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4 Oilfield Review
A Closer Look at Pore Geometry
Andreas Kayser
Cambridge, England
Mark Knackstedt
The Australian National University
Canberra, Australia
Murtaza Ziauddin
Sugar Land, Texas, USA
For help in preparation of this article, thanks toVeronique Barlet-Gouédard, Gabriel Marquette,Olivier Porcherie and Gaetan Rimmelé, Clamart, France;Bruno Goffé, Ecole Normale Supérieure, Paris; andRachel Wood, The University of Edinburgh, Scotland.
Inside Reality and iCenter are marks of Schlumberger.
X-ray computed tomography has advanced the field of medicine for more than 30 years.
For nearly as long, it has also been a valuable tool for geoscientists. Improvements in
the technology are helping geoscientists uncover greater detail in the internal pore
structure of reservoir rock and achieve a better understanding of conditions that
affect production.
Information gained through core analysis is
invaluable for predicting the producibility of a
reservoir pay zone. While other methods enable
petrophysicists to estimate grain size, bulk
volume, saturation, porosity and permeability of
formations, core samples often serve as the
benchmark against which other methods are
calibrated. However, notwithstanding several
hundred thousand feet of whole or slabbed core
residing in core libraries around the world, most
wells are not cored.
The wealth of information obtained from
cores comes at a price. Coring often increases rig
time, lowers penetration rates and increases the
risk of sticking the bottomhole assembly. At some
wells, hostile downhole or surface conditions
make coring too risky. In other cases, correla-
tions are not sufficient to allow geologists to
accurately and confidently pick coring points.
Instead, many operators rely on sidewall cores
obtained through prospective pay zones, and may
compensate for lack of whole core data by
supplementing their usual logging program with
a wider range of measurements.
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As oil companies try to drain aging reservoirs
more efficiently, engineers and geoscientists may
come to regret earlier decisions to forgo coring.
Once a well has been drilled through a pay zone,
it is too late to go back to obtain whole cores,
unless the well is sidetracked. However,
mineralogy, grain size, saturation, permeability,
porosity and other measures of rock fabric can
sometimes be determined without coring.
With improvements on the early medical CAT-
scan technique developed in 1972, geoscientists
can take a series of fine, closely spaced X-ray
scans through a rock sample to obtain important
information about a reservoir.1 Using a
nondestructive technique called microcomputed
tomography, a focused X-ray beam creates
“virtual slices” that can be resolved to a scale of
microns, not just millimeters.2 These refinements
also allow the option of examining smaller
samples of rock; instead of depending on whole
cores for porosity and permeability measure-
ments, geoscientists can now use formation
cuttings to estimate these properties.
3
Althoughmany companies do not core their wells, they
usually employ the services of a mudlogger to
catch formation cuttings as they come over the
shale shaker. When they don’t have core,
geoscientists are finding that a sliver of rock can
be highly revealing.
This article reviews the development of X-ray
computed tomography (CT) and the ensuing
technology transfer from medical to oilfield
applications. We describe how the data can be
evaluated using immersive visualization tech-
niques and discuss a range of oilfield applications
that may benefit from this technology. Finally, we will see how this technology served researchers in
their evaluation of casing cement and well
stimulation treatments.
CT Scan Technology
Originally developed for medical use by Godfrey
Newbold Hounsfield in 1972, computed tomog-
raphy uses X-ray scans to investigate internal
structures within a body, such as those of soft
tissue and bone.4 CT overcomes the problem of
superimposition exhibited in conventional X-ray
radiography when three-dimensional features
of internal organs are obscured by overlying
organs and tissues imaged on two-dimensional
X-ray film.
Rather than projecting X-rays through a
patient and onto a film plate, as with
conventional X-rays, the CT process takes a
different approach. The CT scanner uses a
rotating gantry to which an X-ray tube is
mounted opposite a detector array. The patient is
placed in the center of the gantry, while the
opposing X-ray source and detectors rotate
around the patient. With the patient positioned
roughly in the middle of the source-receiver
plane, the rotating gantry allows a series of
closely spaced radiographic scans to be obtained
from multiple angles. These scans, or
radiographic projections, can then be processed
to obtain a 3D representation of the patient
(below).
