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Abstract
This paper argues that any capacity estimation method requires a combination of geological, engineering and economic analysis in order to provide rigorous capacity estimates. It also argues that the classification of capacity estimates should follow concepts in the existing SPE Petroleum Resource Management System as closely as possible. The Energy & Environmental Research Centre (EERC) (Gorecki et al., 2009) have developed a definition of “practical storage capacity” that parallels the definition of petroleum reserves as “the quantity of hydrocarbons which are anticipated to be commercially recovered from known accumulation at a given date forward”, but the EERC acknowledge that there is currently a problem with implementing a price of carbon. This paper develops the economic analysis further than the EERC. Like the EERC, we demonstrate that analytical and numerical injectivity modelling based on geological models of the subsurface can help determine practical storage capacity.

In doing this, the paper makes observations about methods for estimating storage capacity, shows results of reservoir simulations and economic analyses, draws on SPE and internationally accepted methodologies and definitions of petroleum resources and discusses how equivalent definitions can be applied to storage capacity. Finally, the paper provides recommendations for an improved CO2 storage capacity classification system.

1 Introduction
As the technology surrounding carbon capture and storage moves from research to deployment, there is a need to estimate the amount of CO2 that can be safely and securely stored in the subsurface. There is an ongoing discussion about methods for determining CO2 storage capacity and an appropriate storage capacity classification system.

Much of the literature on estimating the capacity of geological sites for storing CO2 focuses on determining the total pore volume of a formation and then applying “storage efficiency factor” to quantify the portion of this resource that can be most likely accessed for storage. This paper identifies the theoretical and practical pitfalls of such storage capacity estimation methods and argues that the most appropriate means of producing rigorous estimates of CO2 storage capacity is by a combination of geological, reservoir engineering and economic analyses. Such an approach leads directly to definitions of capacity that have direct correspondence with SPE definitions of reserves and resources.

The aims of this paper are to —
(a) Discuss the difficulties in applying existing methods of determining CO2 storage capacity in the subsurface.
(b) Suggest an approach for determining CO2 storage capacity that involves geo-science, engineering and economics.
(c) Illustrate this approach by applying it to different types of potential CO2 storage sites.
(d) Compare our illustrative results with the results obtained using other approaches.
(e) Suggest a CO2 storage capacity classification system that corresponds closely to that used worldwide to classify petroleum resources.