A Phase-Partitioning Model for CO2–Brine Mixtures at Elevated Temperatures and Pressures: Application to CO2-Enhanced Geothermal Systems

Abstract

Correlations are presented to compute the mutual solubilities of CO 2 and chloride brines at temperatures 12-300 • C, pressures 1-600 bar (0.1-60 MPa), and salinities 0-6 m NaCl. The formulation is computationally efficient and primarily intended for numerical simulations of CO 2 -water flow in carbon sequestration and geothermal studies. The phase-partitioning model relies on experimental data from literature for phase partitioning between CO 2 and NaCl brines, and extends the previously published correlations to higher temperatures. The model relies on activity coefficients for the H 2 O-rich (aqueous) phase and fugacity coefficients for the CO 2 -rich phase. Activity coefficients are treated using a Margules expression for CO 2 in pure water, and a Pitzer expression for salting-out effects. Fugacity coefficients are computed using a modified Redlich-Kwong equation of state and mixing rules that incorporate asymmetric binary interaction parameters. Parameters for the calculation of activity and fugacity coefficients were fitted to published solubility data over the P-T range of interest. In doing so, mutual solubilities and gas-phase volumetric data are typically reproduced within the scatter of the available data. An example of multiphase flow simulation implementing the mutual solubility model is presented for the case of a hypothetical, enhanced geothermal system where CO 2 is used as the heat extraction fluid. In this simulation, dry supercritical CO 2 at 20 • C is injected into a 200 • C hot-water reservoir. Results show that the injected CO 2 displaces the formation water relatively quickly, but that the produced CO 2 contains significant water for long periods of time. The amount of water in the CO 2 could have implications for reactivity with reservoir rocks and engineered materials.

Saturated phase densities of (CO 2 + H 2 O) at temperatures from (293 to 450) K and pressures up to 64 MPa

“…At temperatures between (285 and 372) K [16; 17] the volume and the partial fugacity coefficient of the CO2-rich phase were approximated to be that of pure CO2, and the water activity was assumed to be equal to the mole fraction of water in the aqueous phase. At temperatures between (382 and 573) K, these assumptions do not allow sufficient accuracy and a Margules activity coefficient model was used to describe the aqueous phase and asymmetric binary interaction parameters that were used in the mixing rule for the Redlich-Kwong EOS to describe the CO2-rich phase [18]. At temperatures between (372 and 382) K, this extended model [18] gives a weighted value of the properties calculated using the ideal (low temperature) and non-ideal (high-temperature) sets of correlations.…”

“…Spycher et al [16; 17; 18] regressed their RK EOS to compressibility factors for pure CO2 calculated with the EOS of Span and Wagner [30]. The binary interaction parameters within the RK EOS mixing rule used at temperatures above 372 K were fit by Spycher et al to phase equilibrium data for the CO2 + H2O system [18]. In the process of calculating the fugacity coefficients using this tuned RK EOS, the Spycher et al [16; 17; 18] model also calculates the density of the CO2-rich phase.…”

“…Deviations (ρaq -ρcalc) of the saturated aqueous-phase densities ρaq for the CO2 + H2O system from values ρcalc calculated at the same temperature and pressure using the models of Duan et al [14] (for saturated phase composition), Sedlbauer et al [22] (for CO2 partial molar volume) and Wagner and Pruss [20] (for solvent molar volume). (a) Symbols: this work , T = 293 K; , T = 298 K; , T = 308 K; , T = 323 K; , T = 343 K; , T = 373 K; , T = 398 K; , T = 423 K; , T = 448 K. Curves: ρaq predicted using the EOS of Spycher et al [16], [18] (for saturated phase composition) in place of the Duan et al [14] at the experimental conditions studied in this work. (b) Symbols: , this work; , Chiquet et al [9]; , Yan et al [36]; , Hebach et al [7]; ,Tabasinejad et al [10]; , Yaginuma et al [11].…”

Supercritical CO2 flow through a layered silica sand/calcite sand system: Experiment and modified maximal inscribed spheres analysis

“…This is because the CO 2 is displaced by the brine, and dissolved into the passing brine. Under the experimental conditions, the equilibrium CO 2 mole fraction is 0.73E-02, which is equivalent to 0.48 molal [calculated using Spycher and Pruess, 2010]. Following the CO 2 flood, approximately 1.85 g CO 2 was present in the sample, and a maximum of 4.2 g could be removed from the system by dissolution into the 2 pore volumes of brine.…”

Ways to Minimize Water Usage in Engineered Geothermal Systems

“…For instance, in order to utilize a geothermal resource that lacks sufficient subsurface flow, a working fluid could be injected via wells drilled from the surface. Possible examples are the injection of treated municipal waste water such as done at The Geysers (Majer & Peterson 2007) or injection of supercritical CO 2 (Spycher & Pruess 2010). Such activities, though, generate additional questions about the uncertainties related to 1) how to access these deep formations, 2) how to generate a structure within the formation that allows for fluid to pass through and be heated, and 3) what will happen to the structure over time as this flow occurs.…”