Viscosity and density of reservoir fluids with dissolved CO2
Creators: name: Calabrese, Claudio
Publisher: name: Imperial College London
Year: 2017

description: The viscosity and density of a range of aqueous and hydrocarbon reservoir fluids, with and without dissolved CO2, have been studied at high temperature and high pressure conditions. The findings of this research can be applied to the oil and gas industry, for instance to design and operate enhanced oil recovery (EOR) processes using CO2 and large-scale carbon storage in depleted hydrocarbon reservoirs or deep saline aquifers. In this work, the viscosity was measured by means of a vibrating-wire (VW) viscometer while the density was measured with a vibrating U-tube (VT) densimeter. The simultaneous measurements of viscosity and density were carried out in the single-phase compressed liquid region at temperatures between (273 and 466) K and pressures up to 100 MPa. In addition, density measurements were made for four hydraulic fluids up to a maximum pressure of 135 MPa. The viscosity and density measurements of NaCl(aq) and CaCl2(aq) brines under CO2 addition were made at salt molalities of 0.77 mol·kg-1 and 1.00 mol·kg-1, respectively. Additional density measurements were also made for the [CO2 + NaCl(aq) or CaCl2(aq)] systems at salt molalities of 2.50 mol·kg-1. To enable the viscosity measurements, a key contribution of this work was the development of a new modification of the working equation of the VW viscometer which takes into account the electrical conductivity of these brines, and hence expanded the use of this precise technique to an entire new class of conductive fluids. The results for the viscosity and density were correlated as functions of temperature, pressure and the mole fraction of dissolved CO2. For viscosity, a simple modification of the Vogel-Fulcher-Tamman equation was employed while, for density, an equation based on the partial molar volume of CO2(aq) and the molar volume of the CO2-free aqueous solution was used. The viscosity and density of two synthetic crude oil mixtures with dissolved CO2 were also measured. The synthetic dead oil contained a total of 17 components including linear and branched alkanes, cyclo-alkanes and aromatics. A live oil with a gas-to-oil ratio of 58 was obtained from this dead oil by adding solution gas (CH4 + C2H6 + C3H8). For the synthetic dead oil, the mole fractions of dissolved CO2 were x = (0.0, 0.1, 0.2, 0.4, 0.6 and 1.0). The investigated CO2 mole fractions for the synthetic live oil mixture were x = (0.0, 0.1, 0.2 and 0.4). The experimental viscosity and density data were correlated at each CO2 mole fraction as a function of temperature and pressure. A modified Tait equation was used to correlate the densities, while an empirical equation was used for modelling the viscosity of the (CO2 + synthetic crude oil) mixtures. Accurate viscosity and density data were then gathered for two synthetic paraffinic mixtures in order to validate the Vesovic-Wakeham (VW) predictive method for these complex mixtures over a wide range of temperature and pressure, at viscosities up to 2.5 mPa∙s. The two mixtures were referred to as oil #1 and oil #2 and contained a total of 10 and 5 liquid normal alkane components, respectively. The selection criteria for these components were based on the distribution of single carbon number (SCN) of a real light stock tank oil with a molecular weight of 184 g∙mol-1 and density of about 867 kg∙m-3. The mole fraction of C7+ in both mixtures was constrained to 0.9. n-alkane mixtures were chosen because they represent the simplest system to investigate for developing a generic predictive model applicable to more complex and heavy synthetic crude oils. The VW model was able to represent the viscosities of both mixtures with an absolute average deviation of 5 %. The positive results of this work on n-alkane mixtures is an essential precursor for the application of the VW model to more complex fluids encountered in the petrochemical industry. The density of four hydraulic fluids were also studied to test the correlative capability of the modified Tait equation over wide ranges of temperature and pressure. In this case, a correlative approach was preferred to a predictive model because the chemical composition of the above-mentioned fluids was unknown. The modified Tait equation fitted well the experimental density data and was successfully employed to extrapolate densities at 473.15 K, at pressures from (0.1 to 135) MPa. The accurate correlative power of the modified Tait equation over wide ranges of temperature and pressure can be exploited for improving the performance and design of motors and pumps which make use of hydraulic fluids. The results presented in this thesis were carried out as part of the Qatar Carbonates and Carbon Storage Research Centre (QCCSRC) program. The work extends the knowledge of the viscosity and density of reservoir fluids under CO2 addition at higher pressures and temperatures compared to the existing available data in the literature. In addition, it also provides purely empirical or semi-theoretical models which are able to determine the viscosity and density of reservoir fluids with dissolved CO2 with satisfactory accuracies for industrial applications. However, additional research is needed in this field, and for this reason, further experimental investigations have been identified and suggested.