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Abstract

Proper modeling of the multiphase flow of supercritical CO₂ in deep saline aquifers for CO₂ sequestration (both cycles of drainage during injection and imbibition during CO₂ migration)  is critical in being able to understand and predict both the short and long term fate of the injected CO₂ over extended time periods (hundreds to thousands of years). Current numerical models require the use of accurate two-phase CO₂/brine relative permeability data at representative in-situ conditions in order to be able to accurately conduct these calculations. However, there are virtually no published data in the literature on the high temperature and pressure displacement character of CO₂/brine systems in actual reservoir rocks, except for the data published by the authors in the June 2008 issue of the SPE Reservoir Evaluation and Engineering Journal. That data set, although it included a few carbonate cases, contained mostly measurements on clastic rocks. This paper presents a new set of nine relative permeability measurements (both drainage and imbibition) for carbonate rocks (limestone and dolomite) of higher permeability values than those in the initial work (which are thus more likely to be representative for candidates for CO₂ sequestration in deep saline aquifers). The new data set to be presented includes also pre and post-test CAT-scan imaging of selected samples to illustrate potential effects of CO₂ contact on potentially soluble carbonate matrices. The paper compares the new data set of measurements for carbonate rocks with the limited set of data available for carbonates from the previous work, and attempts to determine if specific relative permeability and residual saturation trends can be defined based on other rock characteristics that are easier to measure in routine core analyses, to allow extension of the data set to other carbonate facies elsewhere which have not been tested.

Introduction

Interpretation of the temperature record on a scale of centuries to millennia indicates a slight increase in global annual temperatures in the last 150 years, in the order of 0.76ºC (IPCC, 2007). Predictions are that, if continuing in a business-as-usual (BAU) scenario, humankind is facing significant climate change by the end of this century as a result of warming forecast to be in the range of 1.1 to 6.3 ºC, depending on emissions scenario. It is very likely (>90% likelihood) and generally accepted that the main cause of the observed global warming is the increase in atmospheric concentrations of greenhouse gases, such as carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) (IPCC, 2007). This increase, noticeable since the beginning of the industrial revolution, is due to human activity in land use (agriculture and deforestation), which is the major factor in CH₄ and N₂O concentrations increases, and increasing consumption of fossil energy resources, which accounts for >80% of the increase in CO₂ concentrations (IPCC, 2007). Of all the greenhouse gases, CO₂, whose atmospheric concentration has risen from pre-industrial levels of 280 ppm to 380 ppm in 2005, is the most important greenhouse gas, being responsible for about two-thirds of the enhanced “greenhouse gas” effect (IPCC, 2007). Although a direct causal link between the carbon cycle, including CO₂ and CH₄, and global warming has not been demonstrated, circumstantial evidence points toward this link, which has been generally accepted by a broad segment of the scientific community, population and policy makers.