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With the development of microCT (µ CT), researchers resolutions.9

are attaining much higher Using µ CT, researchers are some-

times able to image their samples with voxel sizes as low as 2.5 µ m. Depending on the size of a sample and the number of pixels used to image it, voxel sizes of one-thousandth of the sample size are being attained. For example, a 1-megapixel camera using 1,000 x 1,000 pixels could conceivably resolve a 1-cubic centimeter sample to about 10 µ m. Similarly, a 16-megapixel camera (4,000 x 4,000 pixels) can resolve the same sample to 2.5 µ m.

At such resolutions, geoscientists can distin- guish density or porosity contrasts inside a rock sample and can study pore space and pore connectivity in great detail. This µ CT technology permits recognition of grains or cements with different mineralogical compositions (right). It has even been used to differentiate grains of the same type, such as those found in carbonates, where microporosity may vary between different grain types in the same rock.1 0

The Scanning Process The scanning process to acquire µ CT data is in some respects analogous to acquiring 3D seismic data. A seismic crew shoots a series of regularly spaced seismic lines. Coordinates of the starting and ending points of each line are surveyed, making it possible to infer the distance between each line in the series. It is therefore possible to determine the position of any point along any line as well as the distance between points within the series of lines. With this knowledge, a position between any two points or lines can be interpolated when the data are processed. For µ CT, a regular series of closely spaced scans are acquired to obtain high-resolution virtual slices of a sample. Each pixel in the slice represents a scanned point and has coordinates that correspond to an actual point in the sample. Because coordinates of each point are known, distances between each point and each slice can be determined. And just like the seismic line, points or slices can be interpolated between existing slices. By stacking the series of slices close together to make up a volume of data, each pixel in a slice becomes part of the stack and takes on a third dimension. In this way, each pixel can be treated as a voxel.

The scanning process is carried out by highly specialized X-ray systems. Though several companies offer research-grade systems, many X-ray microtomography devices are custom-built. Regardless of whether they are off-the-shelf or specially designed, all rely on three primary

Barite cem ent: 1%

Pore space: 16% Sandstone grains and q uartz cem ent: 7 8%

Calcite cem ent: 5 %

> Three-dim ensional q uanti cation and spatial distrib ution of sandstone com ponents. While m ost sandstones consist prim arily of q uartz grains and cem ent, X -ray im agery helps put other com ponents into perspective. Differences in X -ray attenuation throughout the sam ple indicate changes in density caused b y porosity and various m ineral constituents of the rock . Once m apped, these characteristics can b e isolated for further scrutiny .

components: an X-ray source, a rotating stage on which the sample is placed and an X-ray camera to record the pattern of X-ray attenuation within a sample. To scan a sample, it must be placed on the rotating stage, positioned between the X-ray source and the camera. X-rays emitted from the source are attenuated through scattering or absorption before being recorded by the camera.1 1

The camera then records a large series of radiographs as the sample rotates incre- mentally on its stage through 360° . A computer program stacks the digital projection data while maintaining true spacing between pixels and slices. CT algorithms are applied to these data to reconstruct the internal structure of the sample and preserve its scale in three dimensions. One such device was built in 2002 by The Australian National University in Canberra (next

page, top). Its source generates X-rays with a 2- to 5 -µ m focal spot. The X-ray beam expands from the focal point, creating a cone-beam geometry.1 2 Because magnification of the sample increases with proximity to the X-ray source, the rotating stage and camera are designed to slide separately on a rail, allowing researchers to adjust distances between source, sample and camera. The sample stage can rotate the sample with millidegree accuracy and can support up to 120 kg [ 265 lbm] of sample and associated test equipment.1 3 At this facility, the X-ray “ camera” consists of a scintillator that fluoresces green in response to X-rays, and a charge-coupled device (CCD) that converts this green light into electric signals.1 4 The camera has a 70-mm2 active area, containing 4.1 megapixels (2,048 x 2,048 pixels). The system’s large field of view allows researchers to


Oilfield Review

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