fraction of net coal thickness contacted by CO2 as a result of CO2 buoyancy 

compared with the water in coal cleats: 0.9 to 1.0 

- Pore-scale displacement efficiency, reflecting the achievable degree of saturation for 

in-situ coal compared with the theoretical maximum predicted by the adsorption 

isotherm: 0.75 to 0.95. 

These ranges of values were chosen to reflect various coals, with the maximum and minimum 

meant to be reasonable high and low values for each parameter. 

Comparison of relations (1)-(3) and (4) indicates that the two proposed methodologies are 

practically identical, where: 

(1 ) ~ a m C C C = n ×G × − f − f and E = Rf ×C

Applicability/Screening Criteria. Both the CSLF Task Force and the USDOE Capacity and 

Fairways Subgroup propose some screening criteria for identifying coal beds suitable for CO2

storage. 
8 

Both the CSLF Task Force and the USDOE Capacity and Fairways Subgroup recommend the 

depth where coal permeability, which generally decreases with depth, becomes less than 1 

mD as the maximum depth for coal beds to be considered for CO2 storage. Similarly, a 

minimum depth is suggested by both as the depth dictated by water-quality standards to 

ensure that potentially freshwater-bearing coals are not included (i.e., only coals deeper than 

the depth of groundwater protection should be considered). The USDOE Capacity and 

Fairways Subgroup recommends considering only coal seams with a water TDS 

concentration of 10,000 mg/l (ppm) and higher. Although this is a regulatory constraint, 

which moves the effective storage capacity estimate into the practical storage capacity 

estimate (or, in USDOE parlance, moves the resource estimate into capacity estimate), it is 

believed that regulations will always be in place to protect potable groundwater and, therefore, 

this criterion should be applied from the beginning to avoid including in assessments coal 

beds that will never be available for CO2 storage. Thus, only coals in this depth interval, 

between the depth of groundwater protection and the depth where coal permeability is less 

than 1 md, should be considered for CO2 storage. In addition, the CSLF Task Force proposed 

to consider only coals where, based on in-situ pressure and temperature, CO2 is in gaseous 

phase, and this criterion may reduce further the coal beds considered for CO2 storage, but this 

is a screening criterion that can be debated. 

In regard to coal mineability, within the depth interval established above, the USDOE 

Capacity and Fairways Subgroup recommends that coal mineability should be established 

based on today’s standards of technology and economic profitability. This criterion implies 

use of economic criteria, which again would move the estimate in the resource-reserves 

pyramid from effective to practical (or from resource estimate to capacity estimate); however 

use of this constraint is necessary because of safety and regulatory concerns for mining coals 

that have been used to store CO2. Thus, coal reserves should be excluded from CO2 storage 

capacity estimates. These recommendations are generally consistent with the 

recommendations made by the CSLF Task Force. 

This analysis shows that there is complete identity between the methodologies and the 

applicability criteria proposed by the CSLF Task Force and the USDOE Capacity and 

Fairways Subgroup of the Regional Carbon Sequestration Partnerships for estimating CO2

storage capacity in coal beds. 
9 

3.2 Oil and Gas Reservoirs

Estimation of the CO2 storage capacity in oil and gas reservoirs is the simplest, relatively 

speaking, and most straightforward of the three media considered for CO2 geological storage. 

This is because, unlike coals and aquifers, oil and gas reservoirs are better known and 

characterized than the other two as a result of exploration for and production of hydrocarbons. 

Also unlike coal beds and deep saline aquifers, oil and gas reservoirs are discrete rather than 

continuous, such that the capacity for CO2 storage in hydrocarbon reservoirs in any particular 

region at any scale is given by the sum of the capacities of all reservoirs in that area, 

calculated on the basis of reservoir properties such as original oil or gas in place, recovery 

factor, temperature, pressure, rock volume and porosity, as well as in situ CO2 characteristics 

such as phase behaviour and density. 

