The effect of mechanical rock properties on CO 2 storage capacity Domagoj Vulin * , Tomislav Kurevija, Iva Kolenkovic Faculty of Mining, Geology and Petroleum Engineering, Pierottijeva 6, Zagreb, Croatia article info Article history: Received 23 August 2011 Received in revised form 2 December 2011 Accepted 22 January 2012 Available online 24 February 2012 Keywords: CCS Deep saline aquifers Pore compressibility Storage efficiency Pressure buildup abstract One of the most important issues when estimating CO 2 storage capacity, especially in the case of the storage into deep saline aquifers, is the mechanical integrity of rock, i.e. estimate of cap rock fracture pressure. In the case of storage into mature oil and gas reservoirs, reservoir pressure should not present an issue since it was significantly decreased due to hydrocarbon production, so it is reasonable to assume that the rock integrity would not be disturbed by injecting CO 2 to the initial reservoir pressure. Because estimates of fracture pressure are necessary, but not convenient for regional aquifers, the analysis of pore volume changes due to pressure buildup for the chosen CO 2 site in Croatia has been made. Pressure buildup depends on reservoir fluid (brine) compressibility, CO 2 compressibility at given reservoir conditions before CO 2 injection and on rock compressibility i.e. pore compressibility. Implementing the simple method for analysis of pressure buildup considering elastic properties of fluids and rock, larger CO 2 storage capacity estimate was achieved than by using storage efficiency coefficient (E), as defined by U.S. DOE [1] and which was used for estimating storage capacities in regional deep saline aquifers in Croatia as the part of EU GeoCapacity project [2]. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Significant mitigation of climate change effects caused by greenhouse gas emissions is possible by geological storage of carbon dioxide captured at large stationary point sources (primarily coal and hydrocarbon power plants). The reliability of storage capacity estimates depends on the level of research undertaken. Storage capacity estimates for depleted oil and gas reservoirs are generally more accurate than aquifer estimates, due to numerous data collected during the hydrocarbon exploration and production. Aquifers are expected to be significantly larger carbon dioxide storage objects. According to Tek [3] minimum requirements for temporary aquifer storage of gases (in order to preserve and monitor volume of gas in storage) are: existence of structural trap, sufficient porosity, sufficient permeability and cap rock of adequate integrity to ensure against leak and migration. If no closure exists, or if the porosity or permeability is too low or if the cap rock strength is insufficient to persist poro-elastic effects caused by changes in pore fluid pressure, there would be no need for further evaluation of a storage site. After USDOE [1], in order to calculate storage capacity in a regional aquifer, simple volumetric approach can be used: m CO2 ¼ A  h  f  E  r CO2 ðp;T Þ (1) where m CO2 is the geological storage capacity in kg, A is the areal distribution of the aquifer (m 2 ), h is the cumulative thickness of reservoir rocks (m), f is the effective porosity, E is storage capacity coefficient, which is in matter of fact assumption of a volumetric sweep efficiency and r CO2 is the density (kgm 3 ) of pure CO 2 under reservoir conditions. After USDOE [1], for saline aquifers in USA and Canada storage estimated value of capacity coefficient varies between 0.01 and 0.04, these values providing 15%e85% confidence range. Difference between real aquifer data and assumed average data inevitably burden these storage estimations with considerable errors. CSLF [4] defined equations for static storage of CO 2 . Besides static trapping (in structural and stratigraphic traps), time depen- dent mechanisms were defined: residual-gas saturation trapping, dissolution, precipitation and hydrodynamic trapping. The theoretical volume available for volumetric (static) CO 2 storage in structural or stratigraphic traps, V CO2t , can be calculated using the formula [4]: V CO2t ¼ V trap  f ð1eS wirr ÞhA  h  f ð1eS wirr Þ (2) where A and h are the trap area and average thickness, respectively. S wirr is irreducible (immobile) water saturation, and V trap denotes bulk volume of the trap. * Corresponding author. Tel.: þ385 95 518 50 29; fax: þ385 1 483 60 74. E-mail addresses: dvulin@rgn.hr, dvulin@yahoo.com (D. Vulin). Contents lists available at SciVerse ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy 0360-5442/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2012.01.059 Energy 45 (2012) 512e518