Sampling Viscous Oil Sands Sampling, Preserving and Testing Heavy Oil Sand Cores Maurice Dusseault.

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Sampling Viscous Oil Sands

Sampling, Preserving and Testing Heavy Sampling, Preserving and Testing Heavy Oil Sand Cores Oil Sand Cores

Maurice Dusseault

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Canadian ExperienceCanadian Experience

Heavy oil core samples exhibit expansion of 1% to as much as 12%

Lab values of = 34-40% are quite common

Porosities in the ground, calculated from geophysical logs, are consistent – 29-31%

Reservoir parameters based on expanded core samples can give serious problems

The expansion cannot be fully reversed by using overburden pressures on specimens

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International ExperienceInternational Experience

Kazakhstan - Karazhanbas - largest heavy oil field in the FSU – core damage issues

Venezuela – Orinoco Extra Heavy Oil Sands – largest extra heavy oil deposit in the world – severe core damage issues

China – Liaohe and Karamay – damaged heavy oil sands core leads to problems

Colombia – Magdalena heavy oil fields – damage leads to poor choice of technology

Oman, Ecuador, USA, …

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Karazhanbasmunai(i.e. Karazhanbas

Oilfield)

Aktau

KARAZHANBASMUNAI - KZKARAZHANBASMUNAI - KZ

Kashagan Tengiz

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Study AreaStudy Area

Roads

O il P ipelines

Ku lsa ry

TENG IZ

N. BUZACHI (Texaco)

ARM AN(Kerr-M cG ee)

KARAZHANBA SSHE LF

Te n giz

Aktau

C A S P I A NS E A

KALAM KAS(M M G )

As trakhan

Atyrau

K arazhanbas(NECL )

PreCaspian Basin

Roads

O il P ipelines

Ku lsa ry

TENG IZ

N. BUZACHI (Texaco)

ARM AN(Kerr-M cG ee)

KARAZHANBA SSHE LF

Te n giz

Aktau

C A S P I A NS E A

KALAM KAS(M M G )

As trakhan

Atyrau

K arazhanbas(NECL )

PreCaspian Basin

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The ProblemsThe Problems

Data, based on core analyses only, gave… Porosities of 33-38% (~31%) Permeabilities of 4-8 D (~2-3D) Gas saturation of 1-8% (Sg = 0)

High kw (low kw)

The field was operated on a 50% production share basis. Issues… Service company problems (core tests…) Regulatory agency problems (KZ - CDC) Poor predictions of recovery and rates Poor choice of technology

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Faja del Orinoco (XH Oil)Faja del Orinoco (XH Oil)

Extra-heavy crude oil deposits

> 1.21012 BOIP ~ 0.30, z ~ 450-

800 m, So ~ 0.88 Unconsolidated Estimated 25%

recoverable with current methods

SAGD – CHOPS – HWCS …

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Faja del OrinocoFaja del Orinoco

-On the order of 200109 m3 OOIP

-1000-6000 cP oil, <10ºAPI

- = 30%, k = 1-15 D

-z = 300-700 m

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Faja StratigraphyFaja Stratigraphy

Cambrian/Cretaceous sedimentary rocks

Architecture

Lo

wer

del

ta p

lain

Up

per

del

ta p

lain

Seq.

Strat.

MFS

Top F

Top E2

Top E1

Top D3

Top D2

Top D1

M14

Base level cycles GR

200

ft

Lo

ng

ter

m b

ase

leve

l ris

e

Lo

ng

ter

m b

ase

leve

l fal

l

E2F

E1

D3

D1

C2

C1

B

A

D2

TS/MFS

Unconformity

M9

M1

MFS

Base F.