In the case of oil and gas reservoirs, the fundamental assumption is that the volume 

previously occupied by the produced hydrocarbons becomes, by and large, available for CO2

storage. This assumption is generally valid for reservoirs that are not in hydrodynamic 

contact with an aquifer, or that are not flooded during secondary and tertiary oil recovery. In 

reservoirs that are in hydrodynamic contact with an underlying aquifer, formation water 

invades the reservoir as the pressure declines because of production, leading to a decrease in 

the pore space available for CO2 storage. Carbon dioxide injection can partially reverse the 

aquifer influx, thus making more pore space available for CO2. However, not all the 

previously hydrocarbon-saturated pore space will become available for CO2 because some 

residual water may be trapped in the pore space due to capillarity, viscous fingering and 

gravity effects. Another important assumption is that CO2 will be injected into depleted oil 

and gas reservoirs until the reservoir pressure is brought back to the original, or virgin, 

reservoir pressure. In some cases reservoir depletion may damage the integrity of the 

reservoir and/or caprock, in which case the pressure cannot be brought back to the initial 

reservoir pressure and the capacity would be lower, while in other cases the pressure can be 

raised beyond the original reservoir pressure as long as it remains safely below the lesser of 

the capillary entry pressure and the threshold rock-fracturing pressure of the seal (caprock), in 

which case the CO2 storage capacity would be higher due to CO2 compression. In many cases 

the structure that hosts a hydrocarbon reservoir is not filled with oil and/or gas to the spill 

point, and the additional pore space down to the spill point can also be used for CO2 storage, 

but, to achieve this, the pressure has to be increased beyond the original reservoir pressure. 

However, raising the storage pressure to or beyond the original reservoir pressure, and 

evaluating the effects of water invasion, reservoir heterogeneity, mobility contrast and 

buoyancy require a case-by-case reservoir analysis that is not practical for country-, regionaland basin-scale evaluations. In this respect there is complete agreement between the CSLF 

and USDOE proposed approaches, assumptions and methodologies. 

CSLF-Proposed Methodology. Consistent with the resource-reserves pyramid concept, both 

theoretical and effective CO2 storage capacities are calculated according to: 

MCO2t = ρCO2r × Rf × (1 – FIG) × OGIP× [(Ps × Zr × Tr) / (Pr × Zs × Ts)] (5) 

for gas reservoirs, and by: 

MCO2t = ρCO2r ×[ Rf × OOIP / Bf - Viw + Vpw] (6) 

for oil reservoirs. 
10 

An alternate equation for calculating the CO2 storage capacity in oil and gas reservoirs is 

based on the geometry of the reservoir (areal extent and thickness) as given in reserves 

databases: 

MCO2t = ρCO2r× [Rf × A × h × φ × (1 – Sw) – Viw + Vpw] (7) 

In the above equations OGIP and OOIP are the initial gas and oil in place, respectively, Rf is 

the recovery factor, FIG is the fraction of injected gas, P, T and Z denote pressure, 

temperature and the gas compressibility factor, respectively, Bf is the formation volume factor 

that brings the oil volume from standard conditions to in-situ conditions, Viw and Vpw are the 

volumes of injected and produced water, respectively (applicable in the case of oil reservoirs), 

and A, h, φ and Sw are reservoir area, thickness, porosity and water saturation, respectively. If 

gas or miscible solvent is injected in oil reservoirs in tertiary recovery, then the mass balance 

of these should be added to eq. (6) or (7). The subscripts “r” and “s” in eq. (5) denote 

reservoir and surface conditions, respectively. The volumes of injected and/or produced water, 

solvent or gas can be calculated from production records. In the case of reservoirs with strong 

aquifer support (water drive), the volumes of injected and produced water may be negligible 

by comparison with the amount of invading water. 

