4 Oilfield Review

A Closer Look at Pore Geometry

Andreas Kayser Cambridge, England

Mark KnackstedtThe Australian National UniversityCanberra, Australia

Murtaza ZiauddinSugar Land, Texas, USA

For help in preparation of this article, thanks to Veronique Barlet-Gouédard, Gabriel Marquette, Olivier Porcherie and Gaetan Rimmelé, Clamart, France;Bruno Goffé, Ecole Normale Supérieure, Paris; and Rachel 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 isinvaluable for predicting the producibility of areservoir pay zone. While other methods enablepetrophysicists to estimate grain size, bulkvolume, saturation, porosity and permeability offormations, core samples often serve as thebenchmark against which other methods arecalibrated. However, notwithstanding severalhundred thousand feet of whole or slabbed coreresiding in core libraries around the world, mostwells are not cored.

The wealth of information obtained fromcores comes at a price. Coring often increases rigtime, lowers penetration rates and increases therisk of sticking the bottomhole assembly. At somewells, hostile downhole or surface conditionsmake coring too risky. In other cases, correla-tions are not sufficient to allow geologists toaccurately and confidently pick coring points.Instead, many operators rely on sidewall coresobtained through prospective pay zones, and maycompensate for lack of whole core data bysupplementing their usual logging program witha wider range of measurements.

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As oil companies try to drain aging reservoirsmore efficiently, engineers and geoscientists maycome 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 cansometimes be determined without coring.

With improvements on the early medical CAT-scan technique developed in 1972, geoscientistscan take a series of fine, closely spaced X-rayscans through a rock sample to obtain importantinformation about a reservoir.1 Using anondestructive technique called microcomputedtomography, a focused X-ray beam creates“virtual slices” that can be resolved to a scale ofmicrons, not just millimeters.2 These refinementsalso allow the option of examining smallersamples of rock; instead of depending on wholecores for porosity and permeability measure-ments, geoscientists can now use formationcuttings to estimate these properties.3 Althoughmany companies do not core their wells, theyusually employ the services of a mudlogger tocatch formation cuttings as they come over theshale shaker. When they don’t have core,geoscientists are finding that a sliver of rock canbe highly revealing.

This article reviews the development of X-raycomputed tomography (CT) and the ensuingtechnology transfer from medical to oilfieldapplications. We describe how the data can beevaluated using immersive visualization tech-niques and discuss a range of oilfield applicationsthat may benefit from this technology. Finally, wewill see how this technology served researchers intheir evaluation of casing cement and wellstimulation treatments.

CT Scan TechnologyOriginally developed for medical use by GodfreyNewbold Hounsfield in 1972, computed tomog-raphy uses X-ray scans to investigate internalstructures within a body, such as those of softtissue and bone.4 CT overcomes the problem ofsuperimposition exhibited in conventional X-rayradiography when three-dimensional features of internal organs are obscured by overlyingorgans and tissues imaged on two-dimensional X-ray film.

Rather than projecting X-rays through apatient and onto a film plate, as withconventional X-rays, the CT process takes adifferent approach. The CT scanner uses arotating gantry to which an X-ray tube ismounted opposite a detector array. The patient is

placed in the center of the gantry, while theopposing X-ray source and detectors rotatearound the patient. With the patient positionedroughly in the middle of the source-receiverplane, the rotating gantry allows a series ofclosely spaced radiographic scans to be obtainedfrom multiple angles. These scans, orradiographic projections, can then be processedto obtain a 3D representation of the patient(below).

CT radiographic projections are based on thedifferential attenuation of X-rays caused bydensity contrasts within a patient’s body. This

patient from this equation, attenuation is afunction of the energy of the X-ray as well as thedensity and atomic number of the elementsthrough which the X-ray passes. The correlationis fairly straightforward: lower-energy X-rays,higher densities and higher atomic numbersgenerally result in greater attenuation.5

Digital projection data are converted into acomputer-generated image using tomographic-reconstruction algorithms to map the distribu-tion of attenuation coefficients.6 This distributioncan be displayed in 2D slices, composed of pointsthat 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 ameter, 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 Density and 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 TechnicalConference and Exhibition, Dallas, October 9–12, 2005.