CT radiographic projections are based on the
differential attenuation of X-rays caused by
density contrasts within a patient’s body. This
patient from this equation, attenuation is a
function of the energy of the X-ray as well as the
density and atomic number of the elements
through which the X-ray passes. The correlation
is fairly straightforward: lower-energy X-rays
higher densities and higher atomic numbers
generally result in greater attenuation.5
Digital projection data are converted into a
computer-generated image using tomographic
reconstruction algorithms to map the distribu
tion of attenuation coefficients.6 This distribution
can be displayed in 2D slices, composed of point
that are shaded according to their attenuation
1. In the medical field, the computerized axial tomography (CAT) scan is sometimes also calledcomputer-assisted tomography, and is synonymous
with computed tomography.2. A micron, or micrometer, is equal to one millionth of a
meter, or more commonly, one thousandth of a millimeter.It is abbreviated as µ, µm or mc. In English measure, amicron equals 3.937 x 10-5 in.
3. Siddiqui S, Grader AS, Touati M, Loermans AM andFunk JJ: “Techniques for Extracting Reliable Densityand Porosity Data from Cuttings,” paper SPE 96918,presented at the SPE Annual Technical Conference andExhibition, Dallas, October 9–12, 2005.
Bauget F, Arns CH, Saadatfar M, Sheppard AP, Sok RM,Turner ML, Pinczewski WV and Knackstedt MA: “What
is the Characteristic Length Scale for Permeability?Direct Analysis from Microtomographic Data,” paperSPE 95950, presented at the SPE Annual Technical
Conference and Exhibition, Dallas, October 9–12, 2005.4. Hounsfield GN: “A Method of and Apparatus for
Examination of a Body by Radiation such as X- orGamma Radiation,” British Patent No. 1,283,915(August 2, 1972).
5. For more on X-ray CT: Publication ServicesDepartment of the ODP Science Operator. http://www-odp.tamu.edu/publications/185_SR/005/005_5.htm(accessed January 27, 2006).
6. Feldkamp LA, Davis LC and Kress JW: “PracticalCone-Beam Algorithm,” Journal of the Optical Societyof America A1, no. 6 (June 1984): 612–619.
> Thoracic CAT scan. Manipulating color and opacity values of different tissues provides physicians with an unobstructed view of a patient’s lungs
and skeletal system. (Image courtesy of Ajay Limaye, VizLab, The AustralianNational University.)
attenuation represents a decrease in energy as
X-rays pass through various parts of the body.
Some tissues scatter or absorb X-rays better than
others: thick tissue absorbs more X-rays than
thin; bone absorbs more X-rays than soft tissue,
while fat, muscle or organs allow more X-rays to
pass through to the detectors. Removing the
values (see “Moving from 2D Points to 3D
Volumes,” page 6). Thus, in hospital scans, bon
would typically be assigned a light color t
correspond with its comparatively high
attenuation value, while air-filled lung tissu
might be assigned a darker color correspondin
to low attenuation values.
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6 Oilfield Review
In the mid-1880s, Neo-Impressionist artist
Georges Seurat perfected a revolutionary
technique of painting with tiny dots of color.
Like Michel Chevrul before him, Seurat recog-
nized that from a distance, the eye would
naturally blend together tiny dots of primary
colors to produce secondary shades. Using tiny
brush strokes, Seurat and his contemporaries
captured scenes of cityscapes, harbors and
people at work and leisure. This technique
came to be known as pointillism.
Computers use a similar technique to display
text and images; however, they work at a much
finer scale. Every image portrayed on a com-puter monitor or video screen is composed of
many, almost imperceptibly tiny dots, spaced
at extremely close intervals. In a 2D picture
screen, each dot, or pixel (a word formed
from the contraction of picture element) can
be defined by its horizontal (x) and vertical
(y) screen coordinates. It is also defined by its
color value. In color images, each pixel is also
assigned its own brightness.
The number of shades that a pixel can take
on depends on the computer and the number
of bits per pixel (bpp) it is capable of process-
ing. Common values range from 8 bpp (28 bits, which translates to 256 colors) to 24 bpp
(224 bits, or 16,777,216 colors). On an eight-bit
gray-scale image, for instance, each pixel
would be assigned a value corresponding to a
shade of gray, ranging from 0 to 255, where 0
represents black and 255 represents white.
The number of pixels used to create an
image controls its resolution (above right).
As more pixels are used, the image can be por-
trayed in greater detail, or higher resolution.
Resolution is thus initially impacted by the
image acquisition system and later, by the
image display system.
Resolution in digital image acquisition
systems is largely governed by the number of
light-sensitive photoreceptor cells, known as
photosites, which are used to record an image.