All the processes and reservoir characteristics, such as buoyancy, gravity override, mobility, 

heterogeneity, water saturation and strength of the underlying aquifer if present, that reduce 

the actual volume available for CO2 storage can be expressed by capacity coefficients (C < 1) 

in the form: 

MCO2e = Cm × Cb × Ch × Cw × Ca × MCO2t ≡ Ce × MCO2t (8) 

where MCO2e is the effective reservoir capacity for CO2 storage, the subscripts m, b, h, w and 

a stand for mobility, buoyancy, heterogeneity, water saturation, and aquifer strength, 

respectively, and the coefficient Ce is a single effective capacity coefficient that incorporates 

the cumulative effects of all the other. 

USDOE-Proposed Methodology. The volumetric relationship proposed by the Capacity and 

Fairways Subgroup of the Regional Carbon Sequestration Partnerships is consistent with the 

definition of CO2 resource estimate (i.e., effective storage capacity): 

MCO2t = ρCO2r× A × h × φ × (1 – Sw) ×B×E (9) 

Where E is a CO2 storage efficiency factor that reflects the fraction of the total reservoir pore 

volume from which oil and/or gas has been produced and that can be filled by CO2. The CO2

storage efficiency factor E involves the original oil or gas in place and recovery factor, and 

can be derived based on experience or reservoir simulations. Factors not explicitly considered 

include CO2 miscibility into oil, dissolution of CO2 into residual and associated water, 

hysteretic effects during hydrocarbon production and CO2 injection, waterflooding, and 

solution gas. Most of these factors were not considered by the CSLF Task Force either 

because they can be evaluated only on a case-by-case basis at the site-specific scale. 

Comparison of the methodologies proposed by the CSLF Task Force and the USDOE 

Capacity and Fairways Subgroup indicates that the two are equivalent. Basically, relation (9) 
11 

is equivalent to relations (7) and (8) if injected and produced water are not considered, and 

where E = Rf×Ce. 

In the cases where good production (and injection) records are available, and particularly 

when cumulative production is greater than the original oil or gas in place, a productionbased method can be used to estimate CO2 storage capacity, in which basically the product Rf

×OGIP or Rf ×OOIP in relations (5) and (6) is replaced by the produced gas or oil, 

respectively. 

Neither the CSLF Task Force nor the USDOE Capacity and Fairways Subgroup recommends 

any methodology for estimating CO2 storage capacity in CO2 enhanced oil recovery (EOR) 

operations because these are based on reservoir simulations at the site-specific scale. 

Applicability/Screening Criteria. While the CSLF Task Force did not recommend any 

specific screening criteria in the case of oil and gas reservoirs, the USDOE Capacity and 

Fairways Subgroup recommends excluding from CO2 resource and capacity estimates those 

oil and gas reservoirs whose water has a salinity less than 10,000 mg/l (ppm), to ensure that 

potentially freshwater-bearing units are not included, although it is recognized that the 

reservoir water is very likely to be classified as non-potable due to oil and/or gas 

contamination. The number of oil and gas reservoirs thus excluded from the CO2 resource 

and capacity estimation is expected to be very small. 

In addition, the USDOE Capacity and Fairways Subgroup recommends aggregating reservoir 

CO2 resource or capacity estimation to the field scale. Field-scale estimates can then be easily 

summed up at either the basin or state scale, depending on interest by geological entity or by 

jurisdiction. 

3.3 Deep Saline Aquifers

Both the CSLF Task Force and the USDOE Capacity and Fairways Subgroup recognize that 

a deep saline aquifer is a body of porous rock that meets the depth conditions for CO2 storage 

and that contains water with total dissolved solids (TDS) greater than 10,000 mg/l (ppm), and 

that it may include more than one geological formation or be defined only as part of a 

formation. Furthermore, more often than not more than one deep saline aquifer is present 

within a sedimentary succession at a given location. 

