01 Pore Geometry

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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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Spring 2006 5

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

Documents

01 Pore Geometry