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Formation Damage due to CO₂ Sequestration in Saline Aquifers
Carbon dioxide (CO₂) sequestration is defined as the removal of gas that would be emitted into the atmosphere and its subsequent storage in a safe, sound place. CO₂ sequestration in underground formations is currently being considered to reduce the amount of CO₂ emitted into the atmosphere. However, a better understanding of the chemical and physical interactions between CO₂, water, and formation rock is necessary before sequestration. These interactions can be evaluated by the change in mineral content in the water before and after injection, or from the change in well injectivity during CO₂ injection. It may affect the permeability positively due to rock dissolution, or negatively due to precipitation. Several physical and chemical processes cover the CO₂ injection operations; multiphase flow in porous media is represented by the flow of the brine and CO₂, solute transportation is represented by CO₂ dissolution in the brine forming weak carbonic acid, dissolution-deposition kinetics can be seen in the rock dissolution by the carbonic acid and the deposition of the reaction products, hydrodynamic instabilities due to displacement of less viscous brine with more viscous CO₂ (viscous fingering), capillary effects and upward movement of CO₂ due to gravity effect. The objective of the proposed work is to correlate the formation damage to the other variables, i.e. pressure, temperature, formation rock type, rock porosity, water composition, sulfates concentration in the water, CO₂ volume injected, water volume injected, CO₂ to water volumetric ratio, CO₂ injection rate, and water injection rate. In order to achieve the proposed objective, lab experiments will be conducted on different rock types (carbonates, limestone and dolomite, and sandstone) under pressure and temperature that simulate the field conditions. CO₂ will be used at the supercritical phase and different CO₂-water-rock chemical interactions will be addressed. Quantitative analysis of the experimental results using a geochemical simulator (CMG-GEM) will also be performed. The results showed that for carbonate cores, maintaining the CO₂/brine volumetric ratio above 1.0 reduced bicarbonate formation in the formation brine and helped in minimizing precipitation of calcium carbonate. Additionally, increasing cycle volume in WAG injection reduced the damage introduced to the core. Sulfate precipitation during CO₂ sequestration was primarily controlled by temperature. For formation brine with high total dissolved solids (TDS), calcium sulfate precipitation occurs, even at a low sulfate concentration. For dolomite rock, temperature, injection flow rate, and injection scheme don't have a clear impact on the core permeability, the main factor that affects the change in core permeability is the initial core permeability. Sandstone cores showed significant damage; between 35% and 55% loss in core permeability was observed after CO₂ injection. For shorter WAG injection the damage was higher; decreasing the brine volume injected per cycle, decreased the damage. At higher temperatures, 200 and 250°F, more damage was noted than at 70°F. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/148153
Doctor of Philosophy Texas A & M University 2012 http://hdl.handle.net/1969.1/148153