Multi-phase Equilibrium in a CO2-filled Observation Well at the Ketzin Pilot Site

Abstract

As part of the Ketzin pilot test site, carbon dioxide (CO2) has been injected into a saline formation around 650 m deep since June 2008. Analysis of measured well temperature and pressure data at two observation wells after the arrival of CO2 has shown that two-phase fluid conditions are prevailing in the upper 400 m of the wells. This significantly hampers the ability to accurately describe the density – and temperature – profile of a well, which is essential to predict the relationship between well-head and reservoir pressure. The pressure and temperature profile depends crucially on the depth of the liquid CO2 free surface, which can be obtained by wrapping a radial temperature simulator in an optimization algorithm that imposes closure of the heat transport at the well scale. Vertical heat transport, necessary for balancing the heat in- and outflow, relies on the upward movement of CO2 vapor bubbles in the liquid-dominated zone and by a descending liquid CO2 film in the upper vapor-dominated zone. A vertical bubble velocity of around 45 mm/s was derived from heat balance and confirmed by camera inspection movie. Temperature and pressure profiles also showed that Ktzi 200 acted as a distillation column: the injected CO2 (with purity >99.9%), picked up water during the residence in the storage horizon. The extremely accurate equation of state for CO2 can accurately predict fluid properties in the two-phase zone and reveals that both light and heavy components in the gas entering the well are not present in the two-phase zone. This, together with an analysis of the thermodynamic behavior of the gas mixture suggests that a distillation process purified CO2, at least in the upper part of the well.