4. Hounsfield GN: “A Method of and Apparatus forExamination 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 Services Department 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: “Practical Cone-Beam Algorithm,” Journal of the Optical Society of America A1, no. 6 (June 1984): 612–619.

> Thoracic CAT scan. Manipulating color and opacity values of differenttissues provides physicians with an unobstructed view of a patient’s lungsand 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 thanothers: thick tissue absorbs more X-rays thanthin; bone absorbs more X-rays than soft tissue,while fat, muscle or organs allow more X-rays topass through to the detectors. Removing the

values (see “Moving from 2D Points to 3DVolumes,” page 6). Thus, in hospital scans, bonewould typically be assigned a light color tocorrespond with its comparatively highattenuation value, while air-filled lung tissuemight be assigned a darker color correspondingto low attenuation values.

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In the mid-1880s, Neo-Impressionist artistGeorges Seurat perfected a revolutionarytechnique of painting with tiny dots of color.Like Michel Chevrul before him, Seurat recog-nized that from a distance, the eye wouldnaturally blend together tiny dots of primarycolors to produce secondary shades. Using tinybrush strokes, Seurat and his contemporariescaptured scenes of cityscapes, harbors andpeople at work and leisure. This techniquecame to be known as pointillism.

Computers use a similar technique to displaytext and images; however, they work at a muchfiner scale. Every image portrayed on a com-puter monitor or video screen is composed ofmany, almost imperceptibly tiny dots, spacedat extremely close intervals. In a 2D picturescreen, each dot, or pixel (a word formedfrom the contraction of picture element) canbe defined by its horizontal (x) and vertical(y) screen coordinates. It is also defined by itscolor value. In color images, each pixel is alsoassigned its own brightness.

The number of shades that a pixel can take on depends on the computer and the numberof 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-bitgray-scale image, for instance, each pixelwould be assigned a value corresponding to ashade of gray, ranging from 0 to 255, where 0represents black and 255 represents white.

The number of pixels used to create animage 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 theimage acquisition system and later, by theimage display system.

Resolution in digital image acquisition systems is largely governed by the number oflight-sensitive photoreceptor cells, known asphotosites, which are used to record an image.These photosites (more commonly referred to

as pixels) accumulate charges correspondingto the amount of light passing through thelens and onto each cell.1 As more light fallsonto a photosite, the charge grows. Light isshut off to the lens once the shutter closes, at

which point the charge in each cell isrecorded by a processing chip and convertedto a digital value that determines the colorand 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

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cal c

oord

inat

es, y

200 400 600Horizontal coordinates, x

800 1,000

color

x

y

Pixel

0

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400

600

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1,000

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oord

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0 200 400 600Horizontal coordinates, x

Slice number, z

800 1,000

Voxelx

y

z

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Evolving to Industrial StrengthDensity contrasts within a rock body can beimaged just as they can within a human body(below). By the mid 1980s, CT technology wasmaking significant inroads into geoscienceapplications. In addition to quantitativedetermination of bulk density of rock samples,CT scanning was adapted to visualize microbialdesulfurization of coal, displacement of heavy oil,and oil flow through carbonate cores.7

It didn’t take long for those outside themedical field to recognize the potential of CTtechnology for nondestructive evaluation ofmaterials. Geoscientists soon joined the ranks ofother researchers, particularly those in the fieldof materials testing, who sought increasinglyfiner detail for imaging internal structures. Thiscapability has largely been realized throughdevelopment of industrial-strength CT systems,which can employ more powerful X-rays, a tighterfocal point and longer exposure times than thoseused 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): PetroleumGeology: 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 havean area of 1,280 x 960 (1,228,800 pixels), whilehigher 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 bythe medium on which it is displayed. A rela-tively low-resolution computer monitor mightbe described as a 640 x 480 display. Thismeans that the monitor has a width of 640 pix-els, spread across a height of 480 lines,totaling 307,200 pixels. If those pixels werespread across a 15-inch monitor, then anyimage displayed on that monitor would beallotted 50 dots per inch. To increase resolu-tion, either the screen size must be reducedor more pixels must be packed into thescreen. Modern applications generally takeboth approaches, squeezing a huge number of pixels into a smaller area.