MFS

MFS

16.8

17.0

17.3

17.1

19.1

23.8

Ma

Allu

vial

/Up

per

del

ta p

lain

M12

DELTAIC

FLUVIAL

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The ProblemsThe Problems

Core damage led to: Excessively high permeabilities High lab compressibilities (40-10010-6 psi-1)

This led to a “belief” in compaction drive Lake Maracaibo heavy oil reservoirs benefit

substantially from compaction drive Vast sums of money and field experiments

Based on expanded core properties Finally, in the 1990’s, the issue disappeared,

but only after much time and money was spent

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Evidence of Core DamageEvidence of Core Damage

Porosity vs. Permeability - Edam Core

0.25

0.270.29

0.310.33

0.350.37

0.390.41

0.43

0 1000 2000 3000 4000 5000 6000 7000

Permeability - mD

Po

ros

ity

Typical value, good heavy oil sands: 31%

EDAM Field, Well 15-29EDAM Field, Well 15-29

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Edam Core, Well 7-30Edam Core, Well 7-30

Porosity vs. Permeability - Edam Core: 7-30

0.20.220.240.260.28

0.30.320.340.360.38

0.4

0 2000 4000 6000 8000

Permeability - mD

Po

ros

ity


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Well 13-29 – Edam FieldWell 13-29 – Edam Field


0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 2000 4000 6000 8000 10000

Permeability - mD

Po

ros

ity


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Edam Well 6-29 Edam Well 6-29


0.2

0.220.24

0.26

0.280.3

0.32

0.340.36

0.38

0 1000 2000 3000 4000 5000 6000 7000

Permeability - mD

Po

ros

ity


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Conclusions from Edam CoreConclusions from Edam Core

Lab porosities are consistently too high The “correction factor” is not consistent among

different wells Different coring practices and operators Different hardware Different treatment in transport, storage, lab

The differences are not trivial We may expect other properties to be affected,

often to the detriment of the company Can these issues be resolved?

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Evidence of Core ExpansionEvidence of Core Expansion

Observed Expansions of 89mm Core: Ironstone 89 mm Basal clays, clayey silts 89-91 mm Oil-poor to oil-free silty sands 90-93 mm Fine-grained oil-rich sand 91-95 mm Coarse-grained oil-rich sand 94-95 mm

ref. Dusseault (1980) Fig. 5 & 6

Gas pressure inside liner

Core has expanded from 120.7mm to 127mm diameter and is now acting like a piston in a cylinder

Radially Axially

Schematic Diagram of Expansion of an 89 mm Core

PVC

liner

Oil sandIronstone band, no expansion

Oil-poor to oil-free silty sands, expansion much less than other material

Corrugated surface characteristic of thinly-bedded and laminated fine-grained sands of variable oil saturation

Cores separate readily along cracks which form between zones of differing expansion potential

89 mm90-91 mm

Oil-rich sample expands to completely fill the liner

95 mm

127 mm

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Reasons for Core ExpansionReasons for Core Expansion

CH4 present in solution, exsolves during the Δp in bringing core to surface

The high oil content means a lot of gas High μ oil means gas cannot drain; no

continuous gas phase is formed without ΔV The sand is cohesionless (To = 0); it cannot

resist internal expansion The core barrel liners are 7%-13% oversize,

allowing a lot of expansion

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Heavy Oil Cores, MR

Scans

Courtesy of Glen Brook, Nexen and

Apostolos Kantzas, U of

Calgary

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CT-Scan Evidence of Damage in Heavy Oil CoresCourtesy of Glen Brook, Nexen and Apostolos Kantzas, U of Calgary

decr

easi

ng

den

sity

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Core Damage ConsequencesCore Damage Consequences

Porosity overestimated Permeability measurements in the lab are too

high by a factor of ~1.5 to 2 Laboratory data for So, Sw, Sg are wrong Reserve estimation can be out by 5-10% Predicted productivity index by factor of 2 All rock mechanics data are in error

Compressibilities are too high by a factor of >10 Rock strength predictions far too low Etc…

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History…History…

“Evaluation of the Alberta Tar Sands”Sah, Chase, & WellsSPE 5034 (1974)

Old Problems… These issues are “re-

discovered” repeatedly However, the issue is

relatively well-documented (SPE, CJPT, conferences…)

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Empirical EvidenceEmpirical EvidenceZwicky and Eade (Shell) UNITAR, 1977Zwicky and Eade (Shell) UNITAR, 1977

-39-121931Water Saturation

+17+128169Tar Saturation

-10-3.532.035.5Porosity

Percent change

DifferenceDensity Log

Core Analysis

Table 3. Comparison of results from core analysis alone and density from logs, Lease 13, average data

