document preview

Abstract
A series of steady state multiphase flow experiments at a range of fractional flows and flow rates have been conducted using Berea Sandstone. Using the multiphase flow simulator TOUGH2 MP with ECO2N module, carbon dioxide saturation distributions, average saturations, and pressure gradients across the core were calculated to determine the influences of subcore scale heterogeneity, gravity and flow rate on brine displacement efficiency. It is found that measured CO2 saturation patterns can be replicated using simulation models that include spatially varying porosity, permeability and capillary pressure curves.

The interplay of viscous, capillary and gravity forces in core flood experiments are also investigated at different Gravity and Capillary numbers representative of those expected for a typical sequestration project (Gravity numbers: 0~1000 while Capillary numbers: 10-6~10-10). These dimensionless numbers span the range of conditions expected in the near-well region to leading of the plume which may be up to 5 km or more from the injection well. Simulations show that the efficiency of brine displacement and saturation distributions during vertical displacement fall into three separate regimes. (1) At high flow rates representative of the near-well region, the brine displacement efficiency is nearly independent of flowrate. (2) When the Capillary number drops below 10-7 and the gravity number is 2, both the heterogeneous and homogenous cores display flow rate dependent saturation distributions, with brine displacement efficiency dropping by about 80%. Most of this effect is caused by the influence of gravity, as the decrease in brine displacement efficiency is only slightly smaller for heterogeneous cores. (3) At very low capillary numbers, the brine displacement efficiency appears to asymptotically approach a constant value which is independent of flowrate. In this regime, heterogeneity leads to a large reduction in brine displacement efficiency, which is independent of gravity.

1. Introduction
Increasing atmospheric concentrations of greenhouse gases, such as carbon dioxide and methane, have been shown to cause global warming and hence climate change. The major contribution to increasing emissions of CO2 is human activity due to the use of fossil fuels. To mitigate this phenomenon, reducing greenhouse gas emissions is needed to stabilize or decrease the concentration of CO2 in the atmosphere. Carbon dioxide capture and sequestration in deep geological formations is one of the most important technologies for climate change mitigation (IPCC, 2005).

Although depleting or abandoned oil and gas reservoirs are available in some regions like Texas in US, the Middle East, Russia and Alberta in Canada, they have lower available capacity than the amount of CO2 emissions from large stationary sources and these depleted reservoirs are not common all over the world. Moreover, their capacities are available only when the reservoirs are depleted or if CO2 sequestration is combined with CO2–EOR. Thus storage of CO2 in depleted or abandoned oil and gas fields is limited. Therefore, the large storage capacity of worldwide distributed saline aquifers makes them good locations to store CO2 captured from power generation stations and industrial sources, which are the large main sources emitting CO2 (IPCC, 2005). Moreover, sequestration of CO2 in the deep underground formations is immediately accessible compared to the other options.