To image a 3D object, the pixel is expandedinto another dimension. A third coordinate(z) is added to the x-y location to preciselydefine the pixel’s position within the volumeof a 3D object, thereby creating a voxel—short for volume pixel. In CT images, thez-coordinate often denotes depth, and is dic-tated merely by the position that atomographic slice holds within a volumeformed by stacking together numerous closelyspaced slices (previous page, bottom). Inaddition to x, y and z coordinates, a voxel candefine a point by a given attribute value. Inthe 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 tiedto a color spectrum, while a range of intensi-ties can control the opacity of a voxel on acomputer screen. With this information and3D rendering software, a two-dimensionalimage of a 3D object can be generated forviewing 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 Density, 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 notunusual for geoscientists to work out agreementswith the only institution in town that couldprovide access to such sophisticated technology.Often in the dark of night, with as little attentionas possible, core samples from the oilpatch wouldbe wheeled into the pristine and sterile setting ofa hospital CAT-scanning facility for imaging andanalysis (below).

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With the development of microCT (µCT),researchers are attaining much higherresolutions.9 Using µCT, researchers are some-times able to image their samples with voxelsizes as low as 2.5 µm. Depending on the size of asample and the number of pixels used to image it,voxel sizes of one-thousandth of the sample sizeare being attained. For example, a 1-megapixelcamera using 1,000 x 1,000 pixels couldconceivably resolve a 1-cubic centimeter sampleto about 10 µm. Similarly, a 16-megapixel camera(4,000 x 4,000 pixels) can resolve the samesample to 2.5 µm.

At such resolutions, geoscientists can distin-guish density or porosity contrasts inside a rocksample and can study pore space and poreconnectivity in great detail. This µCT technologypermits recognition of grains or cements withdifferent mineralogical compositions (right). Ithas even been used to differentiate grains of thesame type, such as those found in carbonates,where microporosity may vary between differentgrain types in the same rock.10

The Scanning ProcessThe scanning process to acquire µCT data is insome respects analogous to acquiring 3D seismicdata. A seismic crew shoots a series of regularlyspaced seismic lines. Coordinates of the startingand ending points of each line are surveyed,making it possible to infer the distance betweeneach line in the series. It is therefore possible todetermine the position of any point along anyline as well as the distance between pointswithin the series of lines. With this knowledge, aposition between any two points or lines can beinterpolated when the data are processed.

For µCT, a regular series of closely spacedscans are acquired to obtain high-resolutionvirtual slices of a sample. Each pixel in the slicerepresents a scanned point and has coordinatesthat correspond to an actual point in the sample.Because coordinates of each point are known,distances between each point and each slice canbe determined. And just like the seismic line,points or slices can be interpolated betweenexisting slices. By stacking the series of slicesclose together to make up a volume of data, eachpixel in a slice becomes part of the stack andtakes on a third dimension. In this way, eachpixel can be treated as a voxel.

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

components: an X-ray source, a rotating stage onwhich the sample is placed and an X-ray camerato record the pattern of X-ray attenuation withina sample.

To scan a sample, it must be placed on therotating stage, positioned between the X-raysource and the camera. X-rays emitted from thesource are attenuated through scattering orabsorption before being recorded by thecamera.11 The camera then records a large seriesof radiographs as the sample rotates incre-mentally on its stage through 360°. A computerprogram stacks the digital projection data whilemaintaining true spacing between pixels andslices. CT algorithms are applied to these data toreconstruct the internal structure of the sampleand preserve its scale in three dimensions.

