Viscosity and density of aqueous fluids with dissolved CO2

Abstract

Carbon Capture and Storage (CCS) is considered a technology which can help ease the emissions associated with the use of fossil fuels for energy and industrial processes. In Qatar, rapid industrial expansion has established a need for CCS. Here, saline aquifers represent a potential sink for geologically storing the associated CO2 emissions. CCS is an important component of meeting carbon emission targets; CO2 storage in the carbonate reservoirs of the Middle East and elsewhere will become increasingly important. CO2 can also be used for enhanced oil recovery (EOR), which consequently brings an economic benefit. However, we know little about how CO2 behaves when it is mixed with reservoir fluids (hydrocarbons, water, brines) or with carbonate rocks; so we need more research in this area. In particular, the injection of CO2 into these reservoirs/aquifers depends crucially on the viscosity of the fluids involved. Hence the focus of this research has been on determining experimentally the viscosity and density of aqueous fluids with dissolved CO2. This thesis details the design, construction, testing and utilisation of a new experimental apparatus for performing measurements at high pressure and high temperature (HPHT). Measurements were made at temperatures between (274.15 and 448.15) K, and at pressures up to 100 MPa in the single-phase compressed liquid region. The vibrating-wire technique was used for viscosity determination, with simultaneous measurements of density by means of a vibrating-tube densimeter. Wetted parts were made from Hastelloy-C276 and Pt-Ir alloy, chosen to resist corrosion. Measurements were first performed on the system (water + CO2). Following this, mixtures of salt solutions with dissolved CO2 were measured and the results correlated in useful models which allowed for comparison with literature data where this was possible. Salts investigated include NaCl(aq), CaCl2(aq), each at two different concentrations, and lastly, a synthetic Qatari reservoir brine. A correlation based on the Vogel-Fulcher-Tammann (VFT) equation for viscosity is presented which represents all the data to within ±3%, including the pure Qatari brine and its modification by dissolved CO2. Density is correlated using the partial molar volume of CO2 and gives the densities of the mixtures to within ±0.1 %. The results presented in this thesis extend our knowledge of the viscosity of CO2-water-brine systems to significantly higher temperatures and pressures than are currently available in the literature. The effect of temperature on density and viscosity is significant whereas pressure effects are relatively small. Since rigorous theories for gas-saline mixtures are not yet available, semi-empirical correlations with some underlying physicochemical basis have been used to represent the new results. These are capable of estimating the density and viscosity of the fluid mixtures involved to an accuracy that is more than adequate for the design and control of CCS and EOR subsurface processes.