11

Pore Geometry Effects in Pore Geometry Effects in Carbonate ReservoirsCarbonate Reservoirs

22

Common Cretaceous Carbonate Rock Textures

Unlike sandstones, carbonate pore systems do not

generally exhibit a relationship between pore throat size and

pore body size.

The connectivity between pores in

carbonates is generally fairly

chaotic.

33

Pore Geometry Model

Hi K

Low K

PoreBody

Plug Scale

Pore Throat

PoreBodyPore

Body

PoreBody

PoreBody

t = o + e

44

Rock Type 1

•Largest pore throats•Well connected•Lowest Swi•Highest K•Intermediate porosity•Little or no microporosity

Rock Typing Pore Geometry

55

Back-ups?

Rock Type 3

•Strongly Bimodal -Large and small pore throats•Some macro pores connected via micropores•Higher Swi at a given H•Intermediate K•Intermediate to low porosity•Abundant microporosity within grains and matrix (where present)

Rock Typing Pore Geometry

66

Pressure @ 400 psi

Sw>0.0 & <=0.2

Sw>0.2 & <=0.4

Sw>0.4 & <=0.6

Sw>0.6 & <=0.8

Sw>0.8 & <=1.0

K (

md)

Phi (%)

0.01

10000

0.1

1

10

100

1000

0 5 10 15 20 25 30

Pore Body SizePore Body Size

Throat Size Throat Size

Greater probability of

large pore bodies

77

Pressure @ 2 psi

Sw>0.0 & <=0.2

Sw>0.2 & <=0.4

Sw>0.4 & <=0.6

Sw>0.6 & <=0.8

Sw>0.8 & <=1.0

K (

md)

Phi (%)

0.01

10000

0.1

1

10

100

1000

0 5 10 15 20 25 30

88

Pressure @ 5 psi

Sw>0.0 & <=0.2

Sw>0.2 & <=0.4

Sw>0.4 & <=0.6

Sw>0.6 & <=0.8

Sw>0.8 & <=1.0

K (

md)

Phi (%)

0.01

10000

0.1

1

10

100

1000

0 5 10 15 20 25 30

Winland Rock Type

f(k/)

99

Pressure @ 10 psi

Sw>0.0 & <=0.2

Sw>0.2 & <=0.4

Sw>0.4 & <=0.6

Sw>0.6 & <=0.8

Sw>0.8 & <=1.0

K (

md)

Phi (%)

0.01

10000

0.1

1

10

100

1000

0 5 10 15 20 25 30

1010

Pressure @ 20 psi

Sw>0.0 & <=0.2

Sw>0.2 & <=0.4

Sw>0.4 & <=0.6

Sw>0.6 & <=0.8

Sw>0.8 & <=1.0

K (

md)

Phi (%)

0.01

10000

0.1

1

10

100

1000

0 5 10 15 20 25 30

1111

Pressure @ 40 psi

Sw>0.0 & <=0.2

Sw>0.2 & <=0.4

Sw>0.4 & <=0.6

Sw>0.6 & <=0.8

Sw>0.8 & <=1.0

K (

md)

Phi (%)

0.01

10000

0.1

1

10

100

1000

0 5 10 15 20 25 30

1212

Pressure @ 60 psi

Sw>0.0 & <=0.2

Sw>0.2 & <=0.4

Sw>0.4 & <=0.6

Sw>0.6 & <=0.8

Sw>0.8 & <=1.0

K (

md)

Phi (%)

0.01

10000

0.1

1

10

100

1000

0 5 10 15 20 25 30

Non Winland Behavior

1313

Pressure @ 80 psi

Sw>0.0 & <=0.2

Sw>0.2 & <=0.4

Sw>0.4 & <=0.6

Sw>0.6 & <=0.8

Sw>0.8 & <=1.0

K (

md)

Phi (%)

0.01

10000

0.1

1

10

100

1000

0 5 10 15 20 25 30

1414

Pressure @ 100 psi

Sw>0.0 & <=0.2

Sw>0.2 & <=0.4

Sw>0.4 & <=0.6

Sw>0.6 & <=0.8

Sw>0.8 & <=1.0

K (

md)

