state-of-the-art review of nondestructive testing with computer-assited tomography

SPESociety of Petroleum Engineers

SPE 22127

State-of-the-Art Review of Nondestructive Testing WithComputer-Assisted TomographyA. Bansal, U. of Alaska Fairbanks, and M.R. Islam, EMERTEC Developments Inc.SPE Members

Copyright 1991, Society of Petroleum Engineers, Inc.

This paper was prepared for presentation at the International Arctic Technology Conference held in Anchorage, Alaska, May 29-31, 1991.

This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper,as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessanly reflectany position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Societyof Petroleum Engineers. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgmentof where and by whom the paper is presented. Write Publications Manager, SPE, P.O. Box 833836, Richardson, TX 75083-3836 U.S.A. Telex, 730989 SPEDAL.

ABSTRACT

Computer assisted tomography (CAT) has been a

revolutionary technique in medical radiology.

Recently, CAT scanners started to be used as

non-destructive testing facilities in different

industrial applications. Most of the applications

of the new technology are in the areas of petroleum

engineering. Majority of this CAT scanner

technology has been developed in the U.S.A.

However, several Canadian companies have acquired

CAT scanners and are using for novel commercial as

well as research applications. This paper presentsa comprehensive review of advances made in the

areas of CAT scan technology as applied to thepetroleum industry.

Recent efforts in developing CAT scan technology

has been in the areas of core analysis methods.

The ultimate objective is the quantitative

monitoring of dynamic two- and three-phase

experiments in porous media. Even though advances

have been made in using CAT scan for quantitative

evaluation of fluid flow, more research has to be

done before gaining confidence m such a technique.

This paper presents a critical review of recent

research publications. Also, discussed is the

References at end of paper

523

possibility of using CAT scan as a unique reservoir

characterization tool. Reservoir properties, such

as direction and frequency, of fractures, thepresence of vuggs, stratification in the bedding

plane, etc. may be identified only through

non-destructive testing of reservoir cores.

Various applications of CAT scanning to oil

industry are discussed in detail.

PRINCIPLES OF CAT

Recently, Kantzas1 has outlined in detail the

principle of computer assisted tomography.

Consequently, we will describe the principle only

briefly. The original theory for reconstructing a

complete image of an object from numerous views

around the object was first described by Radon in

19172• The principle of CAT is based on emitting

x-ray from a source which revolves around the

object in consideration while one-dimensional

projections of attenuated x-rays are collected by a

detector on the other side of the source. These

projections are collected as the sample travels

through the scanner longitudinally and are used to

reconstruct a three-dimensional image of the

object. Intensity values of attenuated x-rays are

collected from small volumetric elements, called

pixels. These elements are typically O.76mmxO.76mm

2STATE-OF-THE-ART REVIEW OF NON-DESTRUCTIVE TESTING

WITH COMPUTER-ASSISTED TOMOGRAPHY SPE 22127

2n 00

f(r,lP) = #(r,lP) = J J F(R,O) e2niRrcos(IP-0)IRldRdO

o 0

where 1 is the distance along the scan line, ° is

the angle describing the scan line, R is the

conjugate Fourier frequency variable and p(l,O) is

the transmission of a particular ray described by

(1,0). The distribution is then found by

transforming back into real space, given by:

where M(r, IP) is the linear attenuation coefficient

given as a function of r and IP. The variable r is

obtained from 1 by r = 1 cos( IP-O) and arises from

the Jacobian of a coordinate transformation. The

angle IP represents a point in real space and the

angle ° represents a point in Fourier space. The

above two equations are sufficient for

reconstructing a two-dimensional object from its

Here dl IdE represents the spectral distribution ofo

intensity, e(E) the efficiency of the detector at a

given energy E, el the lower energy and eh the

higher energy. Also, # is now a function of:

spatial coordinates as well as photon energy. One:

can see immediately that the volume of data as well

the complexity of data processing increases

drastically by implementing this relationship. As

a consequence, this relationship is not commonly

used. In stead, an effective energy is assumed to

replace a spectrum of energy and the resulting

error is corrected by manipulating the data for

beam hardening effects. The resulting problem,

thus, involves in determining two-dimensional

distribution of p(x,y) from the line integrals.