The CSLF Task Force differentiated between the various CO2-trapping mechanisms that 

operate in deep saline aquifers: structural and stratigraphic (volumetric) trapping, residual-gas 

saturation trapping, dissolution, precipitation and hydrodynamic trapping. Because CO2

storage through residual gas trapping, mineral precipitation and hydrodynamic trapping are 

time-dependent processes, and because CO2 storage capacity can be evaluated in these cases 

only at the local and site-specific scales using numerical simulations (CSLF, 2007; Bachu et 

al., 2007), they are not discussed in this comparative analysis. The USDOE Capacity and 

Fairways Subgroup also does not consider these trapping mechanisms in calculating CO2

storage resources and capacity at the basin and regional scales. 

CSLF-Proposed Methodology. Storing CO2 in structural and stratigraphic traps is similar to 

storing CO2 in depleted oil and gas reservoirs. If the geometric volume Vtrap of the structural 

or stratigraphic trap down to the spill point is known, as well as its porosity φ and the 
12 

irreducible water saturation Swirr, then the theoretical volume available for CO2 storage, VCO2t, 

can be calculated with the formula: 

VCO2t = Vtrap × φ × (1 – Swirr) ≡ A × h × φ × (1 – Swirr) (10) 

where A and h are the trap area and average thickness, respectively. 

The effective storage volume, VCO2e, is given by: 

VCO2e = Cc × VCO2t (11) 

where Cc is a capacity coefficient that incorporates the cumulative effects of trap 

heterogeneity, CO2 buoyancy and sweep efficiency. 

Calculating the mass of CO2 that corresponds to the effective storage volume is more difficult 

because CO2 density, ρCO2, depends on the pressure in the trap once it is filled with CO2, and 

this pressure is not known a priori but depends on permeability, relative permeability to 

formation water and CO2, dimensions and volume, and the nature of trap boundaries, and 

may vary with the injection strategy (number and/or inclination of injection wells, etc.). 

However, this pressure has to be higher than the initial water pressure in the trap, Pi, in order 

to achieve CO2 injection, but it has to be lower than the maximum bottomhole injection 

pressure, Pmax, that regulatory agencies usually impose in order to avoid rock fracturing or 

breaching of the capillary seal. Thus, the mass of CO2 that would be stored in a structural or 

stratigraphic trap would be between these two limits: 

minMCO2e = ρCO2(Pi, T) × VCO2e ≤ MCO2e ≤ maxMCO2e = ρCO2(Pmax, T) × VCO2e (12) 

where T is the average temperature in the trap. The mass capacity of a trap may vary in time 

if pressure varies because, although the volume of the trap remains constant, CO2 density 

varies with varying pressure. 

Relations (10) – (12) can be applied to both theoretical and effective storage capacity 

estimates for basin- and regional-scale assessments by applying them individually to all the 

structural and stratigraphic traps identified as potential candidates for CO2 storage and 

summing the resulting individual capacities. They can be applied also to the case of a plume 

of CO2 that is not necessarily contained in a stratigraphic or structural trap. 

Solubility trapping is based on CO2 dissolution into formation water, which depends on 

pressure, temperature and water salinity. Solubility trapping is a continuous, time-dependent 

process estimated to be most effective over time periods in the order of centuries. Therefore, 

the CO2 storage capacity through solubility trapping has to be evaluated for a specified period 

or periods of time. The rate at which solubility trapping occurs depends principally on the 

amount of free-phase CO2 coming into contact with formation water unsaturated with CO2. 

Generally, the CO2 storage capacity through solubility trapping has to be determined through 

numerical modeling at the local and site-specific scales. However, at the basin- and regionalscale, the theoretical CO2 storage capacity in solution can be estimated using the relation 

(after Bachu and Adams, 2003): 

MCO2t = X X dxdydz CO CO

S ( S )

2

0 0 2 φ ρ − ρ ∫∫∫ (13) 
13 

where φ is porosity, ρ is the density of formation water, CO2 X is the carbon dioxide content 

