Micro-CT imaging and analysis of reservoir samples for EOR applications
Andrew Fogden and Rudolf Held
Over the past decade, the use of laboratory micro-CT in geoscience and hydrocarbon extraction applications has progressed from scanning of dry rock samples under ambient conditions to in-situ imaging of multiphase flow processes under realistic reservoir conditions. This opens the way to study and better understand complex recovery processes.
This contribution summarizes case studies in which small rock subsamples were scanned through a sequence of laboratory-prepared states to provide pore-scale insight into EOR recovery mechanisms and estimation of flow responses. The 3D images were post-processed using registration and segmentation to quantitatively identify the distribution of fluid phases, on which flow properties were directly calculated for comparison to, or anchoring of, predictions from multiphase flow simulations. A first case study deals with a restored reservoir sandstone which displayed a large incremental waterflooding recovery using low salinity brine. Micro-CT image analysis integrated with SEM-BSE-EDS 2D imaging of a polished section showed that most of the tertiary recovery locally occurred from quartz grain surfaces as opposed to kaolinite aggregates, to which oil remained adhered. The results suggest a two-step mechanism in which low salinity induces kaolinite cation exchange and pH increase, which triggers oil release from neighboring grain surfaces. A second case study focuses on a Middle East reservoir carbonate, which was restored to mixed-wet state and imaged after forced imbibition by ultra-centrifugation to demonstrate the fast estimation of waterflooding oil-brine relative permeability curves. The third study assesses the potential for wettability modification of sandstone reservoirs using a producer wellbore surfactant treatment to mitigate condensate banking. An analog Berea sandstone was imaged after imbibition and drainage without and with surfactant treatment to estimate the associated shift in gas-condensate relative permeabilities through image analysis and simulation.