Several methods are available for solving this

problem. One such method is the Fourier method

derived from the Central Projection Theorem. This

theorem states that the Fourier transforms of a

one-dimensional projection of a two-dimensional

distribution are mathematically, identical to a

section through a two-dimensional Fourier transform

of the original distribution. The Fourier

transform of the transmission profiles is given in

polar coordinates as

+00

J p(1,O) e-2niRl d1

-00

F(R,O)

eh d

I = J(dI/dE) e(E) exp [ - J#(x,y,E) ds ] dE

e l s

where #* is the linear attenuation coefficient, L

is the path length, I is the incident intensityo

and I the transmitted intensity. Of course, #* is

characteristics of the medium and reflects the

nature of the medium. This relationship applies

only for a narrow monoenergetic beam of x-ray

photons which travels across a homogeneous medium.

If the medium in consideration is heterogeneous,

the above equation holds true while replaced by the

line integral of the linear attenuation

coefficients. The modified form is:

d

In(l/I) = J#(x,y)dL

s

,/ = (lIL) In(III)o

in area and 1 cm in depth (along the direction of

the x-ray beam) for a second generation CAT

scanner. Once these elements are all assigned an

intensity values after a complete radial and

longitudinal scan, these data are processed by a

computer. This processing constitutes the major

part of the CAT. The inlet intensity and the

outlet intensity are related through the following

relationship,

where dL is the differential of path length along

the beam, s is the x-ray source, and d is the

detector. As compared to the equation, a unique

value of # is replaced by a distribution #(x,y) in

plane perpendicular to the direction of the x-ray

beam. This relationship calls for a large memory

for storing all the information even for a

relatively small sample. Besides, this

relationship is still quite idealized as it assumes

the x-ray beam to be monoenergetic. In reality,

the beam includes a wide range of photons ranging

from 20 keY up to 120 keY in some instances. Also,

the detector system in itself is energy dependent.

Consequently, a rigorous representation of the

intensity relationship is given by

524

SPE 22127 A. Bansal and M.R. Islam 3

one-dimensional projections. Before this

conversion can be used, the Fourier coefficients

need to be converted from polar to rectangular grid

coordinates and the two-dimensional Fourier

transform needs to be taken to obtain the density

distribution on a Cartesian grid which is usually

used for computer display systems.

Another method, called the Convolution Method or

Back Projection, is more commonly used for

commercial CAT systems. This method is

mathematically close to the Fourier method. In

this method, the process is represented by

where p is the projection value and LI Ok is the

angular increment between projections. The above

relationship is equivalent to throwing back the

projection data for each of the various views into

the reconstruction plane and summing them for

obtaining an image. Obviously, this is a very

crude state of reconstruction and is known to bethe distorted version of the correct solution.

This distortion is removed by the two-dimensional

convolution of the degraded image with an

appropriate correlation function or by taking the

two-dimensional Fourier transform of the image

while multiplying by an appropriate filter and

performing the inverse Fourier transform. These

methods require considerably large computer time

and are known to give less than optimal results

with real data. This calls for an alternative

method, called the Filtered Back-Projection method.

This method uses correction and filtering of data

prior to back projecting. The filtering is done as

a convolution operation in the spatial domain or as

a functional multiplication in the Fourier

frequency domain. The Fourier filtered back

projection method multiplies the Fourier transform

of the projection data by a ramp-shaped filter

which has a magnitude proportional to the Fourier

frequency variable associated with the linear

position of the projection data, 1. The

convolution theorem dictates that this spatial

I frequency filtering is equivalent to convoluting

the projection data with a corresponding filter

525

function in the spatial domain. The corrected

projection data, p. (r.,O) is obtained from the1

measured projection data, p(r.,O), for any given1

projection angle 0, by the following relationship

p·(r.,O) = (p(r.,O)/4a) - (1I1r2a)[Z p(r.,O)/(i-j)2]1 1 J J

while counting only odd values of (i-j). In this

.relationship, a is the sampling interval. The

image is formed by back projecting these corrected

projection data onto a suitable two-dimensional

matrix by transforming back into real space as

shown by an earlier equation. The result of the

reconstruction process is, thus, a two-dimensionalarray of numbers. Several methods may be used for

representing the reconstruction matrix. These

matrix values are directly related to the linear

attenuation coefficient distribution of the object

at a given effective x-ray energy. Before storing

this information into a computer, the linear

attenuation coefficients are scaled to integer

values covering a given range and commonly adjusted

so that the value of water is zero. The following

equation relates the linear attenuation

coefficients to the number stored in computer

(known as the CT numbers or CTn)

where p and p are the linear attenuationwater

coefficients of the object scanned and of water at

the effective energy of the beam, and K is a

scaling factor equal to the number of CT values·

between air and water. Most present scanners use a

value of 1000 for K. Therefore, CT numbers range

from -1000 to +1000 with water corresponding to a

CT number of O. Also, each CT number corresponds

to 0.1 % of the linear attenuation coefficient of

water.