These photosites (more commonly referred to
as pixels) accumulate charges corresponding
to the amount of light passing through the
lens and onto each cell.1 As more light falls
onto a photosite, the charge grows. Light is
shut off to the lens once the shutter closes, at
which point the charge in each cell is
recorded by a processing chip and converted
to a digital value that determines the color
and intensity of individual pixels used to dis-
play the image on screen. Resolution in these
Moving from 2D Points to 3D Volumes
> Pixel resolution. The sharpness and clarity of an image are affected by pixel count and the sizeof the pixels. To increase the number of pixels within a fixed space, pixel size must be reduced.As pixel size (in white) progressively decreases (left to right), more pixels can be used to providegreater detail in the image.
> Pixel to voxel. A flat pixel ( left ) takes on a new dimension when the slice on which it resides isstacked with other slices to form a volume (right ). Adding the z-coordinate of the slice numberessentially assigns a depth-value to the pixel, thus creating a voxel within the stack of slices.
0
0 Color bar 256
0
200
400
600
800
1,000
V e r t i c a l c o o r d i n a t e
s ,
y
200 400 600
Horizontal coordinates, x
800 1,000
color
x
y
Pixel
0
200
400
600
800
1,000
V e r t i c a l c o o r d i n a t e
s ,
y
0 200 400 600
Horizontal coordinates, x
Slice
number
, z
800 1,000
Voxelx
y
z
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Spring 2006 7
Evolving to Industrial Strength
Density contrasts within a rock body can be
imaged just as they can within a human body
(below). By the mid 1980s, CT technology was
making significant inroads into geoscience
applications. In addition to quantitative
determination of bulk density of rock samples,
CT scanning was adapted to visualize microbial
desulfurization of coal, displacement of heavy oil,
and oil flow through carbonate cores.7
It didn’t take long for those outside the
medical field to recognize the potential of CT
technology for nondestructive evaluation o
materials. Geoscientists soon joined the ranks o
other researchers, particularly those in the field
of materials testing, who sought increasingly
finer detail for imaging internal structures. Thi
capability has largely been realized through
development of industrial-strength CT systems
which can employ more powerful X-rays, a tighte
focal point and longer exposure times than those
used in the medical field.8
7. Kayser A, Kellner A, Holzapfel H-W, van der Bilt G,Warner S and Gras R: “3D Visualization of a RockSample,” in Doré AG and Vining BA (eds): Petroleum Geology: North-West Europe and Global Perspectives – Proceedings of the 6th Petroleum Geology Conference.London: The Geological Society (2005): 1613–1620.
Vinegar HJ: “X-ray CT and NMR Imaging of Rocks,”Journal of Petroleum Technology 38, no. 3 (March 1986):257–259.
8. For more on high-resolution X-ray CT: University of TexasHigh-Resolution X-ray Computed Tomography Facility.http://www.ctlab.geo.utexas.edu/overview/index.php# anchor1-1 (accessed January 30, 2006).
devices is often expressed not in terms of pho-
tosites, but rather in megapixels. A
1.2-megapixel device, for instance, might have
an area of 1,280 x 960 (1,228,800 pixels), while
higher resolution would be attained by a 3.1-
megapixel device measuring 2,048 x 1,536
(3,145,728 pixels).
Image resolution can then be affected by
the medium on which it is displayed. A rela-
tively low-resolution computer monitor might
be described as a 640 x 480 display. This
means that the monitor has a width of 640 pix-
els, spread across a height of 480 lines,
totaling 307,200 pixels. If those pixels were
spread across a 15-inch monitor, then any
image displayed on that monitor would be
allotted 50 dots per inch. To increase resolu-
tion, either the screen size must be reduced
or more pixels must be packed into thescreen. Modern applications generally take
both approaches, squeezing a huge number
of pixels into a smaller area.
To image a 3D object, the pixel is expanded
into another dimension. A third coordinate
(z) is added to the x-y location to precisely
define the pixel’s position within the volume
of a 3D object, thereby creating a voxel—
short for volume pixel. In CT images, the
z-coordinate often denotes depth, and is dic-
tated merely by the position that a
tomographic slice holds within a volume
formed by stacking together numerous closely spaced slices (previous page, bottom). In
addition to x, y and z coordinates, a voxel can
define a point by a given attribute value. In
the case of CT scans, that value is density,
which is a function of the sample’s trans-
parency to X-rays. Density values can be tied
to a color spectrum, while a range of intensi-
ties can control the opacity of a voxel on a
computer screen. With this information and
3D rendering software, a two-dimensional
image of a 3D object can be generated for
viewing at various angles on a computer screen.
