Experimental multi-phase H2O-CO2 brine interactions at elevated temperature and pressure: Implications for CO2 sequestration in deep-saline aquifers

Introduction The burning of fossil fuel and other anthropogenic activities have caused a continuous and dramatic 30% increase of atmospheric carbon dioxide (CO2) over the past 150 years. CO2 sequestration is increasingly being viewed as a tool for managing these anthropogenic CO2 emissions to the atmosphere. The disposal of this excess CO2 into deep-saline aquifers is one of several potential storage repositories, but the details of the geochemical reactions between supercritical CO2 and potential host fluids and formation rocks are largely unknown. The initial reaction between liquid CO2 and the aquifer fluid is the dissolution of CO2 (eqn. 1) and is fundamentally important because it is the aqueous, not the supercritical form of CO2 that is reactive toward the formation rocks. CO2 +H 2O↔ H 2CO3 (1) The aqueous solubility of CO2 is temperature, pressure, and ionicstrength dependent. At 25°C, the solubility of CO2 in an aquifer fluid with total dissolved solids of ~22% is approximately threefold less than in pure water. The dissociation of carbonic acid into reactive hydrogen ion and bicarbonate (eqn. 2) potentially initiates a complex series of reactions with aquifer fluids and formation rocks to fix CO2 in mineral phases. H 2CO3↔ H + +HCO3 − (2) The dissociation of carbonic acid is also temperature dependent. There is a maximum in the log K of reaction 2 at about 50°C beyond which log K decreases continuously with increasing temperature such that a weak acid becomes increasingly weak at elevated temperature. Reactions involving supercritical CO2 and carbonic acid with aquifer fluids and formation rocks are many and varied, depending on the matrix of the fluid and the composition of the rock. In general, thermodynamics favor the dissolution of carbonate phases in limestones and dissolution of silicates and precipitation of carbonates in arkosic sandstones. Reactions in limestone (ionic trapping). Reactions of CO2 saturated aquifer fluids with limestone are characterized by dissolution of calcite due to the increased acidity produced by the dissociation of carbonic acid (eqn. 3) CaCO3 +CO2 +H2O→Ca +2 + 2HCO3 − (3) for a net increase of an additional mole of CO2 stored as bicarbonate relative to the simple solubility of CO2. Calcite has a retrograde solubility, becoming less soluble and less efficient at trapping CO2 at elevated temperature. Similar reactions can be written for the dissolution of dolomite and siderite. Reactions in arkosic sandstones (mineral trapping). Reactions of CO2 saturated aquifer fluids with arkosic sandstones are characterized by dissolution of silicates due to the increased acidity produced by the dissociation of carbonic acid and precipitation of carbonates. An example of the mineral trapping of CO2 is the dissolution of the anorthitic component of palgioclase (eqn. 4) 2H +CaAl2Si2O8 +H2O→Ca +2 +Al2Si2O5 (OH)4 (4) and the subsequent precipitation of calcite (eqn. 5) Ca +HCO3 − →CaCO3 +H + (5) for a net reaction shown in equation 6. CaAl2Si2O8 +H 2CO3 +H 2O→CaCO3 +Al2Si2O5 (OH )4 (6) The log K of this reaction decreases with increasing temperature, resulting in competing effects of favorable thermodynamics versus kinetic limitations. We present here the results of CO2-saturated brine-rock experiments carried out to evaluate the effects of multiphase H2OCO2 fluids on mineral equilibria and the potential for CO2 sequestration in mineral phases within deep-saline aquifers