DETERMINATION OF PHYSICAL PROPERTIES

WITH CATSCAN

One of the most successful application of CAT

scanning lies in determining physical properties of

rocks non-destructively. The CAT Scanner produces

two-dimensional arrays of data that are called CT

numbers. In order to correlate the CT numbers to



oillwater system, saturations are given by

S = A/Co

S 1 - Sg 1

For a liquid gas system, the following equations

provide estimation of saturations:

cP(CT -CT )o w

S = 1 - Sw 0

(CT -CT ) - cP(CT -CT )mew 3S =

o

For a three-phase system, saturations are given by

the following equations:

CTm-CT1

ij) (CT - CT)1 •

where subscripts a,w,0 denote air, water, and oil,

respectively, whereas m and e represent saturated

and dry state of the core, respectively. CT refers

to CT numbers of corresponding fluids (as given by

the subscripts).

Z3 0 8

JJ = p (a + P E3 0 2)

where a, p are constants, E is the mean photon

energy in keV, p is the bulk density and Z is the

effective atomic number of the sample. For

energies above 100 keY, x-rays interact with

materials more vigorously by Compton scattering

which is dependent on bulk density of the sample.

For x-ray energies below 100 keY, photoelectric

absorption becomes a dominant phenomenon and

depends on effective atomic number of each pixel.

Therefore, a dual energy system is used for usual

CAT scanning. The high energy scan can be used for

determining bulk density while the second scan at

low energy can be used for determining effective

atomic numbers. At this point, the porosity of the

pixel is given by

specific physical properties, one has to seek

relationship between linear attenuation

coefficients and other physical properties. Vinegar

and Wellington3 provided a detailed discussion on

this technique. They showed that the linear

attenuation coefficient is a function of the bulk

density and the effective atomic number of the

sample, given by

S = B/Cw

where, Pb is the bulk density, Pg is the mineral or

grain density, and Pf is the fluid density. Two

scans of the same porous medium would provide one

with bulk density, grain density (mineralogy) as

well as porosity. This would, in turn, provide one

with information on heterogeneities in a porous

medium.

S =1-S-Sg 0 W

where A, B, and C are defined by:

A = (CT -CT) (CT -CT) - (CT -CT) (CT -CT)m el W g2 m e2 w gl

B = (CT -CT) (CT -CT) - (CT -CT )I(CT -CT)2m e2 0 gl mew ~

Fluid saturation in a dynamic or static system can

be measured with CAT scanning as well. However,

typically scanning period has to be faster than

movement of fluid flow in a porous medium. This

could be a severe limitation if one wishes to study

the effect of high flow rates in immiscible or

miscible flood experiments. For a two-phase

C = cP«CT -CT ) (CT -CT) - (CT -CT )2(CT -CT )1)o gl w g2 0 g w g

Subscripts 1 and 2 refer to the scan at energy

levels of 1 and 2, respectively. These equations

are somewhat different from those of Wellington and

Vinegar4 in the sense that CT numbers are used of

those of fluids rather than cores saturated with

respective fluids. Kantzas~ as shown these two

formulations to be equivalent to each other.

526


APPLICATION OF CAT IN OIL INDUSTRY

Multiphase Flow Visualization and

Relative Permeability Measurements

Wang et aI.6 are reportedly the first researchers

to publish on CAT application of multiphase fluid

flow in porous media. They reproduced the image of

saturation distribution during an immiscible

liquid-liquid displacement in a Berea sandstone

core7

• The technique developed by them proved to

be very useful when they were successful in

reproducing images of viscous fingering during a

dynamic displacement test8• They also reported

time derivatives of local composition and residual

oil distribution during waterflooding experiments.