> Density values of various minerals commonly found in sedimentary rock.X-rays used to visualize rock structures are affected, in part, by differencesin density and mineralogy within a sample.
Quartz
Calcite
Anhydrite
Barite
Celestite
Mineral Density, g/cm3 Mineral Densi ty, g/cm3
2.64
2.71
2.98
4.09
3.79
Gypsum
Dolomite
Illite
Chlorite
Hematite
2.35
2.85
2.52
2.76
5.18
> A different kind of patient. A section of whole core is placed on a sliding gurney prior to imaging ata hospital CAT-scan facility.
1. Although experts may correctly assert that photositesare not actually pixels, the terms are becomingincreasingly interchangeable in the popular vernacu-lar, thanks largely to the broad appeal of digitalphotography, in which manufacturers of digital cam-eras describe resolution in terms of megapixels.
In the early days of CT rock scans, it was not
unusual for geoscientists to work out agreements
with the only institution in town that could
provide access to such sophisticated technology.
Often in the dark of night, with as little attention
as possible, core samples from the oilpatch would
be wheeled into the pristine and sterile setting of
a hospital CAT-scanning facility for imaging and
analysis (below).
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With the development of microCT (µCT),
researchers are attaining much higher
resolutions.9 Using µCT, researchers are some-
times able to image their samples with voxel
sizes as low as 2.5 µm. 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 µm. Similarly, a 16-megapixel camera
(4,000 x 4,000 pixels) can resolve the same
sample to 2.5 µm.
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 µCT 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.10
The Scanning Process
The scanning process to acquire µCT 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 beinterpolated when the data are processed.
For µCT, 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
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.11 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-µm focal spot. The X-ray beam expands from
the focal point, creating a cone-beam geometry.12
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.13
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.14
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
8 Oilfield Review
> Three-dimensional quantification and spatial distribution of sandstonecomponents. While most sandstones consist primarily of quartz grains andcement, X-ray imagery helps put other components into perspective.Differences in X-ray attenuation throughout the sample indicate changes indensity caused by porosity and various mineral constituents of the rock.Once mapped, these characteristics can be isolated for further scrutiny.
Sandstone grains and quar tz cement: 78%
Barite cement: 1%
Pore space:16%
Calcite cement: 5%
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Spring 2006 9
image a 60-mm specimen with a 30-micron pixel
size. They can also zoom in for high-resolution
scanning to image a 4-mm specimen with
2-micron pixels.
Approximately 3,000 projections are needed
to generate a 2,0483 voxel tomogram. Between
each projection, the sample stage is rotated0.12°. The entire process takes 12 to 24 hours,
depending on the type of sample and the filtering
steps required to reduce sampling artifacts. The
resulting 24 gigabytes of projection data are
processed by supercomputer, and it takes 128
central processing units about 2 hours to
generate the tomogram.
Visualization Technology
Once individual radiographic projections have
been compiled into a 3D data volume file, thedata can be loaded into an immersive visuali-
zation environment for detailed examination.
With Inside Reality virtual reality technology, the
data can be imaged and manipulated like any
other volume of 3D data. Originally developed to
help visualize seismic volumes based on miles or
kilometers of data, Inside Reality technology can
also handle data volumes based on much finer,
submillimeter scales.
Geoscientists utilize this advanced visuali-
zation technology to view a data volume from any
direction. This capability enables bedding planes
and fracture planes of rock samples to be viewed
orthogonally, even when the physical sample ha
been cut obliquely to these planes. Sedimentary
and structural features of the rock sample are
typically analyzed in the form of slices or
transparency views through a volume.
While the scanning process relies on density
differences to distinguish features within a
sample, the visualization process depends largely
on opacity differences. One way to expose
features deep within a volume comprising
millions of voxels is to render surrounding voxels
invisible. Opacity rendering is the key to
visualization. Each voxel is assigned a value along
a transparency-opacity spectrum, thus making
some voxels stand out while others fade away
Without this capability, the opacity of outer voxel
would hide all features lying within the volume.
Voxel-based technology can be used to
determine the volume and geometry of rock
grains, cement, matrix and pore space within a
sample. Using Inside Reality opacity-rendering
tools, geoscientists can assign different values othe opacity-transparency spectrum to variou
components within a volume. This technique
allows geoscientists to distinguish between
materials of different density values. For
example, the distribution of cement between
mineral grains shows up as a distinctive color
while setting pore space to zero-opacity makes i
transparent, thus showing the spaces between
grains. This allows the viewer to separate rock
grains from cement, matrix and pore space to
reveal internal sedimentary and structura
features (below).