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Log Derived PorosityLog Derived Porosity

Neutron porosity is not a true measure of porosity (it actually is a measure of H)

Determine the true density using a gamma-gamma density log (average over 1 m)

Calculate based on this density number Heavy oil sands in situ are almost always

liquid saturated (i.e.: Sg = 0) Determine saturations from cores Some factors (grain size, mineralogy, liquid

densities…) are not affected by expansion

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Density RelationshipDensity Relationship

}log = Soo + Sww + (1 – ) Gm

Where:So = oil saturation

Sw = water saturation

log= density from logs

o = density of oil

w = density of water

Gm = matrix density

(1 - )

Intact rock

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Calculate Porosity from Calculate Porosity from LogLog

= Gm - log Gm - Soo - Sww

So, Sw, o, w from core (Sg = 0)

Gm is measured on a grain sample

Then, do quality control assessments

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Quality Control AssessmentsQuality Control Assessments

Obtain So and Sw from log analyses, compare to lab data to decide if saturations are correct Perhaps some water invasion occurred Always assume Sg = 0, but check on logs

Decide which to use When you have lab-derived porosities and

geophysical-derived porosities: Cross plot of the two Examine the data to see what is happening Make decisions…

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Porosity Cross PlotPorosity Cross Plot

0.40

0.38

0.36

0.34

0.32

0.30

0.280.400.380.360.340.320.30

Log

-deri

ved p

oro

sity

Core-derived porosity

“average”error in

Reasonable data, acceptable scatter

“Typical” heavy oil case

Porosity in serious error

Is it valid to apply an “average” correction factor to core data??

Probably not…

Best is to use individual values of log-derived porosity information

Adjust your core data as required

0.28

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Quality ControlQuality Control

X-plots are useful Careful with “average” corrections You can even check using neutron porosity,

but you must be careful! CNL log numbers may be wrong in heavy oils,

which tend to be deficient in hydrogen, as compared to conventional oil correlations

If there is a lot of clay, the CNL data may also be off the mark somewhat (1-2 porosity units)

Get your logging company to help calibrate your field case

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More Methods More Methods

Needle penetrometer for core consistency Use of sonic travel time transducers in the

laboratory as a QC method Visual examination

Extrusion when core is cut and boxed? Extrusion from cut ends in the lab? Core recoveries of 100% always reported? Gas bubbling from core surface, fluids extruding?

And so on…

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““Correcting” the Data - ACorrecting” the Data - A

Clearly, permeability overestimated as well This is considerably harder to correct If you have some oil-free, undamaged core

with similar characteristics in your field Do lab tests on undisturbed specimens (check) Use these to determine k equation

Compare log k values with core values Is there a useful “correction factor”? Can log data be considered reliable enough?

Find some other way to correct E.g. Kozeny-Carman correlation, Archie plot…

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““Correcting” the Data - BCorrecting” the Data - B

Recalculate your reserves, volumes, etc. You can eventually develop a better

empirical log equation to determine porosity, volume factors, etc. directly

Always test out relationships on specimens that are undisturbed (if this is possible)

I used outcrop samples to do this High quality coring may help, but…

Can’t avoid expansion in heavy oil sands Even core plugging at room temp is damaging Nevertheless, do the best possible…

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A Few More Slides…A Few More Slides…

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Index of DisturbanceIndex of Disturbance

logdensity

logdensity

coreDI

A quantitative measure of core disturbance.

e.g.: Dusseault, 1980, used by others, e.g. Settari, et al. (1993) ID = [10.2%, 18.4%]

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I D =

40%

I D =

30%

I D =

20%

I D =

10%

I D =

0%

A neg

ativ

e va

lue

of I D in

dicat

es

poor c

orrela

tion to

the

logs

Index of DisturbanceIndex of Disturbance

Dusseault & van Domselaar (1982) Fig. 2

25

30

35

20

PO

RO

SIT

Y (

%)

