A New Classification System For Evaluating CO2 Storage Resource/Capacity Estimates

Abstract

Carbon dioxide (CO2) storage estimates are a critical component of the decision-making process when considering the implementation of large-scale CO2 storage in the subsurface. In order to compare storage resource/capacity estimates, both the scale of the estimate and the type of estimate must be considered. To date, the classification of resources and commodities has been used almost exclusively for valuable materials that can be economically extracted from the subsurface, e.g., hydrocarbons, metal ore, coal, etc. These industries have benefited from the establishment of classification systems with consistent terms and definitions that have gained international acceptance and allow for systematic accounting and comparison of resources across geological, geographical, and jurisdictional boundaries. As the carbon capture and storage (CCS) industry grows, there is increasing need for an accepted classification system that describes the available CO2 storage resource. While the classification systems used in the mining and hydrocarbon industries have elements that are instructive and sometimes indirectly applicable with respect to CO2, the direct application of those systems is largely insufficient. This is because, in the context of geological CO2 storage, the desired resource is not something to be removed from a subsurface reservoir but rather the accessible pore volume of the reservoir itself. Although much work has been accomplished, particularly by the Carbon Sequestration Leadership Forum (CSLF), and the U.S. Department of Energy (DOE) in the Carbon Sequestration Atlas of the United States and Canada, inconsistencies in definitions related to CCS exist between groups, and a widely accepted set of definitions for discussing CO2 storage resource and capacity has not yet been established. In order to move the CCS industry toward a useful set of definitions and provide a consistent set of terms, an improved classification system has been developed to not only address the level of the assessment but also the scale at which the assessment was made. Introduction Carbon capture and storage (CCS) represents an emerging set of technologies aimed at reducing the amount of anthropogenic carbon dioxide (CO2) that reaches the atmosphere by injecting and storing it in geological formations. Several targets have been identified as amenable to the injection and storage of CO2, most notably oil and gas reservoirs, saline formations, and unminable coal seams. Other potential targets have been proposed for future use, including basalts and organic shales; however, they are not considered for this paper. Within these target formations, a variety of trapping mechanisms must be considered, e.g., physical, solubility, and mineral. Each of the trapping mechanisms varies in its permanence and relevance over different timescales, with physical trapping beneath a low-permeability cap rock the most notable in the short term.

Review of CO2 storage efficiency in deep saline aquifers

“…Since this "pyramid" concept of CO 2 storage capacity has been introduced, attempts were made to align the definitions and estimates of CO 2 storage capacity with the classifications for resources and reserves used in extractive industries for non-renewable commodities (mining and oil & gas industries; see Ahlbrandt et al, 2004). To eliminate the ambiguity of the term "storage capacity", the terms "CO 2 resource" and "CO 2 reserve" were introduced by analogy with the classifications used by geological surveys and the oil and gas industry (see, e.g., Burruss, 2009;Frailey and Finley, 2009;Gorecki et al, 2009b). Using this analogy, the effective and practical storage capacities defined by Bachu et al (2007) are equivalent, respectively, to CO 2 storage resources and reserves, while the matched storage capacity would be equivalent to the produced commodity (e.g., produced oil).…”

Uncertainty in static CO2 storage capacity estimates: Case study from the North Sea, UK

“…For example, the parameter 'fraction of total basin/region area that has a suitable formation present' may be considered to be 100% on a prospect scale that has been thoroughly investigated, rather than the 20-80% range used by the Atlas. Recent authors 15,16 have refi ned the input parameters over those used within the original methodology to remove these regional scale variables. Despite this modifi cation, effi ciency factors improve only by a value of 8% pointing to a fl aw inherent in the method, i.e.…”

“…multiplying a fraction by a further fraction resulting in an ever-decreasing value. Where suffi cient data are available, the method of calculating effi ciency by relying on dynamic reservoir simulations requiring irreducible water saturation values as detailed by Gorecki et al 16,17 and Allinson et al 10 would appear to give more accurate results based upon a site-specifi c basis and result in estimated E factors of up to 16.5% for thin low permeability reservoirs, and up to 25% 4-way dip closed structures.…”

“…Map indicating location of study site within UKCS Quads 28 and 29 correlated to approximate topographical extent of the Northern Permian Basin (NPB) indicated by dashed grey line (adapted from Legler and Schneider, 2008) and related to major faults (red line) and selected grabens of the Central North Sea. Well logs used in this study (Table ) are indicated by red dots with well name.…”

“…Initial work undertaken by the US Department of Energy (DoE) 12 devised a simple methodology for calculating storage capacity of regional scale saline aquifers by calculating the total aquifer volume and applying a series of effi ciency factors that attempt to correct for the presence of geologic heterogeneity in the form of a probabilistic multiplicative sum of fractions. Significant work has been undertaken to refi ne and improve this approach by a number of authors 10,13,[15][16][17][18][19][20][21][22] specifically on refi ning the use of effi ciency factors. Two commonly implemented methodologies have since been devised drawing upon the DoE method 12,21 and that of the Carbon Sequestration Leadership Forum 22 that have been summarized by Kopp et al 15 A further controversial method was proposed by Ehlig-Economides and Economides 11 stating that geological formations acted like sealed containers and thus injection into such formations would result in a rapid pressure increase drastically reducing the potential storage volume.…”