Abstract

A modeling of CO2 sequestration in aquifers from an analytical point of view was studied. The driving force behind this work is based on a description of a model that analyzes the mechanisms of supercritical CO2 disposal in aquifers. The CO2 presence was analyzed from two different aspects, as gas bubble formation and as dissolution in brine. In our model, for the determination of the pressure distribution in the gas bubble, a constant injection rate was taken into consideration. With the formation of the gas bubble, the gravitational effects cause the CO2 to raise and accumulate under the caprock. For this accumulation process a maximum bubble thickness was determined to avoid leakage through the fractures in the caprock. For the CO2 that is in the dissolved phase in the aquifer, the salinity calculations were taken into consideration. One effect of salinity was that it causes an increase in brine viscosity resulting in a decrease in sweep efficiency.

Introduction

With the technology developing every second, the demand for fossil fuels is increasing drastically. Although more environmental friendly fossil fuels are being used today, there is still an excess release of CO2, which is a greenhouse gas, to the atmosphere causing the earth to warm up. As a consequence, global warming is becoming a threat for our lives day by day. In order to reduce the CO2 emission into the atmosphere, several techniques are being developed and geological sequestration is one of these techniques. The geological sequestration process can be done in aquifers, salt caverns, coal seams, hard rock caverns and in depleted oil and gas reservoirs. Due to the great potential of storage while being wide spread avaliable and having lack of effective uses, the most promising places for sequestration are aquifers. The CO2 storage capacities of potential reservoirs can be seen in Table126.

In this paper, the main focus is on CO2 sequestration in saline aquifers from an analytical point of view. In calculations, thermodynamic properties of CO2 such as compressibility factor (z), density (ρg), viscosity (µg) are determined as a function of pressure and temperature. Peng-Robinson equation of state is used in the determination of compressibility factor (z), while viscosity (µg) is calculated by using Vulkovich-Altunin equation1. Also, Corey's approach has been used for relative permeability calculations2. CO2 properties are investigated by defining sample aquifer properties, which are used in determination of CO2 solubility in water as a function of salinity, temperature and pressure 3,4,5.

The topics covered in this paper include; the pressure and saturation distributions with respect to radius and injection time6,7,8, growth of the bubble radius, the changes in viscosity and density of CO2 with temperature and pressure and changes in viscosity and density of water with changing salinity, pressure and temperature4,9.

Physical Properties of CO2

Depending on pressure and temperature CO2 may exists in three phases under it's critical point: solid, liquid and /or gas with a triple point at 56.6 °C and 75.1 psi CO2 is in supercritical phase at temperature values greater than 31.04 °C and pressure values Above 1071psi34.

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