- L

ab

ora

tory

40

POROSITY (%) - Geophysical Density Log

ID < 10% Intact or slightly disturbed

10% < ID < 20% Intermediate disturbance

20% < ID < 40% Highly disturbed

40% < ID Disrupted generally

25 30

Suitable

only for q

ualitativ

e or

descriptiv

e purp

oses

Sampl

es s

uita

ble fo

r hig

h-qu

ality

geom

echa

nical

test

s

Sampl

e qu

ality

ade

quat

e fo

r

mos

t pet

roph

ysic

al re

sear

ch

35 40

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IIDD - O - Oldakowski Table 4.3ldakowski Table 4.3

25

30

35

I D =

40%

20

PO

RO

SIT

Y -

Lab

ora

tory

(%

)

40

POROSITY - Geophysical Density Log (%)

I D =

30%

I D =

20%

25 30

I D =

10%

I D =

0%

35 40

Oldakowski (1994)

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Heavy Oil Core DataHeavy Oil Core Data

Weight percent bitumen Mining application

Summation of fluids Grain weight and Total weight Dean-Stark water (sometimes) Dean-Stark bitumen – corrections

Do not correlate with well logs because of the core dilation problem…

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Core PorosityCore Porosityv.v.Log PorosityLog Porosity

“Evaluation of the Alberta Tar Sands”“Evaluation of the Alberta Tar Sands”Sah, Chase, & WellsSah, Chase, & Wells

SPE 5034 (1974)SPE 5034 (1974)

!!

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Low Disturbance SamplesLow Disturbance Samples

Dr. Amin Touhidi-Baghini, PhD thesis (1998)

McMurray sample from river valley outcrop Minimal disturbance: no gas ex-solution Absolute Permeability measured:

at low confining stress during shear failure

Best laboratory data available (to the present time)

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Absolute Absolute PermeabilityPermeability Increase Increase

ref. Touhidi-Baghini (1998) Fig.8.21 & 8.22

-4 -2 0

4

2

6

8 1042 6

1

5

3

Volumetric Strainv (%)

Ka

Mu

ltip

lier

K2 / K

1

Volumetric Strainv (%)-2 0 42 6

Experimental

Kozeny-Carmen

Chardabellas B=2

Chardabellas B=5

1.6x

6x

Vertical core

specimens with an

average porosity of 33.9%

Vertical

Horizontal2.5x

5%Horizontal core

specimens with an

average porosity of 33.7%

5%

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What Has Been Tried?What Has Been Tried?

Pressure core barrels Non-invasive fluids Special core catchers Special freezing during transport Re-stressing before testing And so on and so forth None of these methods has been satisfactory. The best results have been obtained by…

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Best ResultsBest Results

Sampling in a tunnel through an outcrop where gas pressure was depleted

In other outcrops (but Sw is incorrect)

In very shallow boreholes where pgas is low Core barrels of short L, small potential for

radial expansion, + axial restraint Use of analogue materials from outcrops

(e.g. the oil-free outcrops along riverbanks) Intact samples of deep heavy oil UCS sands

is highly problematic. Is it worth doing??

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Then What?Then What?

If “successful” core has been brought to surface… Freeze to dry ice T for transport Keep fully sealed

Prepare test specimens in cold room (-25°C) Do not plug with a fluid (heat–expansion–etc) Slow lathes for trimming to diameter OK Trim ends flat in the lathe as well

Mount specimens while cold, thaw only when under pressure…

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CoringCoring

Only very shallow cores (<100 m) have achieved any reasonable ID values

A specialized, short length core barrel is advised

Internal flush (no radial expansion) Protruding cutting edge (avoid fluid contact) Some method for axial restraint “Rigid” core sleeve Etc.

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ConclusionsConclusions

Core damage can be a very serious issue Mis-estimation of reserves by 10-15% Over-estimate permeabilities by factor of 2 to 4 And so on…

Geophysical log-derived is best Use lab or log So, Sw?

Calibrated neutron porosity is “OK” Put into place quality control measures on

coring, testing, lab procedures, log analysis Then, just do the best you can…

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Sampling Viscous Oil Sands Sampling, Preserving and Testing Heavy Oil Sand Cores Maurice Dusseault.

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