PHASE EQUILIBRIA IN CO2-BRINE SYSTEM FOR CO2 STORAGE

Open Access
Author:
Zhao, Haining
Graduate Program:
Petroleum and Natural Gas Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 29, 2014
Committee Members:
  • Serguei Lvov, Dissertation Advisor
  • Serguei Lvov, Committee Chair
  • William D Burgos, Special Member
  • Li Li, Committee Member
  • Yilin Wang, Committee Member
  • Luis F Ayala H, Committee Member
Keywords:
  • CO2 solubility
  • CO2-brine system
  • Equation of state
  • NaCl
  • CaCl2
  • Na2SO4
  • MgCl2
  • KCl
  • Pitzer model
  • activity coefficient
  • PSUCO2
Abstract:
The apparatus for measuring CO2 solubility at elevated temperatures and pressures were developed in this study; new experimental CO2 solubility in the following brines were collected: (1). Single-salt aqueous NaCl, CaCl2, Na2SO4, MgCl2, and KCl with ionic strength from 0.5 to 6 mol/kg; (2). Synthetic mixed-salt (NaCl and CaCl2) brines with ionic strength 1.712 and 4.984 mol/kg; (3). Synthetic mixed-salt (NaCl, CaCl2, Na2SO4, MgCl2, KCl, SrCl2, and NaBr) brines with ionic strength 1.712 and 4.984 mol/kg; (4). A natural Mt. Simon formation brine with ionic strength 1.815 mol/kg. The experimental data are used to develop the proposed PSUCO2 model. The model is capable of calculating mutual solubilities of CO2 and H2O for the system of CO2-salt-H2O containing NaCl, CaCl2, Na2SO4, MgCl2, or KCl or a mixture of them. Comparisons against literature data reveal a clear improvement of the proposed PSUCO2 model among the published models in predicting CO2 solubility in the aforementioned brines. A comparison of modeling results with experimental values on the P-x diagram (Figure 2. 14) revealed a pressure-bounded "transition zone" in which the CO2 solubility decreases to a minimum and then increases as the temperature increases. CO2 solubility is not a monotonic function of temperature in the transition zone but outside of that transition zone, the CO2 solubility decreases or increases monotonically in response to increased temperature. The similar phenomenon is also observed by plotting CO2 solubility contours in P-T diagram (Figure 4. 9), where the path of the maximum gradient among the CO2 solubility contours is defined to divide the P-T diagram into two regions: in Region I, the CO2 solubility in the aqueous phase decreases monotonically in response to increased temperature, but in region II, the behavior of the CO2 solubility is the opposite of that in Region I as the temperature increases. A web-based computation interface of the model PSUCO2 is developed by the author and can be accessed via the link: http://www.carbonlab.org/psuco2/.