NMR studies of supercritical CO 2 in carbon sequestration and immiscible two phase flow in porous media
Prather, Cody Allen
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Nuclear magnetic resonance (NMR) was used to research mechanisms related to two-phase flow in porous media. Experiments were conducted to further understand; 1) the capillary trapping mechanism that occurs during sequestration of CO 2 in deep underground sandstone reservoirs, 2) the viscous fingering phenomena that occurs when scCO 2 convectively dissolves in brine under reservoir conditions, and 3) flow patterns and fluid mechanisms in immiscible two-phase flow in porous media for the two pressure gradient regimes formed under different capillary numbers. Capillary trapping is a prominent mechanism for initially trapping CO 2 in pore structures of deep underground rock formations during the sequestration process. Because of its significant role in securing CO 2 underground, it is important to characterize and understand the residual saturation and distribution of CO 2 within the pore structure. A setup was developed in which drainage and imbibition of a Berea Sandstone core takes place within an NMR spectrometer under reservoir conditions. NMR results provide comparisons between the different nonwetting fluids used and help characterize the capillary trapping of each nonwetting fluid. In conclusion, scCO 2 is trapped 13% less efficiently than air or CO 2, and the nonwetting fluid is preferentially trapped in larger pores. Viscous fingering is a significant long-term trapping mechanism that further increases storage security by enhancing mass transfer through convective dissolution. A setup was developed in which scCO 2 could dissolve into a water saturated bead pack, under reservoir conditions, within the NMR spectrometer. NMR results track spatial changes in T 2 relaxation time and signal intensity. The results are inconclusive and the phenomena could not be directly observed but results do suggest dissolution is occurring during the experiment. Immiscible two-phase flow in porous media is unpredictable and existent in many industries. Therefore, determining flow patterns and understanding the fluid mechanisms from a capillary number/pressure gradient relationship could prove valuable. A setup was developed in which an immiscible two-phase flow through a bead pack was monitored, for different capillary numbers, with NMR techniques. NMR results provide snapshots of the water saturation distribution within the bead pack. The results suggest there's a consistent slug-type flow pattern during the steady state.