Carbon Dioxide (CO₂) sequestration requires that the injected CO₂ be permanently trapped in the subsurface and not leak from the target location. To accomplish this, it is important to understand the main mechanisms associated with CO₂ flow and transport in the subsurface once CO₂ is injected. In this work CO₂ seeps at the Crystal Geyser site were studied using modeling and simulation to determine how CO₂ geochemically reacts with formation brines and how these interactions impact the migration of CO₂. Furthermore different scenarios for CO₂ migration and seepage along the Grand Wash fault are studied and the possible outcomes for these different scenarios are documented. The GEM (Generalized Equation-of-State Model) from CMG Ltd. was used to perform the simulation studies. A 2-D model was built without geochemical reactions to mainly study the mechanism associated with dissolution of CO₂ gas. The process of CO₂ release from the brine as the fluid mixture flows up along the fault was modeled. Then, 3-D models with geochemical reactions were built for CO₂ migration corresponding to two different sources of CO₂ - deep crustal ₂ and CO₂-dissolved in groundwater. In both these cases, CO₂ reacted with the aqueous components and minerals of the formation and caused carbonate mineralization. In the case of deep crustal CO₂ source, there were vertical patterns of calcite mineralization simulated along the fault that indicated that calcite mineralization might be localized to isolated vertical flow paths due to vertical channeling of CO₂ from the crust. In the case of CO₂-dissolved groundwater flowing along the sandstone layers, calcite mineralization is spread over the entire fault surface. In this case, the groundwater flow is interrupted by the fault and there is vertical flow along the fault until a permeable sandstone layer is encountered on the other side of the fault. This vertical migration of CO₂-saturated brine causes a release in pressure and subsequent ex-solution of CO₂. As a result, modeling allowed us to establish difference in surface expression of CO₂ leakage due to two different CO₂ migrations scenarios along the fault and helped develop a scheme for selecting appropriate model for CO₂ leakage based on surface observation of travertine mounds. A key observation at the Crystal Geyser site is the lateral migration of CO₂ seep sites over time. These migrations have been confirmed by isotope studies. In this modeling study, the mechanism for migration of seep sites was studied. A model for permeability reduction due to precipitation of calcite was developed. It is shown using percolation calculations that flow re-routing due to permeability alterations can result in lateral migration of CO₂ seeps at rates comparable to those established by isotope dating. / text

The Little Grand Wash normal fault near Green River, eastern Utah, hosts a series of naturally occurring CO₂ seeps in the form of active and extinct CO₂-charged springs distributed along the fault zone. I have studied the association of fault structure with CO₂-related alteration as an analogue for the long-term (1,000- to 10,000-year) effects of leakage through faults in CO₂ sequestration reservoirs. Structure and alteration in a portion of the Little Grand Wash fault zone were mapped at a 1:700 scale in order to determine the association of faulting with CO₂-related diagenesis. I combined structural and diagenetic mapping were combined with laboratory analyses of mineralogical, isotopic and textural changes in order to assess controls on the migration of CO₂ traveling up the fault and its effects on the fault itself. The fault zone is 200 m wide at its widest and contains 4-5 major subparallel fault segments that form multiple soft- and hard-linked relay ramps. The area includes a travertine deposit and related sandstone alteration: outcrop-visible coloration, porosity-occluding calcite cement and veins occasionally so abundant that they obliterate the rock fabric. Structural mapping shows that the travertine is located at an intersection of major fault segments constituting the hard link of a 450-meter-long relay ramp. Sandstone alteration is confirmed to be related to the CO₂ seep by mapping its distribution, which shows a decrease in concentration away from the travertine, and by the unique isotopic signature of calcite cement near the travertine. At distances greater than 25 m from the travertine intense alteration disappears, though scattered fault-subparallel veins and patchy, burial-related calcite cement remain. Intense alteration is limited to major fault overlaps and does not permeate the fault zone along its entire length, nor does it extend outside the zone. This indicates that rising CO₂-laden fluids do not flow uniformly through the entire fault zone, but that vertical flow is channeled at fault intersections. In thin section, porosity near the travertine has been extensively or completely occluded by calcite cement. Permeability in some conduit samples is less than 1 mD, three or four orders of magnitude lower than sandstone away from the travertine. In active CO₂ conduits, such reduction in porosity and permeability would occlude the preferred flow conduit and ultimately restrict upward flow of CO₂-charged water. X-ray diffraction detects small amounts of goethite and hematite and a decrease in chlorite-smectite in altered conduit sandstones. Calcite is abundant, but many authigenic minerals predicted by geochemical models of CO₂ influx into sandstone reservoirs are not observed, including kaolinite, aragonite, dolomite, siderite, ankerite or dawsonite. This difference between observed and predicted mineral occurrence likely results from differences in mineral kinetics between natural and laboratory systems. Prediction of leakage risk based on fault geometry improves the ability to assess the suitability of potential carbon sequestration reservoirs, many of which will be faulted. The point seep nature of leakage through a fault zone limits the amount of CO₂ that can escape over time and also enables targeted surface monitoring for CO₂ escape into the atmosphere--both critical for ensuring the effectiveness of injection projects and earning the trust necessary for carbon sequestration to gain public acceptance. The point seep nature of leakage also accelerates the rate at which conduits may seal through mineralization, since precipitation from a large volume of fluid is focused in a narrow conduit. The presence of multiple fossil and active seep locations along the Little Grand Wash fault, active at different times in the geologic past, indicates that cementation may be effective in sealing single conduits but that fault systems with complex geometry such as Little Grand Wash may continue to leak for a long period of time. / text