Nanometer-scale imaging and pore-scale fluid flow modeling inchalk Page: 1 of 16
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NANOMETER-SCALE IMAGING AND PORE-SCALE FLUID FLOW
MODELING IN CHALK
LIVIU TOMUTSA (SPE), DMITRIY SILIN (SPE) AND VELIMIR RADMILOVIC
Lawrence Berkeley National Laboratory, Berkeley, CA, U.S.A.
For many rocks of high economic interest such as chalk, diatomite, tight gas sands or coal, nanometer scale
resolution is needed to resolve the 3D-pore structure, which controls the flow and trapping of fluids in the rocks.
Such resolutions cannot be achieved with existing tomographic technologies. A new 3D imaging method,
based on serial sectioning and using the Focused Ion Beam (FIB) technology has been developed. FIB allows
for the milling of layers as thin as 10 nanometers by using accelerated Ga+ ions to sputter atoms from the
sample surface. After each milling step, as a new surface is exposed, a 2D image of this surface is generated.
Next, the 2D images are stacked to reconstruct the 3D pore or grain structure. Resolutions as high as 10 nm are
achievable using this technique. A new image processing method uses direct morphological analysis of the
pore space to characterize the petrophysical properties of diverse formations. In addition to estimation of the
petrophysical properties (porosity, permeability, relative permeability and capillary pressures), the method is
used for simulation of fluid displacement processes, such as those encountered in various improved oil recovery
(IOR) approaches. Computed with the new method capillary pressure curves are in good agreement with
laboratory data. The method has also been applied for visualization of the fluid distribution at various
saturations from the new FIB data.
Field-scale oil recovery processes are result of countless events happening in individual pores. To model
multiphase flow in porous media at pore scale, 3D data are needed with a resolution adequate for the rock of
interest. Chalk formations are encountered in oil fields in the Texas, Middle East, North Sea, etc. Chalk
reservoirs hold significant oil reserves. The extremely small typical pore sizes in chalk impose very high
requirement on imaging resolution. In the last decade, X-ray microtomography has been used extensively for
direct visualization of pore system and the fluids within sandstone (Jasty, J.K. et al, 1993, Coles et al, 1996,
Wildenschild et al, 2002, Seright et al, 2003). While this approach is relatively fast and nondestructive, its
applicability is mostly limited to micron resolutions, although recent developments are bringing the resolution
to submicron range (Stampanoni et al, 2002). For chalk pore systems, which are characterized by submicron to
nanometer length scales, 3D stochastic methods based on 2D scanning electron microscope (SEM) images of
thin sections have been used to reconstruct the pore system (Talkudar et al, 2001). The advent of the Focused
Ion Beam technology has made it possible to reconstruct submicron 3D pore systems. 2D and 3D images for
diatomite and chalk pore structures are shown in Figure 1. (Tomutsa and Radmilovic, 2003). FIB technology is
used in microelectronics to access individual components with nanoscale accuracy for design verification,
failure analysis and circuit modification (Orloff et al, 2002). It also has been used in material sciences for
sectional sample preparation for SEM and for 3D imaging of alloys components (Kubis et al, 2004). In earth
sciences, FIB has been used for sample preparation for SEM and to access inner regions for performing
microanalysis (Heaney et al, 2001). To access the pore structure at submicron scale, FIB mills successive layers
of the rock material as thin as 10 nm. As successive 2D surfaces are exposed, they are imaged using either the
electron or the ion beam. After processing, the images are stacked to reconstruct the 3D pore structure. To
analyze the 3D chalk images obtained by the FIB method, we applied a recently developed technique (Silin et
al., 2003). A distinctive feature of this approach is that the 3D pore space image is analyzed directly, without
construction of pore networks. This approach bypasses the nontrivial task of extracting a simple but
representative network of pore throats linking pore bodies from the 3D data (Lindquist, 2002). Moreover, the
pore network extraction methods based on relatively simple grain shapes in sandstones, (Oren and Bakke, 2002)
may be not always feasible for the complex pore structures of carbonates. Although pore-network based flow
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Tomutsa, Liviu; Silin, Dmitriy & Radmilovich, Velimir. Nanometer-scale imaging and pore-scale fluid flow modeling inchalk, article, August 23, 2005; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc893671/m1/1/?rotate=270: accessed May 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.