The mechanisms of low salinity recovery were investigated by micro-CT imaging of the pore-scale distribution of residual crude oil in clay-rich reservoir carbonates after secondary recovery by high salinity flooding and tertiary recovery by low salinity flooding. A workflow is presented for preparing small mini-plugs (5 mm diameter, 19 mm long) in a holder/flow cell and performing restoration, flooding, imaging and image processing and quantitative analysis. In tomograms of one such sandstone mini-plug after high salinity flooding, the half closest to the inlet was well swept with around 20% residual oil, as disconnected blobs contacting pore walls. The half closest to the outlet had much greater residual due to a substantial capillary end effect, reinforced by the close proximity of the mini-plug walls. Low salinity flooding did not change the oil saturation in the inlet half, while that in the outlet half was greatly reduced by lessening of the end effect. Analysis of interfacial areas in the tomograms showed that oil selectively detached from clay in the inlet half, but remained too disconnected for additional recovery. Oil detached equally from clay and grain in the outlet half, where oil-brine interfacial tension lowering was probably responsible for the artificial extra recovery.


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