(mass fraction) in formation water and the subscripts 0 and S stand for initial carbon dioxide 

content and carbon dioxide content at saturation, respectively. The initial carbon dioxide 

content and carbon dioxide content at saturation depend on the pressure, temperature and 

salinity distributions in the aquifer, and, because of their continuous variation, a process of 

volumetric integration needs to be used. If average values are being used for aquifer thickness 

and porosity, and for carbon dioxide content in aquifer water (initial and at saturation), then 

the following simpler relation can be used: 

MCO2t = A × h × φ ×( )

2

0 0

CO2 CO

S ρSX − ρ X (13’) 

where A and h are aquifer area and thickness. The effective storage capacity, MCO2e, is 

determined using a relationship similar to relation (3) for storage capacity in coal beds, and 

relation (8) for storage capacity in oil and gas reservoirs: 

MCO2e = C × MCO2t (14) 

where C is a coefficient that includes the effect of all factors that affect the spread and 

dissolution of CO2 in the whole aquifer volume under consideration. Given the strong timedependence of CO2 dissolution, the coefficient C should arguably be time-dependent. It may 

be possible to evaluate through numerical simulations a functional expression for the 

coefficient C, or even just a single value. 

USDOE-Proposed Methodology. The USDOE Capacity and Fairways Subgroup proposes 

the following relation for calculating the volumetric CO2 storage resource (equivalent to 

effective storage capacity) in deep saline aquifers: 

MCO2 = A × h × φ × ρCO2×E (15) 

where ρCO2 is the average CO2 density evaluated at pressure and temperature that represents 

storage conditions anticipated for a specific deep saline aquifer, and E is a storage efficiency 

factor that reflects the total pore volume filled with CO2. No distinction is made between CO2

stored by various mechanisms. Monte Carlo simulations produce a range for E between 1 and 

4% of the bulk volume of a deep saline aquifer for a 15 to 85% confidence range, with an 

average of 2.4% for 50% confidence. In the Monte Carlo simulations that produced the 

recommended range for E, various calculation components were varied as follows: 

- Fraction of the saline aquifer that is suitable for CO2 storage: 0.2 to 0.8 

- Fraction of the geological unit that has the porosity and permeability required for CO2

injection: 0.25 to 0.75 

- Fraction of interconnected porosity: 0.6 to 0.95 

- Areal displacement efficiency: 0.5 to 0.8 

- Vertical displacement efficiency: 0.6 to 0.9 

- Fraction of net aquifer thickness contacted (occupied) by CO2 as a result of CO2

buoyancy: 0.2 to 0.6 

- Pore-scale displacement efficiency: 0.5 to 0.8. 

These ranges of values were chosen to reflect various lithologies and geological depositional 

systems that occur in North America, with the maximum and minimum meant to be 

reasonable high and low values for each parameter. 
14 

Comparison of the methodologies proposed by the CSLF Task Force and the USDOE 

Capacity and Fairways Subgroup indicates several analogies and differences: 

1) Only volumetric (static) storage of CO2 in free phase is considered and discussed by 

the USDOE Capacity and Fairways Subgroup (no CO2 in solution ); 

2) On the other hand, unlike the CSLF Task Force, the USDOE Capacity and Fairways 

Subgroup does not limit the volumetric trapping in deep saline aquifers only to 

stratigraphic and structural traps; rather the entire aquifer is considered; 

3) The effect of irreducible water saturation is not taken into account explicitly in 

relation (15) proposed by the USDOE Capacity and Fairways Subgroup, but is 

included in the efficiency factor E through the pore-scale displacement efficiency; 

4) The two methodologies are computationally equivalent if E=Cc×(1 – Swirr) and if an 

average CO2 density at in-situ conditions is used in relation (12) rather than minimum 

and maximum values. 

Applicability/Screening Criteria. The USDOE Capacity and Fairways Subgroup explicitly 

recommends considering only saline aquifers (TDS greater than 10,000 ppm) deeper than 800 

m (or the necessary depth to ensure that CO2 is in dense liquid or supercritical phase) that are 

confined by aquitards or aquicludes (caprock) which include shale, anhydrite and evaporite. 