Reservoir engineering research with CAT scanners

was further advanced by Cromwell et aI. 9 who

reproduced images of fluid distributions in Danianchalk and Berea sandstone.

. h' k d k 10-11More recently, WIt ~ac an co-wor ers

described techniques for rock property evaluation

and fluid flow visualization. They determined oilwater relative permeability by using CAT scanners

and the results were comparable to those determined

by conventional relative permeability measurement

techniques. Saturation results were within 2%whereas porosities were within 1% of those

determined by conventional techniques. Further

validation of saturation measurement techniques

with CAT scans was done by Manjanth12 who used the

CAT scan technology for verifying the Buckley

Leverett theory of immiscible fluid displacement.

Mohanty and Miller13 studied the factors affecting

unsteady-state relative permeabilities of a

mixed-wet reservoir rock using a CAT scanner. They

showed remarkable success in determining two-phase

relative permeability using this technology. Their

work revealed a number of regimes for the cross

sectional average saturation profile during

waterfloods at low rates. These regimes were

identified as frontal movement (up to 0.15 PV

injected), a non uniform saturation increase (up to

0.6 PV injected) and a uniform saturation increase.

Cross sectional averages of saturations determined

by CAT scanning revealed that the saturation

527

profiles determined by the JBN method at the outlet

of the core are incorrect during early. stages of

fluid displacement (up to 1 PV throughput).

Chatzis et al. 14 modified a medical CAT scanner to

study the saturation profiles during two- and

three-phase flow in gravity-assisted gas injection

processes. They reported relative success in

defining two-phase flow. The description of

three-phase flow was rather qualitative.

In order to investigate the effect of a third phase. I 1s d 'don capillary pressure, Dehgham et a. etermlDe

the height-saturation plot by CAT scanning using a

fourth generation GE 9800 scanner. They found

excellent agreement of the CAT scanning technique

with the conventional Hassler and Brunner method.

MacAllister et aI. 16 reported gas-oil relative

permeability data obtained through CAT scanning.

They found that two-phase relative permeability

could be very accurate since the in situ

saturation is being determined by solving twoequations only (one phase is measured directly, the

other by difference). However, the calculation of

three-phase relative permeability was found to be

questionable since one phase was estimated

independently. Consequently, determination of the

phase which was determined by difference was deemed

to be less accurate. However, in this study the

authors did not use the dual scanning approach.

Among others, they were able to observe non-uniform

saturation profiles during early stages of fluid

displacement. This was in agreement with Mohantyand Miller13 •

Hicks17 reported porosity and the distribution of

residual oil in heterogeneous carbonate cores from

the Fenn-Big Valley reservoir of Alberta and the

Taylor-Link Field of Texas. They also calculated

semivariographs and porosity distributions in order

to quantify rock heterogeneities.

Lenormand et aI. 18 verified a two-phase network

simulator by comparing numerical results with those

obtained with CAT scanning. Immiscible gas

injection behavior visualized by the CAT scanner

was in good agreement with numerical results.


WITH COMPUTER-ASSISTED TOMOGRAPHY SPE 2212;

Kantzas et al. 19 presented saturation profiles of

both imbibition and drainage immiscibledisplacements using a CAT scanner. They reported

difficulty in determining saturation profiles in a

three-phase flow system. Also, during two-phase

flow, they had to use dopants for better contrast

between fluids. This prohibited them from using

native fluids in the system. In one instance, the

residual oil saturation following a waterflood was

reported to be 20% as determined by CAT scan. Thiscompared to 38% as determined by material balance.Overall, their study showed the poorest agreement

so far reported on two-phase flow. Authors

provided no explanation for this poor measurement

with the CAT scanner.

Applications in Heavy Oil

Fransham and Jelen20 were the first researchers to

report flow visualization of a heavy oil

displacement using CAT scan technology. Their

technique allowed them to determine dynamic changes

in saturations during a waterflood of a heavy oil

saturated core. They also visualized fingeringphenomena both in macroscopic and microscopic

levels. Similar experiments were also reported bySedgwick and Miles-Dixon21

• More recently, Cuthielland Sedgwick? applied the CAT scanner to heavy oil

corefloods. They studied both steady-state and

dynamic waterlbitumen displacement tests using a

CAT scanner.