9. Abbreviations for microcomputerized tomography rangefrom µCT (where the Greek letter mu is a standardsymbol for the prefix “micro”) to uCT (where u is asubstitute for mu ) to mCT (where the m stands for micro) to XMT for X-ray Microtomography.
10. Kayser A, Gras R, Curtis A and Wood R: “VisualizingInternal Rock Structures: New Approach Spans FiveScale-Orders,” Offshore 64, no. 8 (August 2004): 129–131.
11. Ketcham RA and Carlson WD: “Acquisition, Optimizationand Interpretation of X-Ray Computed Tomographic
Imagery: Applications to the Geosciences,” Computers & Geosciences 27, no. 4 (May 2001): 381–400.
12. Sakellariou A, Sawkins TJ, Senden TJ and Limaye A:“X-Ray Tomography for Mesoscale PhysicsApplications,” Physica A 339, no. 1-2 (August 2004):152–158.
Sakellariou A, Sawkins TJ, Senden TJ, Knackstedt MA,Turner ML, Jones AC, Saadatfar M, Roberts RJ,Limaye A, Arns CA, Sheppard AP and Sok RM: “AnX-Ray Tomography Facility for Quantitative Predictionof Mechanical and Transport Properties in Geological,Biological and Synthetic Systems,” in Bonse U (ed):Developments in X-Ray Tomography IV, Proceedings of SPIE—The International Society for Optical Engineering ,Vol. 5535. Bellingham, Washington, USA: SPIE Press(2004): 473–474.
13. This test equipment includes pumps or other devicesused to study fluid flow or mechanical compaction.
14. Rather than exposing film to light, CCD technologycaptures images in a technique similar to commondigital photography. A CCD uses a thin silicon wafer torecord light pulses given off by a scintillator. The CCDsilicon wafer is divided into several thousand individuallight-sensitive cells. When a light pulse from thescintillator impinges on one of these cells, thephotoelectric effect converts the light to a tiny electricalcharge. The charge within a cell increases with everylight pulse that hits the cell. Each cell on the CCD siliconwafer corresponds in size and location to an imagepixel. The pixel’s intensity is determined by themagnitude of the charge within a corresponding cell.
> A high-resolution X-ray tomography device at The Australian National University. The rotatingsample stage and charge-coupled device (CCD) camera slide on a track, enabling adjustment of thedistance between the camera, sample and X-ray source. With this device, a sample can be magnifiedfrom 1.1 to more than 100 times its original size. The stage rotates with millidegree accuracy and canbe fitted with fluid pumps for imaging flow through porous media. (Figure courtesy of The AustralianNational University.)
Approximately 1.5 meters
Rotation stage X-ray sourceScintillator + CCD
> Sandstone pores. An opacity filter is used to render different features in volume windows usingInside Reality software. The left window above and behind the yellow arrow shows only quartz grains(light green) in this eolian sandstone from the Rotliegendes formation in Germany. A volume showingonly pore space (blue) is in the background on the right. The smaller volume in the foreground on theright shows late diagenetic barite cement (red). The slice making up the base image indicates quartz(gray), pore space (blue), barite (red) and carbonate cement (orange). The yellow arrow for scale is1 mm long.
1.0 mm
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The ability to manipulate opacity values plays
an important role in the seedpoint and volume-
grower tools featured as part of the Inside Reality
toolbox. Using the seedpoint tool, the viewer
selects a point within a slice or volume. This
point has a certain X-ray attenuation value. Once
a point is selected, the program can automat-
ically pick all neighboring voxels of a similar
value that are connected to that point. This
feature can help a geoscientist pick a point
within a volume known to represent porosity, for
example, and the volume-grower tool will display all
interconnected porosity within the volume (left).
Because each voxel is defined in part by its
coordinates, the distance between any two voxels
can be measured. To facilitate this process, the
Inside Reality system uses a ruler tool to provide
a visual scale. This tool can be used to measure
grain or pore size in three dimensions, helping
geoscientists estimate pore-volume proportions
and connectivity.
Taking rock samples from the laboratory to an
immersive visualization environment enables an
asset team to share important information and
concepts about reservoir samples so they can make
more informed decisions. Inside Reality virtual
reality technology lets geoscientists share 3D
virtual core data with those in remote sites to help
asset teams collaborate with company experts and
partners around the world (below left).
Applications
Rock fabric and textural data provide geologists with key information used in analyzing facies and
in determining depositional environments.