The CSLF Task Force did not make any specific recommendations in this regard, these 

screening criteria being implicit on the basis of the IPCC Special Report on CO2 Capture and 

Storage (IPCC, 2005). 

4. Conclusions 

Carbon dioxide capture and geological storage is a means for reducing greenhouse gas 

emissions into the atmosphere that, technologically, is immediately available, as 

demonstrated by analogue commercial-scale operations in CO2 enhanced oil recovery (EOR), 

natural gas storage and acid gas disposal, and by several commercial and pilot projects such 

as Sleipner and In Salah. However, for implementation of this technology at the scale needed 

to achieve a significant and meaningful reduction in CO2 emissions, governments and 

industry need to know more about CO2 storage capacity within their respective jurisdictions 

or within economic distance from large CO2 emitters. 

Previous attempts to assess CO2 storage capacity used a variety of approaches and 

methodologies, and data sets of variable size and quality, resulting in widely varying 

estimates of inconsistent quality and reliability. The CSLF Task Force for Review and 

Development of Standard Methodology for Storage Capacity Estimation produced in March 

2007 a report in which a consistent set of methodologies for estimating CO2 storage capacity 

in coal beds, oil and gas reservoirs and deep saline aquifers was recommended. In parallel, 

the USDOE Capacity and Fairways Subgroup within the Geologic Working Group of the 

Regional Carbon Sequestration Partnerships Program developed standards for CO2 storage 

estimation for producing a Carbon Sequestration Atlas of the United States and Canada. At 

the March 2007 CSLF meeting the CSLF Task Force proposed and was given approval to 

conduct a comparison of the methodologies developed in parallel by the CSLF Task Force 

and the USDOE Subgroup. 

The methodologies proposed by the CSLF Task Force and the USDOE Subgroup are 

basically identical, with minor differences in regard to computational formulation. In both 

cases the methods are based on static volumetrics and are applicable to country, regional and 
15 

basin scale storage capacity estimates. Local and site-specific storage capacity estimates 

should be based on numerical modeling that takes into account the dynamic aspect of CO2

injection and of CO2 plume evolution. 

The only difference of significance is that the CSLF Task Force proposed to estimate static 

CO2 storage capacity in deep saline aquifers by considering only stratigraphic and structural 

traps present in these aquifers, while the USDOE Subgroup proposes to consider the entire 

aquifer, not only the traps (i.e., storage in open systems). 

The proposed methodologies are useful in estimating the effective CO2 storage capacity, 

before applying regulatory, land use, economic and other constraining overlays. The only 

regulatory constraint explicitly considered by the USDOE Subgroup is that CO2 storage 

should be at depths greater than the depth of protected groundwater, defined by water salinity 

less than 10,000 mg/l (ppm). This constraint has been recognized also by the CSLF Task 

Force, but no specific value has been recommended, allowing for each jurisdiction to 

establish its own. 

In certain respects the work of the USDOE Capacity and Fairways Subgroup represents an 

advance on the work of the CSLF Task Force. Both the CSLF Task Force and the USDOE 

Subgroup have introduced storage efficiency coefficients in calculations. The CSLF Task 

Force has not provided values for these coefficients, undertaking to compile a table of values 

for these coefficients during Phase III based on literature review. Through Monte Carlo 

simulations of CO2 storage in coal beds and in deep saline aquifers for conditions 

characteristic to North America, the USDOE Subgroup obtained a range of values for these 

storage efficiency coefficients for the 15% and 85% confidence intervals, which are between 

0.28 and 0.40 for coal beds, and between 1% and 4% for deep saline aquifers. 