Reservoir Characterization

Honarpour et al.23 were one the first researchers

to use CAT scanners for reservoir characterization.

They determined rock heterogeneities, permeabilitybarriers, fractures and their orientations in North

Sea Danian chalk and Wyoming Phosphoria formations.

In the same line, Bergosh et al.24 used the CATscan technique for core analysis of naturally

fractured reservoirs. They found the techniquehelpful for characterizing fracture width, spacing,

tortuosity, interconnectivity and drilling mud

invasion. Honarpour et al. 25 further applied theCAT scan technology for reservoir characterization

studies. They used dolomite samples from the Upper

528

Madison Limestone Group. They successfully

identified mineral densities and their distributionwithin a fracture system. This application of CAT

for mineral identification was indeed novel. This

technique requires both density calibration and

contrast. They felt that in order to gain more

confidence, this technique should be compared with

other techniques. Unlike previous idea of rather

idealized fracture infilling, they observed that

fractures exhibited variable densities and

therefore multiple types of mineralization. Theydetected the presence of dolomite, calcite, quartz,

gypsum, anhydrite, illite, and chlorite minerals in

naturally occurring fractures. Bergosh and Lord26

conducted further studies on analysis of naturallyfractured reservoirs. They demonstrated that the

fracture porosity could be easily determined by CATscanning.

Kantzas et al. 19 recently presented a bulk of

information on seven Canadian reservoirs regarding

the use of reservoir characterization with a CAT

scanner. This was a follow-up study of a previous

investigation21• Their study could not be proven

to be useful for any quantitative characterization.

However, they provided useful information about theextent of core heterogeneities, presence offractures and fissures, etc. within the corelength.

A new method for imaging the three dimensionalmicrostructure of porous media was presented by

Jasti et aes. This method was based on high

resolution x-ray CT where a cone shaped divergingx-ray beam is used to generate two dimensional

transmission images. A three-dimensional

reconstruction array is created in this methoddirectly as opposed to series of two dimensional

slices as in conventional CAT. The authors

demonstrated the new technique using an

unconsolidated pack of glass beads. Even thoughthe technique appears to have great potential in

future, very little useful information was provided

at this stage of research.


Core Analysis and Well Logging

H 129unt et a . presented the ftrst detailed

technique for using CAT scan in core analyses.They noted that qualitative core analyses could bedone with relative ease using the CAT scan

technology. They showed application of CATscanning to

- determine the extent of drilling mud invasion

- detection of fractures

- characterization of preserved cores

screening of core prior to laboratory investi

gation

quantitative determination of porosity, permeability and mineralogy.

Kantzas et aeo reported modiftcation of the

hardware on a medical CAT scanner for adopting the

machine to the need of core analysis. They

modifted the CAT scanner for conducting

displacement tests on long cores and at any angle

of inclination. Using this machine, they studied

physical properties of porous rocks.

investigating damage to unconsolidated cores.

Auzerais et al.34 calculated densities and

effective atomic numbers for several anisotropic

rocks using CAT scanning as a part of a complete

core characterization project. Among others, theyconcluded that permeability variations are due to

variations in grain size and packing.

During an investigation into describing dual

porosity systems, Moss et al.35 presented an

algorithm for measurements of the constituent

porosities in a dual porosity matrix. They

saturated the core with a dopant and used an image

subtraction technique during scanning at differenttimes. Their technique is very similar to that of

Withjack et al. 36• Moss et al.35 suggested that

one can identify the easily accessible fractureporosity from the matrix porosity by following the

time sequence of the penetration process. As

examples, they applied the technique on coal and

shales. They observed that the width of the

porosity distribution is a good indicator of

heterogeneity in porosity. They also found that

porosity images can lead to statistically accurateporosity distributions.

One of the rare applications of CT scanning in

unconsolidated reservoir cores has been addressedby Kantzas et al.27

•32

• They used a medical x-ray

CAT scanner for characterizing a Canadian heavy oil

reservoir. Reservoir cores were collected and

rubber sleeves and were frozen in order to preserve

the form of the cores. Using a new post processing

software l, they determined a detailed image of

heterogeneities within each core section. These

heterogeneities included cracks, shale barriers,

and clay clasts. They also determined bulk densityas well as porosity of the reservoir cores. They

suggested one of the more interesting uses of CAT

scanner, Le. to identify locations in a core for

good plug selection. In addition, they reported

three- dimensional mapping of heterogeneities.

