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G ra in S iz e


V ery coarse sand


5 0 4 0 3 0 2 0 1 0 0 -1 0 1 2 3 4 = -log2 ( diameter) > Statistics ob tained from a single slice of a sam ple. More than 4 , 100 grains


w ere virtually disaggregated from a single slice, allow ing researchers to com pile detailed statistical data used to characterize rock fab ric and tex ture. When com pared w ith other sam ples, these statistical m easures can help geologists sort out the depositional environm ent of the rock . ( Adapted from Saadatfar et al, reference 15 . )


> Pore-scale inform ation derived from tom ographic im ages. Pore centers


( b lue spheres) , connected b y pore throats ( b lue cy linders) , are used to m odel porosity w ithin a sam ple of carb onate rock ( y ellow ) . The size and location of pore centers and pore throats in this netw ork re ect actual conditions w ithin the rock m icrostructure. The com plex ity and heterogeneity of carb onate pore netw ork s are b rought to the forefront as part of the rock m atrix is rendered sem itransparent w hile pore space is rendered opaq ue. ( Im age courtesy of The Australian National University . )


measurements (above left). Grain volume is measured by counting the voxels in each distinct grain, from which size is derived and then graded against standard Udden-Wentworth or Krumbein scales of grain sizes.1 6


Automated programs can


track and classify individual grains according to grain shape characteristics of sphericity and roundness or classify according to textural categories, such as sorting, grain contacts, and matrix or grain-support. Some programs can also measure anisotropy in grain orientation to help geoscientists ascertain sediment- transport direction.


More important than the detailed measure- ment of rock grains is the analysis of the space between the grains and the contents therein. Opacity-rendering tools work particularly well in showing what is not rock— that is, its porosity. Researchers can obtain a good picture of porosity by decreasing the opacity of dense voxels representing rock grains and cements, while simultaneously increasing the opacity of low- density voxels (right). This same opacity- rendering technique highlights the extent of interconnected porosity within the rock. Once the porosity is brought up on screen, geo- scientists can measure the size of pore spaces and pore throats using the ruler tool. Pore interconnectivity can also be charted, using pore network models based on tomographic imaging (above right). Pore-throat and pore-size distri- bution, along with interconnectivity, figure


Spring 2006


prominently in determining relative permeability and recovery estimates in reservoir samples— parameters that can be hard to quantify when different fluids compete to flow through the same opening.


A variety of other measurements can be taken from tomographic images, from which important information is derived. Analysts can directly correlate image data on pore structure and connectivity to measures of formation factor, permeability and capillary drainage pressures. Comparisons of results obtained from µ CT images and conventional laboratory measure- ments on the same core material have generally shown good agreement.1 7


Studying Effects of Carbon Dioxide on Casing Cement


In an important application beyond the realm of conventional petrophysics, µ CT was used to study the effects of carbon dioxide [ CO2] on casing cement. Greenhouse gases, particularly CO2, have been linked to rising temperatures around the world.


Capturing CO2 emissions and


sequestering them in the subsurface have been proposed as a measure to reduce atmospheric greenhouse-gas concentrations


until low-


emission energy sources become viable.1 8 However, CO2 becomes supercritical when temperature and pressure conditions exceed


Coarse M edium F ine Silt


O pacity change


G rains and quartz cement


P ores and pore throats


> A w hole lot of nothing. By m anipulating the opacity of a scanned sam ple im age, it is easy to visually ex am ine either sand grains ( green) or pore space ( b lue) . In m any evaluations, this detailed analy sis of pore space can reveal critical clues to future perform ance of a reservoir.


11


F requency


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