Geologists and petrophysicists can now obtain
important information about grain size, shape
and matrix from digital scans of core or core
fragments. A single core-fragment image can
yield thousands of individual grains. By digitally
disaggregating grains in a scanned sample,
analysts can obtain coordinates of all voxels
composing each grain, the number of neighboring
grains and grain-overlap information.15
From such a dataset, geologists can derive a
comprehensive analysis of grain sizes anddistribution to obtain a full suite of statistical
10 Oilfield Review
15. Saadatfar M, Turner ML, Arns CH, Averdunk H,Senden TJ, Sheppard AP, Sok RM, Pinczewski WV,Kelly J and Knackstedt MA: “Rock Fabric and Texturefrom Digital Core Analysis,” Transactions of the SPWLA46th Annual Logging Symposium , New Orleans,June 26–29, 2005, paper ZZ.
16. Both the Udden-Wentworth and the Krumbein scales areused to classify rock samples according to diameter; theformer is a verbal classification while the latter isnumerical. According to the Udden-Wentworth scale,sedimentary particles larger than 64 mm in diameter areclassified as cobbles. Smaller particles are pebbles,granules, sand and silt. Those smaller than 0.0039 mmare designated as clay. Several other grain-size scalesare in use, but the Udden-Wentworth scale (commonlycalled the Wentworth scale) is the one that is mostfrequently used in geology. The Krumbein scale is alogarithmic scale, which assigns a value designated asphi to classify the size of the sediment. Phi is computedby the equation: ø = –log2 (grain size in mm).
17. Arns CH, Averdunk H, Bauget F, Sakellariou A,Senden TJ, Sheppard AP, Sok RM, Pinczewski WV andKnackstedt MA: “Digital Core Laboratory: Analysis ofReservoir Core Fragments from 3D Images,”Transactions of the SPWLA 45th Annual Logging Symposium , Noordwijk, The Netherlands, June 6–9,2004, paper EEE.
18. Bennaceur K, Gupta N, Monea M, Ramakrishnan TS,Tanden T, Sakurai S and Whittaker S: “CO2 Capture andStorage—A Solution Within,” Oilfield Review 16, no. 3(Autumn 2004): 44–61.
> Sandstone tracking. An opacity filter has been used to highlight quartz grains in sandstone from aRotliegendes gas reservoir in Germany. In the volume (light gray), interconnected porosity (blue) isimaged using the volume-grower tool provided by Inside Reality software. Fringe (red) along the edgeof the porosity indicates possible connections to neighboring pores detected automatically by thesoftware. Carbonate cement (orange) is also shown in the volume. The horizontal slice shows quartzgrains (dark gray), pore space (black), carbonate cement (medium gray), and barite cement (white).
1.0 mm
> Visualization using Inside Reality technology. Bringing sample volumes intoan iCenter secure networked collaborative environment allows asset teams to become immersed in their data. Stereo projection creates a perception ofdepth, providing a different perspective on the 3D nature of the rock and itsmicrostructure. Inside Reality visualization software provides a detailedimage of a foraminifera fossil measuring 1.5 x 1.0 mm (inset ). This 3Dvisualization allows examination of the fossil from many different angles. The
animated avatar mirrors the pointing motions and actions of another viewerwho is interacting with these data from a remote site.
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Spring 2006 11
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.16 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 programscan 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
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 µCT
images and conventional laboratory measure-
ments on the same core material have generally
shown good agreement.17
Studying Effects of Carbon Dioxide
on Casing Cement
In an important application beyond the realm o
conventional petrophysics, µCT 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 atmosphericgreenhouse-gas concentrations until low
emission energy sources become viable.1
However, CO2 becomes supercritical when
temperature and pressure conditions exceed
> Statistics obtained from a single slice of a sample. More than 4,100 grainswere virtually disaggregated from a single slice, allowing researchers tocompile detailed statistical data used to characterize rock fabric and texture. When compared with other samples, these statistical measurescan help geologists sort out the depositional environment of the rock.
(Adapted from Saadatfar et al, reference 15.)
F r e q u e n c
y
0-1 0 1 2 3 4
10
20
30
40
50
= -log2 (diameter)
Medium
Grain Size
CoarseVery coarse
sand Fine Silt
> A whole lot of nothing. By manipulating the opacity of a scanned sample image, it is easy to visuallyexamine either sand grains (green) or pore space (blue). In many evaluations, this detailed analysis ofpore space can reveal critical clues to future performance of a reservoir.
