Pore-scale Analysis of Residual Oil in Sandstones and its Dependence on Waterflood Salinity, Analysed by Tomography and Microscopy

Abstract

Understanding of the pore scale phenomena in porous media is a crucial step towards designing a more efficient enhanced oil recovery techniques in large scale oil reservoirs. Three dimensional (3D) x-ray micro CT imaging and recent advanced in 3D image analysis are primary tools that provide unprecedented level of detailed pore scale information of complex rock structures. Combining this technology with laboratory EOR experiments provides valuable insights into the mechanisms behind the oil recovery process. Low salinity water flooding is a relatively convenient and efficient tertiary recovery method which is applicable to the most of the reservoir types. However, its performance prediction is uncertain because this technique is not fully understood. Fundamental understanding of underlying mechanisms of low salinity water flooding, at the pore-scale, by direct visualization and image analysis is the aim of this thesis. In this thesis a technique to quantify fluids and rocks using series of images is developed and applied to 3D images of mini-plugs that underwent Spontaneous imbibition of high and low salinity brine or flooded by high or low salinity brine. Further, oil/rock and oil//brine interfacial areas, oil/brine interfacial mean curvature and oil saturation configuration in each pore was determined for each mini-plug. The image analysis and data interpretation demonstrate that small incremental oil recovery by low salinity brine corresponded to a slight shift towards water-wet in clay-rich outcrop sandstones. Further, the influence of oil composition in the low salinity brine flooding of reservoir sandstones was investigated. Two crude oil which are mainly distinctive in their total acid number (TAN) were used in spontaneous imbibition experiments. All mini-plugs shows strongly oil wet state after spontaneous imbibition of high and low salinity brine. The low salinity effect was observed in mini plug with high TAN oil while the mini-plugs with low TAN oil exhibited much less tertiary recovery. The analysis of mini-plug with high TAN oil shows that the salinity-induced wettability shift was sufficient to displace oil from locations that were already more water-wet in the state after spontaneous imbibition of high salinity brine, but was insufficient to cause oil movement from more oil-wet locations. Pore scale study of core flooded mini-plug shows that the low salinity brine redistributed oil blobs by displacing them from smaller to the larger pores and disconnecting oil. Microscopy studies of mini-plugs after spontaneous imbibition or core flooding provided further insight to the low salinity oil recovery mechanism at local minerals or wettability of pore walls.