On the solubility of acid and sour gases in water and brines under reservoir conditions

Abstract

Carbon Capture and Storage (CCS) has been identified as a central technology to reduce CO2 emissions, but it is burdened by unoptimised economics. The capture and compression activities of a CCS plant represent the largest portion of the added cost of the generated electricity. Reducing the extensiveness of the separation process, in other words lowering the purity of the captured CO2 stream has the potential to substantially cut the energy penalty resulting from this process thus making it more economically viable. Presence of impurities in the captured CO2 stream introduces unknowns that must be understood in order to accurately predict the sequestration process. Thermodynamic properties and, in particular, phase-behaviour measurements, are the keystones to quality understanding of the reservoir performances. It provides scientific proof of the behaviour of injected fluids and their mixtures in the in-situ reservoir components. This thesis focuses on the former. During the first part of this work, the phase-behaviour of binary mixtures composed of impurities (namely O2 and C3H8) and water, were experimentally investigated at pressures and temperatures encountered in deep reservoir conditions, p = (0.1 - 60) MPa and T = (348.15 - 398.15) K. Two equipments were exploited during part of this work: a synthetic apparatus (for brines binaries bubble point measurement over a wide range of temperatures and pressures, T = (298.15 - 473.15) K and p = (0.1 - 50) MPa) and a static analytical apparatus (for pure-water based systems at temperatures ranging from (298.15 to 473.15) K and pressures up to 20 MPa). The synthetic equipment produced reliable solubility for the (CO2 + H2O) system (1% Absolute Average Deviation with the literature data) during the validation phase. However, due to internal dead-volume issues which introduced large uncertainties for very low solute solubilities, as it is the case for C3H8 in H2O, the apparatus failed to generate satisfactory measurements for the (C H +H O) system. The (O2+H2O) was investigated at two isotherms with the analytical equipment: T = (308.15 and 348.15) K, and up to 17.95 MPa. The use of Thermal Conductivity Detector proved not to be adapted to the analysis of this binary system as measurements did not give satisfactory accuracy.

The water-rich phase composition of the (C3H8+H2O) binary generated good results through the parallel use of a Thermal Conductivity Detector and an Flame Ionised Detector. The isotherm T = 308.15 K was measured up to 17 MPa. Measurements for isotherm T = 348.15 K could only be obtained up to = 2.8 MPa. The solubilities obtained for both isotherms were in very good agreement with the literature (average standard deviation of 2%). Experimental limitations encountered with the aforementioned apparatus were overcome in the second part of this work. To do so, a novel semi-analytical equipment was designed, assembled and validated for High Pressure High Temperature (HPHT) phase behaviour analysis. It offers the dual capabilities of solubility measurements for pseudo-binary mixtures composed of sour gases (CO2, H2S, SO2) as well as sparingly soluble gases (C3H8) in highly concentrated brines and for a wide range of temperatures and pressures: T = (298.15 - 473.15) K and p = (5 - 70) MPa. The new semi-analytical apparatus operates in a new laboratory for exclusive use of High Pressure High Temperature (HPHT) sulphuric gases, partially built and commissioned under this work. To validate the new apparatus, the composition of the water-rich phase of the (CO2+H2O) binary was obtained for two isotherms: T = (348.15, 398.15) K for pressures up to 50 MPa and up to 60 MPa, respectively. Isotherm T = 348.15 K was experimentally obtained, which subsequently lead to the statistical re-appraisal of an older data set: isotherm, T = 398.15 K. The two isotherms were consistent with the Spycher-Pruess (SP) solubility model as the AADs was 2% for both temperatures. The CO2 solubility in the water-rich phase of the (CO +H O+NaCl) binary was studied at T = 348.15 K and up to 50 MPa, and with a salt concentration of m = 3 mol/kg. This isotherm, which had not been explored in the literature at these conditions, presented good agreements with the SP predictions (AAD of 1%). This therefore verified the capacity of the new equipment to deliver good quality HPHT solubility measurements for pseudo-binaries composed of gases and concentrated brine. New water-rich phase composition measurements for the (CO2+ H2O + NaHCO3) system were obtained at two isotherms: T = 348.15 K with p = (0.1 - 60) MPa andT = 398.15 K with p = (0.1 - 50) MPa and with a salt concentration of m = 0.8 mol/kg. These studies led to highly repeatable data sets for both isotherms, with an average deviation of 2.7% at T = 348.15 K and 2.2% at T = 398.15 K. As expected for highly concentrated brines, a salting-out effect is noticeable at all temperatures and pressures with less CO2 dissolved in the saline solution than for pure H2O. However this salting-out effect was not correctly predicted by the SP solubility model. A modification of the SP solubility model was performed in terms of an addition to their Pitzer activity-coefficient formulation. A linear dependency in the bicarbonate molality was introduced, which resulted in a better prediction of the CO mole fraction obtained for this system at the conditions of study. For isotherm T = 398.15 K, over the pressure range 5 to 60 MPa and with the optimised Pitzer model, the computed CO solubility in the bicarbonate brine agreed with the data reported here with an average absolute deviation of 2% and a maximum deviation of 6%. For isotherm T = 398.15 K, the average absolute deviation was 1% and the maximum deviation was 2%. For both isotherms, the agreements between the modified SP solubility model and the measured data were within the uncertainty of the experiments, which is a satisfactory conclusion. The new results obtained in this work illustrate the potential of the new semi-analytical apparatus to provide previously unstudied solubility data of sour gases in highly concentrated brines for a wide range of pressures and temperatures: T = (298.15 - 398.15) K and p = (0.1 - 60) MPa. Low-hanging fruits for future high-impact measurements have been also identified.