In addition, the USDOE Capacity and Fairways Subgroup proposes to use a confidence 

indicator from 1 (low) to 9 (high) to express the degree of confidence in the CO2 storage 

capacity estimates based on the amount and quality of the data used in the estimation, and on 

the degree of variability in the geological storage environment. The proposed confidence 

indicator is presented in the following table. 
16 

Table 1: 

Confidence indicator in CO2 storage capacity estimates, proposed by the USDOE 

Capacity and Fairways Subgroup of the Regional Carbon Sequestration Partnership 

Program 

CONFIDENCE INDICATOR 

Complex subsurface, 

numerous structures, 

highly discontinuous 

formation properties, 

typical of tectonically 

deformed areas 

5 3 1 

Moderately heterogeneous 

subsurface structure and 

anisotropy, possible to 

interpolate rock properties 

for up to 10 miles 

7 5 3 

S H 

u e 

b t 

s e 

u r 

r o 

f g 

a e 

c n 

e e 

 i 

 t 

 y Structural complications 

are infrequent and range 

of rock properties can be 

projected more than 10 

miles 

9 7 5 

Average Well Density > 1 well/sq. mi. > 1 well per 

9 sq. mi. 

> 1 well per 

100 sq. mi. 

Average Seismic Survey Spacing > 1 line per 

10 miles 

> 1 line per 

50 miles 

> 1 line per 

100 miles 

Data Density 
17 

The close similarity between the methodologies for CO2 storage capacity estimation at the 

country, regional and basin scales proposed independently by both the CSLF Task Force for 

Review and Development of Standard Methodology for Storage Capacity Estimation and the 

USDOE Capacity and Fairways Subgroup within the Geologic Working Group of the 

Regional Carbon Sequestration Partnerships Program indicates that these methodologies are 

robust and science-and-engineering based, increasing the degree of confidence in their use 

and in the results obtained by using them. Application of these methodologies at various 

appropriate scales should provide decision makers in governments and industry with 

information needed in assessing the potential for CO2 geological storage and in focusing 

further work for site screening and selection. Further work is needed in improving the 

estimates of storage efficiency coefficients. 

5. References 

Bachu, S. and J.J. Adams, 2003: Sequestration of CO2 in geological media in response to 

climate change: Capacity of deep saline aquifers to sequester CO2 in solution. Energy 

Conversion and Management, v. 44, no. 20, p. 3151-3175. 

Bachu, S., D. Bonijoly, J. Bradshaw, R. Burruss, S. Holloway, N.P. Christensen and O.M. 

Maathiassen, 2007. CO2 storage capacity estimation: Methodology and gaps. 

International Journal of Greenhouse Gas Control, v. 1, no. 4, p. 430-443. 

Bradshaw, J., S. Bachu, D. Bonijoly, R. Burruss, S. Holloway, N.P. Christensen and O.M. 

Mathiassen, 2007: CO2 storage capacity estimation: Issues and development of 

standards. International Journal of Greenhouse Gas Control, v. 1, no. 1, p. 62-68. 

CSLF (Carbon Sequestration Leadership Forum), 2005: A taskforce for review and 

development of standards with regards to storage capacity measurement; CSLF-T-2005-

9 15, August 2005, 16p. 

http://www.cslforum.org/documents/Taskforce_Storage_Capacity_Estimation_Version_

2.pdf

CSLF (Carbon Sequestration Leadership Forum), 2007: Estimation of CO2 storage capacity 

in geological media, June 2007, 43 p. 

http://www.cslforum.org/documents/PhaseIIreportStorageCapacityMeasurementTaskFor

ce.pdf

IPCC (Intergovernmental Panel on Climate Change), 2005: IPCC Special Report on Carbon 

Dioxide Capture and Storage (B. Metz, O. Davidson, H.C. de Coninck, M. Loos and 

L.A. Mayer, eds.), Cambridge University Press, Cambridge, U.K., and New York, NY, 

U.S.A., 442 p. 

USDOE (U.S. Department of Energy, Office of Fossil Energy), 2007: Carbon Sequestration 

Atlas of United States and Canada, 86 p.