Journal of Canadian Petroleum Technology
Volume 48, Number 8, August 2009, 22-27

Abstract

CO₂ sequestration in deep geological formations has been suggested as an option to reduce greenhouse gas emissions. Saline aquifers are one of the most promising options for carbon dioxide storage. It has been shown that the dissolution of CO₂ into brine causes the density of the mixture to increase. If the corresponding Rayleigh number of the porous medium is enough to initiate convection currents, the rate of dissolution will increase. Early time dissolution of CO₂ in brine is mainly dominated by molecular diffusion, while late time dissolution is predominantly governed by a convective mixing mechanism. In this paper, linear stability analysis of density-driven miscible flow for carbon dioxide sequestration in deep inclined and homogeneous saline aquifers is presented. The effect of inclination and its influence on the pattern of convection cells has been investigated and the results are compared with the horizontal layer. The current analysis provides approximations for the initial wavelength of the convective instabilities and the onset of convection that helps in selecting suitable candidates for geological CO₂ sequestration sites.

Introduction

Carbon dioxide sequestration is the capture and safe storage of carbon dioxide that would otherwise emit to the atmosphere. Sequestration refers to any storage scheme that can keep CO₂ out of the atmosphere⁽¹⁾. In general, proposed storage sites of carbon dioxide can be divided into two categories: geological sites and marine sites. Carbon dioxide sequestration in deep geological formations has been suggested as a way of reducing greenhouse gas emissions. Geologic sequestration of CO₂ is the capture of CO₂ from major sources, transporting it usually by pipeline, and injecting it into underground formations such as oil and gas reservoirs, saline aquifers and unmineable coal seams for a significant period of time^{(2, 3)}. Unlike coalbed methane reserves and oil reservoirs, sequestration of CO₂ in deep saline aquifers does not produce value-added by-products, but it has other advantages. While there are uncertainties regarding the scope, the world's total capacity to store CO₂ deep underground is large⁽⁴⁾. Underground formations are generally unused and are available in many parts of the world⁽⁵⁾. It has been estimated that deep saline formations in the United States could potentially store up to 500 billion tonnes of CO₂. Most existing large CO₂ point sources are within easy access to a saline formation injection point and, therefore, sequestration in saline formations is compatible with a strategy of transforming large portions of the existing energy and industrial assets to near-zero carbon emissions via low-cost carbon sequestration retrofits⁽³⁾. However, it is important to investigate the behaviour of CO₂ injected into aquifers for effective and safe use of storage. Geological storage of CO₂ as a greenhouse gas mitigation option was proposed in the 1970s⁽⁶⁾, but little research was done until the early 1990s when the idea gained credibility through the work of individual research groups^(7-10).

When CO₂ is injected into the formation above its critical temperature and pressure, the density of supercritical carbon dioxide is usually less than brine. This density difference causes CO₂ to migrate upwards to the top of the formation under an impermeable caprock.

© 2009. Petroleum Society of Canada (now Society of Petroleum Engineers)

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History

• Original manuscript received: 27 March 2008
• Meeting paper published: 17 June 2008
• Revised manuscript received: 30 April 2009
• Manuscript approved: 6 July 2009