This line of work was further advanced by Gililand

and Coles33 who used the CAT scanner as a tool for

Narayanan and Deans3l used a

Deltascan 2020HR to validate

heterogeneous porous media.

fourth generation

a model for

529

Withjack et al.36 studied miscible displacement as

well as heterogeneities using a CAT scanner. The

focus of this study was to study heterogeneitycaused by permeability variation on a sub beddingplane scale. For the first time, they reported a

method for predicting core permeability using the

CAT scanner. The method is based on a bundle tube

model. In order to determine permeability, they

had to use results of miscible displacement tests.

This method assumes linear pressure drop along the

core. Such an assumption, even though a reasonableone for stratified heterogeneities, is not

satisfactory for vuggy and fractured cores. The

authors reported an excellent correlation betweenporosity and permeability.

Miscible Flooding and Mobility Control

CAT scanner was put into use for visualizing both

immiscible and miscible corefloods by Hove et

137-38 Th CAT . .a . e scannmg techmque allowed these

8

STATE-OF-THE-ART REVIEW OF NON-DESTRUCTIVE TESTINGWITH COMPUTER-ASSISTED TOMOGRAPHY SPE 22127

Cuthiel and Sedgwick22 studied two-phase

bitumen/water flow, steam flooding, and foam

propagation with the aid of a CAT scanner.

Withjack and Akervollll used the CAT scan

technology for studying miscible displacement in a

3-dimensional 5-spot scaled laboratory model. They

were very successful in describing miscible

displacement in a qualitative format.

presented suggestions for designing high-pressure

core materials for using CAT under realistic

reservoir conditions. Among others, they derived

fundamental equations for CAT scanning for

reservoir applications and listed examples of

applications of CAT in different areas, such as,

measurement of compressibility and compaction,

characterization of core materials, correlation of

core with well logs, multiphase core flood studies,

etc.

researchers to observe viscous fingering as well as

dispersion phenomena. They conducted the

corefloods using North Sea sandstones.

Some of the first applications of CAT in oil

research were done by the Shell Research Center of

Bellaire. Wellington and Vinegar4 used CAT scan

for studying mobility control with CO2

foam. They

used aluminum vessels for scanning high- pressure,

high-temperature core floods. It is well known

that aluminum exhibit very low x-ray attenuation.

Linear attenuation coefficients were used for

calculating in situ saturations. They used dual

energy scanning in order to image the three-phase

saturations. To obtain both density and effective

atomic number, CAT images are usually taken at two

x-ray energies. One energy is high enough for the

x-rays to be predominantly Compton-scattered and

one is low enough for them to be mostly

photoelectrically absorbed. Typically, a

combination of the two images for each pixel can be

used to generate separate images of bulk density

and effective atomic numbers. Wellington and

Vinegar4 used dual energy scanning to image the

three-phase fluid saturations in a core. Following

equations were used to measure saturations:

31 edNarayanan and Deans report

miscible displacement experiments in

carbonate rock samples under

conditions.

visualization of

highly vugular

high-pressure

miscible

A

cores.of the

They

with

Wellington and Vinegar39 studied mobility control

of CO injection using surfactant to generate foam.2

They reproduced images which identified several

mechanisms which explained the nature of mobility

control foam and how it renders CO2

stable against

gravitational and viscous forces during miscible

flood experiments.

Liu et al. 4O used the CAT scan technology to

monitor transient foam flow in laboratory linear

foam flood experiments. CAT scan results allowed

them to observe that the foam-displacement is not

piston-like. Gas channeling was observed near the

front whereas the foam eventually blocked thesechannels.

Hicks et al.41 studied vertical

displacements in heterogeneous carbonate

They were able to observe the propagation

displacement front using a vertical scanner.

compared numerical simulation results

experimental local and effluent concentrations.

