Drying Rate Measurements in Convection- and Diffusion-Driven Conditions on a Shaly Sandstone Using Nuclear Magnetic Resonance
Carbon Capture and Storage (CCS) is one of the solutions studied to reduce greenhouse gas accumulation in the atmosphere. Depleted oil and gas reservoirs have been studied for potential storage sites but also saline aquifers that have the advantages of much larger pore volume. In this latter case, injection of large volume of anhydrous carbon dioxide will lead to a strong water desaturation of the near wellbore region because of evaporation mechanisms. Even the capillary trapped water can be removed by thermodynamical transfer of water vapor in the CO2 phase. The extension in time and space of the dry zone will be controlled by the drying rate induced by the gas flow. Consequences of drying may induce alteration of the injectivity by salt precipitation and/or alteration of the rock fabric itself, especially for shaly sandstones in the case of clay drying. The context of CCS has raised new interests in the understanding of drying kinetic where the water vapor is evacuated by gas convection. In this study, we investigated experimentally the drying rate evolution with time on a shaly sandstone sample in two conditions of drying: convective and diffusive. In convective conditions, air is injected at different flow rates through the porous media in conditions of drying representative of a CO2 injection site at one million ton per year. In diffusive conditions, no flow is imposed and the water vapor escape by diffusion. Drying rates dynamics in both conditions were measured by Nuclear Magnetic Resonance (NMR) and compared. We varied the temperature and the salinity in diffusive-driven drying and the gas flow rate in convective-driven drying. The water distribution in the pore network and the water saturation profiles were monitored continuously using T2 relaxation and 1D imaging NMR techniques. For the range of temperature and air flow rate used, we show that drying rates in the two drying conditions are similar but not identical. They both present different periods characteristic of the main mechanisms for water mass transfer. Drying rate has a power law dependence on the temperature, as predicted by thermodynamic, and drying rate was found proportional to the flow rate in convective drying. Presence of salt has a complex effect: an increase of the drying rate at early stage of drying followed by a strong decrease for the remaining time of drying.
KeywordsDrying NMR Sandstone
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