The fluid flow consequences of CO2 migration from 1000 to 600 metres upon passing the critical conditions of CO2

The minimum injection depth for the storage of CO(2) is normally set at 800 metres. At and beyond this depth in the subsurface conditions exist where CO(2) is in a so-called critical state. The supercritical CO(2) has a viscosity comparable to that of a normal gas and a liquid-like density, Due to the high density of the supercritical CO(2), storing the CO(2) in the supercritical state is the most efficient geological storage The CO(2) will therefore be injected below the transition zone between subcritical and supercritical conditions. In the case of CO(2) storage in large aquifers, some part of the storage formation may lie at shallower depths where CO(2) occurs in gas phase. The CO(2) will also occur in the subcritical state (gas phase) in the case of an unintentional CO(2) leak from an existing storage site. In both cases it is crucial to understand how the CO(2) will behave when it reaches and passes this transition zone. In the case of intentional CO(2) injection in a shallow aquifer this knowledge is important to determine both the injection strategy and the available storage capacity of the aquifer; in the case of leakage, safety and risk assessment. In this paper we present the results of a reservoir simulation study, supported by a literature study, that considers the principles of CO(2) phase behaviour in the subsurface, and the implications for the injection strategy and storage volumes. (C) 2008 Elsevier Ltd. All rights reserved.