3D Imaging of Residual Saturation in Carbonates; Exploring the Role of Wettability, Rate and Saturation State

Abstract

At the conclusion of flooding in an oil- or gas-bearing carbonate reservoir, a significant fraction of the original hydrocarbon in place remains in the swept region as trapped residual phase. In addition to the amount of trapped phase, its microscopic distribution within the pore space of a reservoir rock is important to gain a better understanding of recovery mechanisms and for the design and implementation of improved or enhanced recovery processes. Despite the importance of the pore scale structure and distribution of residual oil, little quantitative information is currently available. In this study the residual saturation is directly visualized in core material at the pore scale in three dimensions. In particular, we utilize a new technique for imaging the pore-scale distribution of fluids in reservoir cores in 3D; the method allows the same reservoir core material to be imaged under different wettability conditions, saturation states and flooding rates. A range of examples are given for waterflooding of reservoir carbonates. We observe a strong dependence of the residual hydrocarbon saturation and distribution on rate and wettability. The detailed structure of the residual trapped phase is described. This information is correlated to pore structural information from the 3D image data (pore geometry, connectivity), mineralogy and rock type as well as to wettability and flow conditions. These results provide an important platform for the testing, correlation and calibration of pore scale rock typing to multiphase flow properties. This detailed pore scale information of the residual oil saturation is crucial to the design and implementation of improved recovery processes and can be related to conditions required for mobilization of residual oil. Oil recovery mechanisms are directly tested and the differences in the habitat of the residual fluids under different conditions are directly quantified. The role of wettability is particularly studied. Crude oil drainage of simpler analogue materials are considered where flat mineral substrate have been incorporated. After aging and cleaning the planar slabs are removed and analyzed by surface sensitive techniques, in particular interferometric profilometry, to characterize the distribution of oil-wet and water-wet sub-regions. The results give some insight into the wettability conditions associated with waterflooding.