Techniques in helical scanning, dynamic imaging and image segmentation for improved quantitative analysis with X-ray micro-CT

This paper reports on recent advances at the micro-computed tomography facility at the Australian National University. Since 2000 this facility has been a significant centre for developments in imaging hardware and associated software for image reconstruction, image analysis and image-based modelling. In 2010 a new instrument was constructed that utilises theoretically-exact image reconstruction based on helical scanning trajectories, allowing higher cone angles and thus better utilisation of the available X-ray flux. We discuss the technical hurdles that needed to be overcome to allow imaging with cone angles in excess of 60°. We also present dynamic tomography algorithms that enable the changes between one moment and the next to be reconstructed from a sparse set of projections, allowing higher speed imaging of time-varying samples. Researchers at the facility have also created a sizeable distributed-memory image analysis toolkit with capabilities ranging from tomographic image reconstruction to 3D shape characterisation. We show results from image registration and present some of the new imaging and experimental techniques that it enables. Finally, we discuss the crucial question of image segmentation and evaluate some recently proposed techniques for automated segmentation

Systematic pore-scale study of low salinity recovery from Berea sandstone analyzed by micro-CT

The low salinity effect in clay-rich outcrop sandstones is probed by micro-CT imaging and analysis. A set of eight Berea sandstone mini-plugs underwent primary drainage and aging in crude oil to a mixed-wet state, followed by spontaneous imbibition of high and low salinity brines and imaging of this sequence of prepared starting and endpoint states. Tomogram registration and analysis were used to determine the salinity-induced changes in oil volume, oil/rock and oil/brine interfacial areas, and oil/brine interfacial mean curvature. Pore-scale statistics were extracted to explore any local correlation between the low salinity effect and pore geometry/topology. The qualitative observations and quantitative analyses demonstrated that the small oil recovery by the low salinity effect corresponded to a slight shift towards water-wet.Financial support from the member companies of the Digital Core Consortium Wettability Satellite and Statoil are acknowledged

Added insight from image-based wettability characterization

Microtomographic rock and fluid imaging under in-situ conditions is applied for reservoir wettability characterization. The investigation entails careful sample preparation and cleaning of mini-plugs, operation with reservoir fluids, wettability restoration, centrifuge wettability testing cycles, repeated sample scanning and image analysis, parametrization of wettability and digital rocks simulation for input into reservoir modeling. The results are compared to conventional Amott testing performed in core laboratories. Determination of saturations from image analysis, instead of centrifuge production, allows the use of stock tank crude, rather than exchanged mineral oil. Doping of the synthetic formation water (here with 1 M sodium iodide) was applied for enhancement of the X-ray contrast. The digital imaging workflow offers insight on the liquid distributions from the plug scale down to the pore-scale, linked to applied pressure gradients and resulting pore fluid occupancies in the sequence of displacement states. An example is given with the investigation of a North-German oil field, where the image-based workflow led to a revised view of the reservoir conditions for spontaneous imbibition and drainage, and the overall wetting behavior

Removal of crude oil from kaolinite by water flushing at varying salinity and pH

The ability of crude oil to adhere to kaolinite in an aqueous environment by adsorbing or depositing its polar components (asphaltenes and resins), and the ability of subsequent water flushing to remove this bound oil, were compared for two oils and varying salinity and pH of the flushing solution. A scanning electron microscopy technique was used to image the locations of oil residues on and between kaolinite platelets, while fluorescence spectroscopy yielded the corresponding amount of asphaltenics. Consideration of the mechanisms of electrostatic interfacial attraction and surface precipitation giving rise to the adsorption/deposition of asphaltenics on kaolinite were used to interpret the extent of their removal by flushing. The oil exhibiting greater surface precipitation was fairly unresponsive to flushing, with displacement of the bulk oil leaving residues substantially lining the kaolinite platelets and filling the pores between them. The oil which adsorbed by electrostatic attraction was more amenable to removal of residues, to partially reinstate the water-wetness of the pristine kaolinite, by flushing with salt solutions which weakened this attraction. In particular, higher salinity of the flush minimized the attractions between oppositely charged sites on the heterogeneous interfaces, and pH shift to higher or lower levels rendered the interfacial charges more similar and mutually repulsive

Experimental investigation of deposition of crude oil components in brine-filled pores

It is often presumed that fine-scale surface pores and roughness in reservoir rock remain water-wet. To test this assumption, deposition tendency of asphaltenes and resins during crude oil aging of brine-filled pores was investigated. Model frameworks o