Physical Chemistry of CO2 (Reid Grigg) Aqueous Carbonate Chemistry Mineral Dissolution and Precipitation

AC is the boiling point curve. AB defines the freezing point. A is called the Triple Point C is the critical point content/uploads/2007/10/water-phase- diagram.jpg

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What do all these have in common? Ocean Acidification Long term storage by mineralization Solubility storage Degradation of USDW

Dissociation of CO 2 in Water CO 2 + H 2 O = H 2 CO 3 H 2 CO 3 = H + + HCO 3 - HCO 3 - = H + + CO 3 -2 Le Chatelier's Principle: When a system at equilibrium is disturbed, the equilibrium position will shift in the direction which tends to minimize, or counteract, the effect of the disturbance.

Speciation of aqueous CO2 From: Geochemistry: pathways and processes: by Richardson and McSween

So as Temperature goes up K CO2 becomes more smaller aH2CO3 = PCO2/10 exp (1.47)

(aH + ) 2 = K1 K CO2 P CO2

So we see that increasing P CO2 will make an aqueous fluid more acidic. It has been proposed that we monitor the reservoir fluid, or an overlying groundwater for pH changes to detect CO2 leakage

The pH of the ocean can be to a first degree controlled By Equilibrium between water, calcite, and the Atm CO2

16 The pH of pure water in equilibrium with calcite at 25°C as a function of the partial pressure of CO 2. Note that pH decreases linearly with increasing CO 2 partial pressure.

Changes to Ocean pH as a function of PCO2

So we see that increasing P CO2 will make an aqueous fluid more acidic. It has been proposed that we monitor the reservoir fluid, or an overlying groundwater for pH changes to detect CO2 leakage. So lets look at some examples.

EPNG Com A 300 Howell D 351 Howell D 350 S EPNG Com A 300S Howell D 353 Howell A 300 Howell A 301S FC State Com1 Howell D 352 S Howell G 300 Injection San Juan Basin By Y. Li (PRRC)

pH Variation Injection

Bicarbonate (HCO 3 - ) Injection

We all know that acid added to calcite, causes the calcite to dissolve. So what happens when CO2 is added to fluid in a limestone or in a sandstone with calcite cement.

SACROC Study Objective 2 Can injectate CO 2 be detected in SACROC groundwater? Can injectate CO 2 be detected in SACROC groundwater? DIC, pH, HCO 3 -, δ 13 C and Ca 2+ H 2 O + CaCO 3 + CO 2 = 2HCO Ca 2+ From Romanak

Schematic of Study Area, SACROCK From K.Romanak, Texas Bureau of Economic Geology

Carbonate vs Silicate Geochemistry Quartz (60-80%) > K-feldspar (10-20%) > albite > dolomite > calcite smectite and illite clays coating grains

Carbonate Geochemistry Mixing creates a complex carbonate system. Dedolomitization, not calcite dissolution. Ca 2+ drives the carbonate system – not CO 2 gas! Increasing CO 2. What is the source? High CO 2 pressure Calcite dissolution trend Low CO 2 pressure Ca (mmoles) HCO3 (mmoles)

Dedolomitization Ca 2+ + CaMg(CO 3 ) 2 ↔ 2CaCO 3 + Mg Natural: mixing with Permian CaSO 4 waters 2.Land use: Infiltration of NaCl brines with cation exchange Ca/Mg > 1

EPNG COM A 300 Injection FC State Com#1

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Temporal Trends TWDB & BEG Data

pH Along Gradient- parallel Transects A A’ B’ C’ B C C B A SAROC

Aquifer and USDW Atmosphere Biosphere Vadose zone Seal Subsurface Monitoring Zone CO 2 plume Near-Surface Monitoring Zone Shallow groundwater Track CO 2 /brine migration Early detection, small signals, low background variability, Moderate background variability, assurance of no damage in USDW High background variability and dynamic, many challenges Monitoring at Carbon Sequestration Sites