Pu - P S + P S + /l S01 0 wI w gl g

/lu - P S + /l S + /l S02 0 w2w g2 g

S + S + S = 10 w g

where p is the total equivalent attenuation

coefficient of the core with several components

whereas Pw' Po' and P g are the attenuationcoefficients for the core saturation 100% with

water, oil, and gas, respectively. The subscripts

1 and 2 refer to energy level of 120 and 90 keV,

respectively. This was one of the first works

reported on detection of three phase flow with CAT

scanning under high temperature and pressure

conditions. In this study, 40% l-Iodododecane was

used as a dopant for the oil phase. In a later

study, Vinegar and Wellington3 outlined the theory

of CAT applications in reservoir engineering

studies. In this work, they discussed the optimum

energy settings and choice of dopants. They also

530


good agreement between these two results wasobtained.

ACKNOWLEDGEMENTS

REFERENCES

5. Kantzas, A., "A Theoretical and ExperimentalStudy of Gravity Assisted Inert Gas Injection asa Method of Oil Recovery", Ph.D. Thesis,University of Waterloo, Canada, 1988.

4. Wellington, S.L. and Vinegar, H.J., "CT Studiesof Surfactant-Induced CO

2Mobility Control",

paper SPE 14393 presented at the AnnualTechnical Conference an Exhibition of the SPE,Las Vegas, NV, 1985.

This study was made with partial financial support

of the Department of Petroleum Engineering,

University of Alaska, USA and the EMERTEC

Developments Inc., Calgary, Alberta, Canada.

S., and Cryte, C.C.,Tomography for theDisplacement in Porous

J., vol. 24 (1984) 53.

6. Wang, S.Y., Ayral,"Computer-AssistedObservation of OilMedia", Soc. Pet. Eng.

2. Waggener, R.G., Kereiakes, J.G., and Shalek,R.J., (eds.) "CRC Handbook of Medical PhysicsVolume IT", CRS Press (1984).

3. Vine¥ar, H.J. and Wellington, S.L., "TomographicImagmg of Three-Phase Flow Experiments", Rev.Sci. Instrum., vol. 58 (1), (1987) 96.

1. Kantzas, A., "Investigation of PhysicalProperties of Porous Rocks and Fluid FlowPhenomena in Porous Media Using ComputerAssisted Tomography", In Situ, vol. 14 (1)(1990) 77-132.

Withjack et al. 36 provided visualization of

miscible flooding in a well characterized Berea

core for a range of mobility ratios. Flow

visualization of the miscible displacement allowed

them to observe finger propagation in a

heterogeneous porous medium. They observed that

with increasing mobility ratios, displacements

become less dominated by local permeability

variations. A characteristic central finger was

observed for all high mobility ratios. They

concluded that displacement characteristics are lot

better understood by in situ saturation

determination through CAT scanning than that

provided by 2-D theory or by assumptions of

homogeneous systems.

Peters and Hardham42 conducted a series of

immiscible and miscible displacement tests in sand

packs for flow visualization. If a miscible

displacement is stable, they claimed that the

displacement data may be used to determine

longitudinal dispersion coefficient in a porous

medium. They also suggested that numerical models

could be calibrated by the saturation profiles

generated by a CAT scanner.

COMCLUSIONS

A historical review of the computer assisted

tomogrphy (CAT) as applied in the petroleum

industry is provided. Also, discussed is the

principle of CAT and basic density and saturation

measurement technques as applied to rocklfluid

systems. The review shows the usefulness of the

technique in a wide range of areas of petroleum

engineering and petroleum geology. CAT scan

technology continues to be improved and, as this

review reveals, a lot more progress has to be made

before a quantitatively useful information can be

derived in the areas of both reservoir

characterization and saturation profile monitoring.

However, even now the technique is unique in

testing samples in a much desired non-destructive

fashion.

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531



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"Newfrom

17479Reg.

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13. Mohanty, K.K. and Miller, A.E., "FactorsInfluencing Unsteady Relative Permeability of aMixed-Wet Reservoir Rock", SPE 18292 presentedat the Annual Technical Conference andExhibition of the SPE, Houston, TX, 1988.

11. Withjack, E.M. and Akervoll, I., "ComputedTomography Studies of a 3-D MiscibleDisplacement Behavior in a Laboratory Five-SpotModel" , SPE 18096 presented at the AnnualTechnical Conference and Exhibition of the SPE,Houston, TX, 1988.

12. Manjnath, A., "Examination of the BuckleyLeverett Theory Using Computerized Tomography",SPE 17491 (abstract) presented at the SPECalifornia Reg. Meet., Long Beach, CA, 1988.