Grains and quartz cement
Opacity change
Pores and pore throats
> Pore-scale information derived from tomographic images. Pore centers(blue spheres), connected by pore throats (blue cylinders), are used to model porosity within a sample of carbonate rock (yellow). The sizeand location of pore centers and pore throats in this network reflectactual conditions within the rock microstructure. The complexity andheterogeneity of carbonate pore networks are brought to theforefront as part of the rock matrix is rendered semitransparent while
pore space is rendered opaque. (Image courtesy of The AustralianNational University.)
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31.1°C and 73.8 bar [87.9°F and 1,070 psi]—
conditions that are easily exceeded in most
medium to deep wells.19 Therefore, an important
aspect of any CO2 sequestration project is to
know how downhole materials will react to
supercritical CO2 (scCO2).
Scientists at Schlumberger Cambridge
Research in England have collaborated with
their counterparts at Schlumberger Riboud
Product Center in Clamart, France, to investi-
gate long-term effects of CO2 storage on wellbore
integrity. One such experiment sought to
determine how scCO2 would react with casing
cement.20 Long used in oil and gas wells to
hydraulically isolate pay zones from the surface
and other permeable zones, portland-based
cements play a critical role in wellbore integrity.
This study focused on a sample of neat
cement.21 The cylindrical cement sample was
cured for three days at 90°C and 280 bar [194°F
and 4,061 psi]. Scientists obtained CT scans of
the cement cylinder before exposing it to scCO2.
The cement was then subjected to a wet scCO 2environment and kept at 90°C and 280 bar for
30 days. Two sample plugs were cut from the
original cylinder and then scanned.
Using Inside Reality software, researchers
were able to manipulate the data volume to
visualize porosity and microfractures and arbi-
trarily slice through zones of interest. By
comparing scans acquired before and after
treatment, researchers noted significant
changes to the cement plug, resulting from scCO2attack. Of particular interest were the formation
and distribution of microfractures, along with a
zone of aragonite replacement and a zone of mineral alteration characterized by high
secondary porosity.
The reaction between scCO2 and cement
produced an irregular carbonation front,
extending 4 mm [0.16 in.] from the outer edge of
the core toward its center. This lighter colored
carbonation front was readily apparent in the
gray-scale 3D volume, and in a color-coded slice
(above right). Subsequent X-ray diffraction
analysis determined that the alteration front had
a different composition than the original cement,
which had been replaced by aragonite. Porosity
was clearly enhanced in the regions around
the microfractures and the aragonite front (right).
The tests suggested that exposure to scCO2
may cause conventional cement to lose more
than 65% of its strength after only six weeks.
These important observations provided an
impetus for creating new blends of cement.
Schlumberger researchers have developed new
scCO2-resistant cementing materials that display
good mechanical behavior after exposure to
scCO2 gas. Laboratory tests on these new
materials show only a slight decrease in
compressive strength during the first two days,
and essentially no loss for the subsequent
three months.
Examining Wormholes Caused
by Stimulation Treatments
Researchers have also used CT imaging to study
the effects of heterogeneity on carbonate matrix
stimulation. In one experiment, it was instru-
mental in visualizing the effects of the original
porosity distribution on acid-dissolution patterns.
12 Oilfield Review
> A sample plug of neat cement. Only a few centimeters in length, this sample revealed importantinformation concerning the behavior of supercritical CO2 on portland cement. The tomographic gray-scale image of the cement sample (right ), scanned with a resolution of 18.33 µm, shows a highconcentration of aragonite along the edge of a carbonation front, accompanied by an alteration front.An additional dissolution front of high porosity extends farther into the core. Circular holes with adiameter of 500 µm may represent air bubbles. Microfractures are filled with aragonite crystals.Lighter features represent higher CT values, signifying different mineralogy in the case of the filledmicrofracture, or different amounts of microporosity, in the case of the alteration front.
0 1cm
2
Alteration front
CT ImageSample Plug
Carbonation frontZone of very low porosity
Air bubble(Diameter 0.5 mm)
Dissolution front
Filled microfracture
Zone of very high porosity
> Highlighting the extent of supercritical CO2 alteration. Color-coding enhances features that may notbe readily apparent in gray-scale imaging. Microfractures formed during the supercritical CO2 attackserved as conduits for further aragonite alteration. The concentration of aragonite along the fracturesand the edge of the alteration front can be visually distinguished using color-coding provided byInside Reality software. Materials imaged are unaltered neat cement (green), an alteration front(yellow), and mineral-filled microfractures or carbonation front (red). Increased porosity (blue) marks the extent of various dissolution patterns.
