1 Kimberly Gilbert, Phil Bennett, Will Wolfe, Katherine Romanak, Tongwei Zhang, Randall Cygan 1 University of Texas at Austin 2 Sandia National Labs March 03, 2016

Challenge 1: Sustaining Large Storage Rates Challenge 2: Using pore space with unprecedented efficiency Challenge 3: Controlling undesired or unexpected behavior Theme 2: Multifluid Geochemistry --- Geochemistry at the fluid-fluid interface Reservoir dynamics of Bravo Dome natural CO 2 reservoir Reactions of CO 2 with clay minerals CO 2 Solubility: Competition for Water 2 Phil Bennett: CO 2 solubility, precipitation/dissolution experiments, wettability Randy Cygan: molecular dynamics simulations Kim Gilbert: CO 2 solubility Senior PersonnelStudents and Post-Docs

3 Characterize the interactions between injected CO 2 and reservoir brine and develop a mechanistic understanding of CO 2 dissolution. Activity Objectives Storage Efficiency Improve sweep efficiency Enhance capillary (ganglion) trapping Controlling Emergence Prevent unwanted fracturing Control path development Prevent unexpected migration of CO 2 Sustaining Injectivity Control wellbore failure Enhance permeabilty/avoid precipitation during injection Guide injection limits CHALLENGES Predict solubility trapping Predict mineral trapping

Executive Summary (for everyone) Molecules are like people, they compete for resources and steal them when they are under high pressure or have lots of energy.

1.What is the CO 2 solubility (C CO2 ) in brines containing NaCl, CaCl 2, NaHCO 3, and Na 2 SO 4 at high T and CO 2 pressure (PCO 2 )? 2.Do different ions with same ionic strengths result in different C CO2, and if so, what are the mechanisms of those differences? Questions

CO 2 fugacity versus PCO 2 generated from Duan, et al Fugacity ~ Effective PCO 2

First I took some measurements…

Stainless steel reactor Brine CO 2 Vacuum pump Sample Cylinder Pressure transducer Evacuated expansion chamber Calculate CO 2 moles with n=RT/PV

…resulting in a total of 131 CO 2 solubility measurements. Water, NaCl, CaCl 2, Na 2 SO 4 and NaHCO 3. T = °C Up to ~24 MPa CO 2 fugacity Water and NaCl measurements taken by Tongwei Zhang and Katherine Romanak

NaCl Brines Increases with fugacity Decreases with temperature Decreases with salts

How do we quantify the salt effect?

We Typically Use Ionic Strength. I=1 I=3

Ionic Strength is a Poor Predictor of CO 2 Solubility When Salt is Changed. Na 2 SO 4 has 15% less dissolved CO 2 than CaCl 2 60°C

CO 2 Solubility vs. Water Activity 60°C and a fugacity of 6.7 MPa R 2 = 0.74

We need another method to quantify the effects of salts. We look at the fundamental properties of dissolution.

CO 2 Needs Lots of Water. Water doesn’t like non-polar molecules, like CO 2 so it forms a cage around them.

δ-δ- δ+δ+ δ+δ+ Na+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ Cl- δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ Spiro, et al, 1968; Millero, et al, 1969; Millero, et al Ions Need Less But Hold it Tightly.

There are 55.5 moles water/kgw Each ion electostricts h i water molecules (hydration #) h a = Σc i n i h i  Electrostricted by ions water (mol/kgw) Available for CO 2 : FW = 55.5 – h a (mol/kgw) For 0.5 m CaCl 2 C i = 0.5 mol/kgw n Ca = 1, n Cl = 2 h Ca = 8.9, h Cl = 2.7 h a = 7.15 mol/kgw Accounting of Water Molecules For h a =7.15 mol/kgw, then FW = mol/kgw.

R 2 = 0.96 CO 2 Solubility vs. Electrostricted Water T = 60°C CO 2 fugacity = 6.7 MPa

New way of thinking CO 2 solubility. CO 2 and ions compete for water molecules.

Na Cl Na+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ Cl- δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ δ-δ- δ+δ+ δ+δ+ Taking a water molecule from a hydrated ion is the reverse reaction and requires energy. Hydration Thermodynamics ΔH hydr ΔS hydr

R 2 = 0.92 CO 2 Solubility vs. ΔH hydr T = 60°C CO 2 fugacity = 6.7 MPa R 2 = 0.95 ΔS hydr (J/kgw°C) ΔH hydr (kJ/kgw)

Systems with Less FW Dissolve Less CO 2. Higher energy barriers to removing water from ions decrease CO 2 solubility at 6.7 MPa and 60°C. Is the evidence still there when we look at all the data?

Ions Ion Concentration (mol/kgw) T (°C) PCO 2 (MPa) # dataReference Na +, Ca 2+, Cl -, SO & Mixed (Gilbert et al., 2015)Gilbert et al., 2015 Na +, Ca 2+, K +, Cl & Mixed (Liu, Hou et al. 2011) Na +, Cl (Rumpf, Nicolaisen et al. 1994) Na +, Cl (Nighswander, Kalogerakis et al. 1989) Na +, SO (Bermejo, Martin et al. 2005) Na +, Cl (Ellis and Golding 1963) Na +, K +, Ca 2+, Mg 2+, Cl & Mixed (Tong, Trusler et al. 2013) Na +, Cl (Yan, Huang et al. 2011) Collated >500 Data Points

Use statistical methods to determine: If variables are significant (p<0.05) Coefficients for each variable What are the interactions between the variables As a 1 st step: C CO2 = f {temperature (T), fugacity (f), T 2, f 2, T*f} Moderated Multiple Regression

Predicting CO 2 Solubility Using Only f and T

Reduced variables by combining: ΔH hydr - TΔS hydr = ΔG hdyr (kJ/mol) then, ΔG = ΣC i n i ΔG hydr (kJ/kgw) Now my variables are f, T, h a and G We include squares and interaction terms Complete Model

CO 2 Solubility R 2 = 0.92 Simple and mixed solutions containing Na +, K +, Ca 2+, Mg 2+, Cl -, SO 4 2-, (and perhaps HCO 3 - ) Up to 267°C and 40 MPa PCO 2 (23 MPa fugacity) No new experimental work to determine parameters. Mixed Brines This Model Error Pitzer Error Mt. Simon Brine3.8%-3.6% NaCl, CaCl 2, KCl-1.1%-10.1% Bravo Dome15.8%15.7%

1.Developed new equation to quantify amount of electrostricted water (h a ). 2.Developed a predictive model for CO 2 solubility using fundamental properties of molecular interactions. No extra experimental work required Predicts CO 2 solubility for 6 ions in any combination (mixed brines) Provides insight into molecular interactions Conclusions

31 Developed new equation to describe the concentration of electrostricted water. Generated a model to predict CO 2 solubility based on a new mechanistic understanding of CO 2 dissolution. Activity Scientific Findings Storage Efficiency Improve sweep efficiency Enhance capillary (ganglion) trapping Controlling Emergence Prevent unwanted fracturing Control path development Prevent unexpected migration of CO 2 Sustaining Injectivity Control wellbore failure Enhance permeabilty/avoid precipitation during injection Guide injection limits CHALLENGES Predict solubility trapping Predict mineral trapping