| Greenhouse gases emission reduction and energy demand are the two significant issues for the world economy development. Thus, research on the CO2 geological sequestration and enhancing oil recovery is of great theoretical and realistic significance.In this study, quantitative analysis had been realized using MRI technique to obtain the pore structure distribution images of sand pack, artificial core and natural core, respectively. The measurement results of average core porosity based on MRI method agreed well with The measurement results based on the conventional material balance method. The porosity distributions and heterogeneity of those cores were observed clearly in the level of "pore group". The observed porosities at different section were analyzed and characterized quantitatively with the statistics method.It can quantitatively analyze the saturation of oil and water using chemical shift selective spin echo imaging sequence if the peaks of oil and water in porous media can be obviously distinguished.The water was imbibed through small pores and low permeability layer, at the same time, the oil was expelled through large pores and high permeability layer due to the effect of capillary forces during the spontaneous counter-current imbibition process in the water-wet natural reservoir sandstone core. When the imbibition process approaches the equilibrium, the residual oil is mainly trapped in large pores, while the recovery is higher in the small pores area than in the large pore area.Two CO2 migration processes in porous media saturated with water were visualized using the MRI technique. CO2 injection from the bottom caused obvious CO2 channeling along some high permeability parts of the core caused by buoyancy. Once the channeling formed, the CO2 followed along the channels and have no impact on the residual water. When the CO2 was injected from the upper side, the phenomenon of CO2 channeling weakened, but the CO2 front profile was irregular. The velocity of the CO2 front was uniform and the distribution curve of the water saturation in the FOV was regular.During CO2 flooding process, the phenomenon, such as onset of viscous fingering, gravity override, CO2 channelling, and the formation, progress and migration of the piston-like miscible regions and CO2 front can be visually detected using MRI technique.In addition, The measurement results of oil saturation based on MRI method were agreed well with the results based on conventional material balance method, the error rate for the MRI method was less than 10%, Through the analysis of the oil saturation profile, the velocity of the CO2 front has been evaluated. A special core analysis method was applied to in situ oil saturation data to directly evaluate the local Darcy phase velocity of the fluids, and the effect of viscosity, buoyancy, and capillary pressure on CO2 miscible displacement.CO2 miscible displacement enhance oil recovery evidently more than CO2 immiscible displacement. The irreducible water can scarcely change the oil saturation curves. Recovery rate can be determined by wettability, porosity, structure of the sand pack and the homogeneousness of the pore distribution. For the slightly oil-wet homogeneous artificial core plug, CO2 miscible displacement can enhance oil recovery after the water flooding. The heterogeneous of the reservoir and the vertically oriented fracture have a great influence on the oil recovery seriously in three stacked core plugs. Lower permeability causes lower recovery rate. When the displacement finishes, the residual oil spatial distribution is homogeneous in every piece of core. |
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