SECURE, LONG-TERM GEOLOGICAL CARBON SEQUESTRATION IN MAFIC ROCKS: RESULTS FROM FIELD AND LABORATORY EXPERIMENTS

For long-term geological CO₂ sequestration, reservoirs (e.g. deep confined aquifers) with a large potential storage capacity as well as solid end products, such as (Ca, Mg)CO₃ are desired. The conversion of CO₂ into Ca-Mg-CO₃ is favorable because of their chemical stability, non-toxic nature and lack of fluidity for rapid migration. When high pressure CO₂ is injected in deep aquifers, it will acidify the groundwater. If the aquifer is associated with mafic igneous rocks, this acid may be neutralized by CO₂-water-rock interaction. The reactions may be represented by: Mg₂SiO₄ + 2CO₂(g) = 2MgCO₃ + SiO₂ and CaAl₂Si₂O₈ + CO₂(g) + H₂O + 2SiO₂ = CaCO₃ + Al₂Si₄O₁₀(OH)₂. Under high CO₂ pressure these reactions may be driven to the right to form carbonates. We tested the feasibility of this concept with field and laboratory experiments. In order to estimate the sequestration capacity of an aquifer system, hydraulic parameters such as effective porosity and permeability are required. In experimental wells drilled into the 300 m thick Palisades Diabase and the underlying Triassic sediments of the Newark Basin Series, several highly fractured layers were located within the diabase as well as in the sediments. The fracture distribution was determined from digital televiewer logs, the transmissive fracture zones were identified by electromagnetic flowmeter measurements and the transmissivity of the fracture zones were estimated from straddle-packer pump test data. Preliminary results of small scale CO₂ injection tests into the fractured zones combined with monitoring of chemical fluid parameters will be presented. Additionally laboratory experiments were performed to investigate the dissolution rates of diabase samples in acidified aqueous solutions for a pH range from 1 to 4 at temperatures between 20 and 70°C. The dissolution rates were measured by monitoring the concentration of Ca²⁺ and Mg²⁺ in the fluid over several weeks. The experiments show that the dissolution rates increase proportionally with decreasing pH, whereas the temperature effect shows only a minor effect. The dissolution rates were 4x10^-5 mol/cm²/day or 2x10^-2 mol/cm²/yr at pH 3. This suggests that for highly fractured rocks, a sufficient dissolution flux of Ca and Mg may be achieved for the fixation of CO₂ as carbonates in several hundred to thousand years.