Neat cement
Aragonite front
S y s t e m M e n u – M a i n M e n uT o o l s
Res to re S cene
Save Sc ene
Snapsh ot
Sy st em Menu
Colo rmap
Fault
Fence
Reservoir
Rule r
Ske tc h
Sl ice
Surface
ume Es timat ion
Volume Window
Well
GrowingSt ereo
In sid e R eali ty
Vers ion 5.1 [90]
AUTOSAVE
SCR_040917_1736_1
SCR_040917_1847_1
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Spring 2006 13
Stimulation treatments are commonly
performed in wells where poor permeability
limits production due to naturally tight
formations or formation damage. A common
stimulation technique involves the injection of
acid into carbonate formations. Acid dissolves
some of the formation matrix material and
creates flow channels that increase the
permeability of the matrix.
The efficiency of this process depends on the
type of acid used, reaction rates, formation
properties and injection conditions. While
dissolution increases formation permeability, the
relative increase in permeability for a given
amount of acid is greatly influenced by injection
formation to facilitate the flow of oil. Better still,
wormholes require only a small volume of acid to
produce significant increases in permeability.
Researchers are therefore investigating factors
that influence production of wormholes.
CT scanning has proved instrumental in
determining the effects that injection rate and
spatial distribution of porosity have on dissolution
patterns formed during stimulation experiments
(below). Because it is nondestructive, this
technique allows for characterization of the core
before and after the treatment experiment so
the development and shape of the wormhole can
be evaluated.
applications, it is easy to envision the potentia
spread of new applications for µCT.
The technology will no doubt prove
instrumental in improving the interpretation and
application of laboratory and log data. As an
increasingly important tool in nondestructive
testing, its application can be extended to
laboratory testing of unconsolidated or friable
formation samples. The combination of µCT
imaging with numerical calculations may lead to
more accurate predictions of a wide range of rock
properties crucial to exploration, reservoi
characterization and recovery calculations.
Further applications include development o
improved cross-property correlations and
development of libraries of 3D images that wil
19. Above its critical point at 31.1°C and 73.8 bar, CO2becomes a supercritical fluid. In this compressed state,its properties lie between those of a gas and a liquid.With a lower surface tension than its liquid form,supercritical CO2 can easily penetrate cracks andcrevices. Unlike CO2 gas, however, it can dissolvesubstances that are soluble in liquid CO2.
20. Barlet-Gouédard V, Rimmelé G, Goffé B and Porcherie O:“Mitigation Strategies for the Risk of CO2 MigrationThrough Wellbores,” paper IADC/SPE 98924, presentedat the IADC/SPE Drilling Conference, Miami, Florida,USA, February 21–23, 2006.
21. Neat cement has no additives that would alter its setting time or rheological properties.
> Visualizing wormhole development. A sample of Winterset limestone was scanned by CT before (bottom ) and after (top ) acid injection. This data volumeis displayed using Inside Reality visualization technology, in which pore space is rendered opaque, while surrounding voxels are rendered transparent.Initial distribution of pores (bottom ) shows discrete clusters of pores (blue) along the long axis of the core. After acidizing ( top ), the core shows increasedporosity, with a dissolution pattern extending from right to left that further marks the flow of acid during injection.
conditions. At extremely low injection rates, acid
is spent soon after it contacts the formation,
resulting in relatively shallow dissolution along
the face of the injection zone. High flow rates
produce a uniform dissolution pattern because
the acid reacts over a large region. In either case,
the resulting gains in permeability require
relatively large expenditures of acid.
However, at intermediate flow rates, long
conductive channels known as wormholes are
formed. These channels penetrate deep into the
Peering into the Future
Tomography is not new to the oil industry. At the
upstream end of the tomography spectrum lies
crosswell seismic tomography; at the downstream
end is industrial process tomography for
refineries. As a research tool, µCT is used across
a broad suite of industrial applications to monitor
performance of polymer-enhanced foams and
polyethylene resins or to view phase separation
and pore-space characterization in formation
samples. Across this range of tomographic
allow a more rigorous and quantitative descrip
tion of rock type and texture. These quantitative
descriptions can be integrated with classica
sedimentological descriptions. The technology
can also make a significant contribution to the
study of elastic behavior, porosity-permeability
trends and multiphase flow properties such as
capillary pressure, relative permeability and
residual saturations.
Future technological innovations will probably
include higher resolution to overcome problems
in predicting porosity when micropores fal
below the detection capability of the presen
technique. With the improving resolution of their
samples, µCT technology is helping today’
geoscientists to better see their world in a grain
of sand. —MV
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