Pore-Scale Imaging of Oil and Wettability in Native-State, Mixed-Wet Reservoir Carbonates
Authors: Dodd Nicole , Marathe Rohini , Middleton Jill , Fogden Andrew , Carnerup Anna , Knackstedt Mark , Mogensen Kristian , Marquez Xiomara , Frank Soren , Bounoua Noureddine , Noman Rashed 
Published: March 2015

Abstract
3D pore-scale imaging and analysis provides an understanding of microscopic displacement processes and potentially a new set of predictive modeling tools for estimating multiphase flow properties of core material. Reconciliation and integration of the data derived from these models requires accurate characterization of the pore-scale distribution of fluids and a more detailed understanding of the role of wettability in oil recovery.
The current study reports experimental imaging progress in these endeavors for a preserved-state carbonate core from a Middle Eastern waterflooded reservoir. Micro-CT methods were used in combination with novel fluid X-ray contrasting techniques and image registration to visualize the 3D pore-scale distribution of residual oil in mini-plugs. Segmentation of the registered tomograms and their differences facilitated estimation of the residual oil saturation. These predictions from digital analysis agreed reasonably well with laboratory measurements of oil saturation from extraction of sister mini-plugs and spectrophotometry. The tomogram segmentations provide additional information beyond this average value, such as the fractions of oil associated with macroporosity and microporosity.
After the tomogram acquisitions, one of the dried mini-plugs was cut and SEM imaged at this exposed face to provide 2D images of fine features below the micro-CT resolution limit, such as the characteristic dimpled texture of asphaltene films on calcite surfaces due to their local wettability alteration in the reservoir. A new registration procedure was developed to embed the SEM images from the cut plug into the tomogram of the original uncut plug at their correct locations, so that this high-resolution wettability information could be integrated into the 3D pore network description and correlated to the local distribution of residual oil.

Introduction
Although waterflooding has been used for decades to recover oil, the recovery mechanisms at the pore- and molecular-scales remain uncertain. Two poorly characterized and interrelated factors are thought to control multiphase flow in rocks. One is the pore-scale configuration of the fluids and their interfaces within the complex geometry and topology of the pore space. The other is the wettability, which is dictated by the crude oil/brine/rock interactions resulting from the molecular-scale surface chemistry (Salathiel 1973; Anderson 1986; Morrow 1990).
Developments in 3D imaging by X-ray computed micro-tomography (micro-CT) and image processing and analysis have been harnessed to visualize and quantify pore networks in rocks (Sheppard et al. 2005), and more recently to image their partial occupancy by two immiscible fluids. Wet-state imaging typically necessitates the doping of one of the two fluids to accentuate its X-ray attenuation, e.g., by dosing the oil with a heavy halogenated analog or the brine with a heavy salt. Such approaches have been used to map the pore-scale saturation in rocks prepared in a sequence of states (Seright et al. 2006), for which tomogram registration algorithms (Latham et al. 2008) are invaluable to monitor changes in individual pores (Kumar et al. 2009). Recent applications to flow (Blunt et al. 2012) and flooding (Wildenschild and Sheppard, 2013) have led to enhanced insight into pore-scale mechanisms and offer the opportunity to explain trends and uncertainties in laboratory measurements and to calibrate computational models of recovery.