Mapping Residual Phase Saturations in Carbonate Core Material at Multiple Scales
Mark Knackstedt, Tim Senden, and Anna Carnerup
Digitalcore Pty Ltd & Applied Maths, Australian National University, Canberra, Australia
The prediction of multiphase flow properties of carbonate core material requires understanding the heterogeneity and wettability of pore systems comprising scales from centimeters down to tens of nanometers. In this paper we describe multiscale experimental studies of flooding in carbonate samples at scales from plug scales down to tens of nanometers. Imaging allows one to determine the amount of residual phase and knowledge of its microscopic distribution within the rock pore space allows a better understanding of recovery mechanisms. This in turn informs the design and implementation of improved or enhanced recovery processes. While the importance of the pore scale structure, mineralogy and wettability in dictating the residual phase distribution is widely acknowledged, little quantitative information on these properties and dependencies has been directly available. This study bridging the core, plug and pore scales using x-ray (micro)-tomographic imaging, petrographical (SEM and SEM-EDS) imaging and wettability mapping leads to a better understanding of pore scale mechanisms and recovery processes. The experimental techniques used are reviewed, emphasizing the registration technology which enables spatial alignment and integration of 2D SEM-based information with 3D micro-CT images. Application of these techniques to visualization of pore scale oil and brine populations is presented, with a particular focus on characterizing native state carbonate plug material.
A set of native state reservoir carbonate plugs (1 inch diameter), were imaged using the micro-CT facility from which a 3D tomographic image was obtained. To improve oil differentiation in CT, the residual oil was then exposed to a reactive species which attenuates the hydrocarbon, improving overall x-ray attenuation of this phase alone. The plug was then reimaged, and image registration was subsequently performed to directly visualize the attenuation difference in these two conditions. For higher resolution imaging, smaller subplugs (diameter 3-6 mm) were taken from the native state plug and subjected to the same preparation and imaging protocol mentioned above. Following this, the residual oil phase was removed by soxhlet extraction followed by treatment in methanol, after which the subplug was dried in air. The cleaned subplug was reimaged and the resulting 3D tomographic image was registered to the attenuated condition, and a difference map was produced from the density variations in the samples; this gives an accurate mapping of the oil saturation in situ. For even higher resolution, a polished section was prepared from within the CT scanned volume of the cleaned subplugs, and imaged with field emission SEM. The 2D images were registered to their corresponding slice in the subplug tomogram.
AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012