One such device was built in 2002 by TheAustralian 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 fromthe focal point, creating a cone-beam geometry.12

Because magnification of the sample increaseswith proximity to the X-ray source, the rotatingstage and camera are designed to slideseparately on a rail, allowing researchers toadjust distances between source, sample andcamera. The sample stage can rotate the samplewith 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 ofa scintillator that fluoresces green in response toX-rays, and a charge-coupled device (CCD) thatconverts this green light into electric signals.14

The camera has a 70-mm2 active area, containing4.1 megapixels (2,048 x 2,048 pixels). Thesystem’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 quartz cement: 78%

Barite cement: 1%

Pore space:16%

Calcite cement: 5%

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image a 60-mm specimen with a 30-micron pixelsize. They can also zoom in for high-resolutionscanning to image a 4-mm specimen with 2-micron pixels.

Approximately 3,000 projections are neededto generate a 2,0483 voxel tomogram. Betweeneach projection, the sample stage is rotated0.12°. The entire process takes 12 to 24 hours,depending on the type of sample and the filteringsteps required to reduce sampling artifacts. Theresulting 24 gigabytes of projection data are

processed by supercomputer, and it takes 128central processing units about 2 hours togenerate the tomogram.

Visualization Technology Once individual radiographic projections havebeen 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, thedata can be imaged and manipulated like anyother volume of 3D data. Originally developed tohelp visualize seismic volumes based on miles orkilometers of data, Inside Reality technology canalso handle data volumes based on much finer,submillimeter scales.

Geoscientists utilize this advanced visuali-zation technology to view a data volume from anydirection. This capability enables bedding planes

and fracture planes of rock samples to be viewedorthogonally, even when the physical sample hasbeen cut obliquely to these planes. Sedimentaryand structural features of the rock sample aretypically analyzed in the form of slices ortransparency views through a volume.

While the scanning process relies on densitydifferences to distinguish features within asample, the visualization process depends largelyon opacity differences. One way to exposefeatures deep within a volume comprisingmillions of voxels is to render surrounding voxelsinvisible. Opacity rendering is the key tovisualization. Each voxel is assigned a value alonga transparency-opacity spectrum, thus makingsome voxels stand out while others fade away.Without this capability, the opacity of outer voxelswould hide all features lying within the volume.

Voxel-based technology can be used todetermine the volume and geometry of rockgrains, cement, matrix and pore space within asample. Using Inside Reality opacity-renderingtools, geoscientists can assign different values ofthe opacity-transparency spectrum to variouscomponents within a volume. This techniqueallows geoscientists to distinguish betweenmaterials of different density values. Forexample, the distribution of cement betweenmineral grains shows up as a distinctive color,while setting pore space to zero-opacity makes ittransparent, thus showing the spaces betweengrains. This allows the viewer to separate rockgrains from cement, matrix and pore space toreveal internal sedimentary and structuralfeatures (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 TomographicImagery: 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: “An X-Ray Tomography Facility for Quantitative Prediction of Mechanical and Transport Properties in Geological,Biological and Synthetic Systems,” in Bonse U (ed):Developments in X-Ray Tomography IV, Proceedings ofSPIE—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 is 1 mm long.

1.0 mm

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The ability to manipulate opacity values playsan important role in the seedpoint and volume-grower tools featured as part of the Inside Realitytoolbox. Using the seedpoint tool, the viewerselects a point within a slice or volume. This

point has a certain X-ray attenuation value. Oncea point is selected, the program can automat-ically pick all neighboring voxels of a similarvalue that are connected to that point. Thisfeature can help a geoscientist pick a point

within a volume known to represent porosity, forexample, and the volume-grower tool will display allinterconnected porosity within the volume (left).

Because each voxel is defined in part by itscoordinates, the distance between any two voxelscan be measured. To facilitate this process, theInside Reality system uses a ruler tool to providea visual scale. This tool can be used to measuregrain or pore size in three dimensions, helpinggeoscientists estimate pore-volume proportionsand connectivity.

Taking rock samples from the laboratory to animmersive visualization environment enables anasset team to share important information andconcepts about reservoir samples so they can makemore informed decisions. Inside Reality virtualreality technology lets geoscientists share 3Dvirtual core data with those in remote sites to helpasset teams collaborate with company experts andpartners around the world (below left).

