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Study of trapping mechanisms of supercritical carbon dioxide in deep heterogeneous geological formations through intermediate-scale testing and modeling
Trevisan, Luca
Trevisan, Luca
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2015
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Abstract
Carbon Capture and Storage (CCS) represents a technology aimed to reduce atmospheric release of CO2 from power plants and heavy industries by injecting it into deep geological reservoirs, such as saline aquifers. The storage process relies on the effectiveness of safely retaining the injected supercritical fluid in deep underground formations over the long term through a number of trapping mechanisms that include structural, residual, dissolution, and mineral trapping. Improved fundamental understanding of these processes is expected to contribute toward better conceptual models, improved numerical models, more accurate assessment of storage capacities, and optimized placement strategies. It is known from reported studies in literature that the natural heterogeneity of typical sedimentary formations has significant effect on the propagation of the CO2 plume, eventually accumulating mobile phase underneath the caprock. From core flooding experiments in reservoir engineering, it is also known that contrasts in capillary threshold pressure due to different pore size can affect the flow paths of the invading non-wetting fluid. This consequently influences the buildup of non-wetting phase (NWP) at interfaces between geological facies. However, since fully characterizing the geologic variability at all relevant scales and making observations on the spatial and temporal distribution of the migration and trapping of supercritical CO2 is not practical, studying these trapping processes at a fundamental level is not feasible in field settings. To provide insight into the impact of heterogeneous systems on the flow dynamics and trapping efficiency of supercritical CO2 under drainage and imbibition conditions, an experimental investigation is conducted at the meter scale in synthetic sand reservoirs packed in quasi-two-dimensional flow-cells. Through an experimental approach that mimics the interplay of governing forces at deep reservoir conditions via application of surrogate fluids, a series of immiscible displacement experiments is performed to observe the preferential entrapment of NWP in well-defined heterogeneous porous media. Each experiment consisted of a drainage event simulating the injection of supercritical CO2 into a brine-wetted reservoir, followed by a gravity relaxation period characterized by spontaneous imbibition at the trailing edge of the plume. Spatial and temporal variations of NWP saturation represent important features to evaluate capillary trapping; for this reason x-ray attenuation analysis is used as a non-destructive technique that allows a precise measurement of phase saturations throughout the entire flow domain. Analysis of the experimental results complemented by numerical simulations suggests that intermediate-scale experiments with analog fluids can provide quantitative information about large scale phenomena and therefore can be used to predict storage performance of natural reservoirs under a range of boundary conditions.
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