15. Dehghani, K., Bansal, A., Ogbe, D.O., andOstermann, R.D., "The Effect of the Presence ofa Third Phase on Capillary Pressure byCentrifuge Method and CT Scanning", SPE 18793presented at the SPE California Reg. Meet.,Bakersfield, CA, 1989.

16. MacAllister, D.J., Miller, K.C., Graham, S.K.,and Yang, C.T., "Application of X-Ray CTScanning to the Determination of Gas-WaterRelative Permeabilities" , SPE 20494 presentedat the Annual Technical Conference andExhibition of the SPE, New Orleans, LA, 1990.

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18. Lenormand, R., Kalaydijan, F., Bieber, M.T.,and Lombard, J.M., "Use of MultifractalApproach for Multiphase Flow in HeterogeneousPorous Media: Comparison with CT-ScanningExperiment", SPE 20475 presented at the AnnualTechnical Conference and Exhibition of the SPE,New Orleans, LA, 1990.

19. Kantzas, A., Marentette, D.F., and Jha, K.N.,"Computer Assisted Tomography: FromQuantitative Visualization to Quantitative CoreAnalysis", paper CIMIAOSTRA 91-72 presented atthe Technical Conference of the CIMIAOSTRA,Banff, Alberta, Canada, 1991.

532


31. Narayanan, K. and Deans, H.A., "A Flow ModelBased on the Structure of Heterogeneous PorousMedia", SPE 18328 presented at the AnnualTechnical Conference and Exhibition of the SPE,Houston, TX, 1988.

32. Kantzas, A., Marentette, D.F., and Skalinski,M., "Computer Assisted Tomo~raphy as aComplementary Tool to Well Loggmg", paper P,presented at the 12th Formation EvaluationSymposium of the Canadian Well Logging Society,Calgary, Alberta, Canada, 1989.

33. Gililand, R.E. and Coles, M.E., "Use of CTScanning in the Investigation of Damage toUnconsolidated Cores", SPE 19408, presented atthe SPE Formation Damage Control Symposium,Lafayette, LA, 1990.

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35. Moss, R.M., Pepin, G.P., and Davis, L.A.,"Direct Measurement of the ConstituentPorosities in a Dual Porosity Matrix" , paper9003, presented at the Fourth Annual TechnicalConference of the Society of Core Analysis,Dallas, TX, 1990.

36. Withjack, E.M., Graham, S.K., and Yang, C.T.,"Determination of Heterogeneities and MiscibleDisplacement Characteristics in Corefloods byCT Scanning", SPE 20490 presented at the AnnualTechnical Conference and Exhibition of the SPE,New Orleans, LA, 1990.

37. Hove, A.O., Ringen, J.K., and Read, P.A.,"Visualization of Laboratory Corefloods withthe Aid of Computerized Tomography of X-rays",SPEFE, vol. 2 (1987) 148; paper originallypresented as SPE 13654 at the SPE CaliforniaReg. Meet., Bakersfield, CA, 1985.

533

38. Hove, A.O., Nilsen, V., and Jenkes, A.,"Visualization of Xanthan Flood Behavior inCore Samples by Means of X-ray Tomography", SPE17342 presented at the SPE/DOE Enhanced OilRecovery Symposium, Tulsa, OK, 1988.

39. Wellington, S.L. and Vinegar, H.J.,"Surfactant-Induced Mobility Control for CarbonDioxide Studied with Computerized Tomography",Surfactant-Based Mobility Control, Chapter 17,ACS, 1988.

40. Liu, D., Castanier, L.M., and Brigham, W.E.,"Analysis of Transient Foam Flow in I-D PorousMedia with CT" , SPE 20071 presented at theSPE California Regional Meeting, Ventura, CA,1990.

41. Hicks, P.J., Jr., Narayanan, R., and Deans,H.A., "An Experimental Study of MiscibleDisplacements in Heterogeneous Carbonate CoresUsing X-Ray CT", SPE 20492 presented at theAnnual Technical Conference and Exhibition ofthe SPE, New Orleans, LA, 1990.

42. Peters, E.J. and Hardham, W.D., "Visualizationof Fluid Displacements in Porous Media UsingComputed Tomography Imaging", J. Pet. Eng.Sci., vol. 4 (1990) 155.

state-of-the-art review of nondestructive testing with computer-assited tomography

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