Applications Rock fabric and textural data provide geologistswith key information used in analyzing facies andin determining depositional environments.Geologists and petrophysicists can now obtainimportant information about grain size, shapeand matrix from digital scans of core or corefragments. A single core-fragment image canyield thousands of individual grains. By digitallydisaggregating grains in a scanned sample,analysts can obtain coordinates of all voxelscomposing each grain, the number of neighboringgrains and grain-overlap information.15

From such a dataset, geologists can derive acomprehensive 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 LoggingSymposium, 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 teamsto 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. Theanimated avatar mirrors the pointing motions and actions of another viewerwho is interacting with these data from a remote site.

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measurements (above left). Grain volume ismeasured by counting the voxels in each distinctgrain, from which size is derived and then gradedagainst standard Udden-Wentworth or Krumbeinscales of grain sizes.16 Automated programs cantrack and classify individual grains according tograin shape characteristics of sphericity androundness or classify according to texturalcategories, such as sorting, grain contacts, and matrix or grain-support. Some programs can also measure anisotropy in grain orientationto help geoscientists ascertain sediment-transport direction.

More important than the detailed measure-ment of rock grains is the analysis of the spacebetween the grains and the contents therein.Opacity-rendering tools work particularly well inshowing what is not rock—that is, its porosity.Researchers can obtain a good picture of porosityby decreasing the opacity of dense voxelsrepresenting rock grains and cements, whilesimultaneously increasing the opacity of low-density voxels (right). This same opacity-rendering technique highlights the extent ofinterconnected porosity within the rock. Oncethe porosity is brought up on screen, geo-scientists can measure the size of pore spacesand pore throats using the ruler tool. Poreinterconnectivity can also be charted, using porenetwork models based on tomographic imaging(above right). Pore-throat and pore-size distri-bution, along with interconnectivity, figure

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

A variety of other measurements can be takenfrom tomographic images, from which importantinformation is derived. Analysts can directlycorrelate image data on pore structure andconnectivity to measures of formation factor,permeability and capillary drainage pressures.Comparisons of results obtained from µCTimages and conventional laboratory measure-ments on the same core material have generallyshown good agreement.17

Studying Effects of Carbon Dioxide on Casing CementIn an important application beyond the realm ofconventional petrophysics, µCT was used to studythe effects of carbon dioxide [CO2] on casingcement. Greenhouse gases, particularly CO2,have been linked to rising temperatures aroundthe world. Capturing CO2 emissions andsequestering them in the subsurface have beenproposed as a measure to reduce atmosphericgreenhouse-gas concentrations until low-emission energy sources become viable.18

However, CO2 becomes supercritical whentemperature 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 andtexture. 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.)

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> 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

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Pores and pore throats

> Pore-scale information derived from tomographic images. Pore centers(blue spheres), connected by pore throats (blue cylinders), are usedto 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 whilepore 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 mostmedium to deep wells.19 Therefore, an importantaspect of any CO2 sequestration project is toknow how downhole materials will react tosupercritical CO2 (scCO2).

Scientists at Schlumberger CambridgeResearch in England have collaborated withtheir counterparts at Schlumberger RiboudProduct Center in Clamart, France, to investi-gate long-term effects of CO2 storage on wellboreintegrity. One such experiment sought todetermine how scCO2 would react with casingcement.20 Long used in oil and gas wells tohydraulically isolate pay zones from the surfaceand other permeable zones, portland-basedcements play a critical role in wellbore integrity.

This study focused on a sample of neatcement.21 The cylindrical cement sample wascured for three days at 90°C and 280 bar [194°Fand 4,061 psi]. Scientists obtained CT scans ofthe cement cylinder before exposing it to scCO2.The cement was then subjected to a wet scCO2

environment and kept at 90°C and 280 bar for30 days. Two sample plugs were cut from theoriginal cylinder and then scanned.

Using Inside Reality software, researcherswere able to manipulate the data volume tovisualize porosity and microfractures and arbi-trarily slice through zones of interest. Bycomparing scans acquired before and aftertreatment, researchers noted significant changes to the cement plug, resulting from scCO2

attack. Of particular interest were the formationand distribution of microfractures, along with azone of aragonite replacement and a zone ofmineral alteration characterized by highsecondary porosity.

The reaction between scCO2 and cementproduced an irregular carbonation front,extending 4 mm [0.16 in.] from the outer edge ofthe core toward its center. This lighter coloredcarbonation front was readily apparent in thegray-scale 3D volume, and in a color-coded slice(above right). Subsequent X-ray diffractionanalysis determined that the alteration front hada different composition than the original cement,which had been replaced by aragonite. Porositywas 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 morethan 65% of its strength after only six weeks.These important observations provided animpetus for creating new blends of cement.Schlumberger researchers have developed newscCO2-resistant cementing materials that display

good mechanical behavior after exposure toscCO2 gas. Laboratory tests on these newmaterials show only a slight decrease incompressive strength during the first two days,and essentially no loss for the subsequent three months.

Examining Wormholes Caused by Stimulation TreatmentsResearchers have also used CT imaging to studythe effects of heterogeneity on carbonate matrixstimulation. In one experiment, it was instru-mental in visualizing the effects of the originalporosity distribution on acid-dissolution patterns.

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> 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.

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> 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) marksthe extent of various dissolution patterns.

Neat cement

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Stimulation treatments are commonlyperformed in wells where poor permeabilitylimits production due to naturally tightformations or formation damage. A commonstimulation technique involves the injection ofacid into carbonate formations. Acid dissolvessome of the formation matrix material andcreates flow channels that increase thepermeability of the matrix.

The efficiency of this process depends on thetype of acid used, reaction rates, formationproperties and injection conditions. Whiledissolution increases formation permeability, therelative increase in permeability for a givenamount of acid is greatly influenced by injection

formation to facilitate the flow of oil. Better still,wormholes require only a small volume of acid toproduce significant increases in permeability.Researchers are therefore investigating factorsthat influence production of wormholes.

CT scanning has proved instrumental indetermining the effects that injection rate andspatial distribution of porosity have on dissolutionpatterns formed during stimulation experiments(below). Because it is nondestructive, thistechnique allows for characterization of the corebefore and after the treatment experiment so the development and shape of the wormhole canbe evaluated.

applications, it is easy to envision the potentialspread of new applications for µCT.

The technology will no doubt proveinstrumental in improving the interpretation andapplication of laboratory and log data. As anincreasingly important tool in nondestructivetesting, its application can be extended tolaboratory testing of unconsolidated or friableformation samples. The combination of µCTimaging with numerical calculations may lead tomore accurate predictions of a wide range of rockproperties crucial to exploration, reservoircharacterization and recovery calculations.

Further applications include development ofimproved cross-property correlations anddevelopment of libraries of 3D images that will

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 settingtime 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, acidis spent soon after it contacts the formation,resulting in relatively shallow dissolution alongthe face of the injection zone. High flow ratesproduce a uniform dissolution pattern becausethe acid reacts over a large region. In either case,the resulting gains in permeability requirerelatively large expenditures of acid.

However, at intermediate flow rates, longconductive channels known as wormholes areformed. These channels penetrate deep into the

Peering into the FutureTomography is not new to the oil industry. At theupstream end of the tomography spectrum liescrosswell seismic tomography; at the downstreamend is industrial process tomography forrefineries. As a research tool, µCT is used acrossa broad suite of industrial applications to monitorperformance of polymer-enhanced foams andpolyethylene resins or to view phase separationand pore-space characterization in formationsamples. Across this range of tomographic

allow a more rigorous and quantitative descrip-tion of rock type and texture. These quantitativedescriptions can be integrated with classicalsedimentological descriptions. The technologycan also make a significant contribution to thestudy of elastic behavior, porosity-permeabilitytrends and multiphase flow properties such ascapillary pressure, relative permeability andresidual saturations.

Future technological innovations will probablyinclude higher resolution to overcome problemsin predicting porosity when micropores fallbelow the detection capability of the presenttechnique. With the improving resolution of theirsamples, µCT technology is helping today’sgeoscientists to better see their world in a grainof sand. —MV

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