Phi (%)

0.01

10000

0.1

1

10

100

1000

0 5 10 15 20 25 30

1515

Pressure @ 200 psi

Sw>0.0 & <=0.2

Sw>0.2 & <=0.4

Sw>0.4 & <=0.6

Sw>0.6 & <=0.8

Sw>0.8 & <=1.0

K (

md)

Phi (%)

0.01

10000

0.1

1

10

100

1000

0 5 10 15 20 25 30

1616

Pressure @ 400 psi

Sw>0.0 & <=0.2

Sw>0.2 & <=0.4

Sw>0.4 & <=0.6

Sw>0.6 & <=0.8

Sw>0.8 & <=1.0

K (

md)

Phi (%)

0.01

10000

0.1

1

10

100

1000

0 5 10 15 20 25 30

Significant volume of

poorly connected

porosity

1717

Hydrocarbon Habitat Hydrocarbon Habitat

Pc = 0

SwiTransition

Zone

60 to 90 psi

Oil in both efficient and occluded pore

volumesOil in efficient pore volume

only

t = o + e

1818

Impact of Efficient Porosity

Pore Geometry Effects

Total Porosity

Efficient Porosity•Logs & Core sense total porosity.

•Efficient porosity contains hydrocarbons at low buoyancy.

•Pores connected by efficient porosity predominately oil wet.

•Occluded pores water wet and charged only at high buoyancy.

EfficientOccluded

Efficient PhiTotal Phi

t = o + e

1919

Pore Geometry & Formation EvaluationWhat is your confidence on Sw?

m = 2.38

m = 1.75

a=1 m=2.03 n=1.98 Rw=0.018@FT

Uncertainties:•How to vary Archie exponents as a function of rock type (incorporate Sw from TDT?)

•Capillary Pressure variables ( cos )

•Pore Geometry effects upon wettability and water saturation

With confidence:•Connate water properties (Rw, water)

•Oil properties (oil)

•Pc=0

100

150

200

250

300

0% 20% 40% 60% 80% 100%Sw

He

igh

t A

bo

ve

Pc

=0

SW-OH

Sw-TDT

RT-1

RT-2

RT-3

RT-4

Sw

Aquifer Well

2020

Effects of AcidizationEffects of Acidization

Acid Effects on CH CNL

2121

Pore Geometry Model

Hi K

Low K

PoreBody

Plug Scale

Pore Throat

PoreBodyPore

Body

PoreBody

PoreBody

t = o + e

2222

Pore Geometry ModelPore Geometry Model

• Conductivity Equation – conductive matrix model

n

o

ne

e

oSwCw

SwCw

CtFF

n

w

mon

ew

me

o

oe

SwaR

SwaR

Ct

Efficient Efficient

2323

Conductive Matrix ModelConductive Matrix Model

Assumptions:

Swo = 100%

Swe = 20%

t = 25%

Pore Geometry Model

t = o+e

0.1

1

10

0% 10% 20% 30%

Efficient Porosity

Res

isti

vity

Rt

Ro

Ft = 25%

Rw =0.018

me =2.1

mo =1.7

n =2.0

2424

Conductive Matrix ModelConductive Matrix Model

Assumptions:

Swo = 100%

Swe = 20%

t = 25%

Pore Geometry Model

t = o+e

0.1

1

10

0% 10% 20% 30%

Efficient Porosity

Res

isti

vity

0%10%20%30%40%50%60%70%80%90%100%

Rt

Ro

Swt

2525

Pore Geometry ModelPore Geometry Model

Efficient filled low in

column

Multiple rock types with

similar degrees of Efficient

Occluded filling slowly with height

•Multiple Rock Types can exhibit the same type of Efficient , making them indistinguishable at low buoyancy pressures.

•The high conductivity of the Occluded dominates the resistivity measurement, especially at low column heights.

Occludedincreasing

2626

Questions & ConsiderationsQuestions & Considerations

DiscussionDiscussion

2727

Full Cycle Impact of Pore GeometryFull Cycle Impact of Pore Geometry

• Well Planning– The position of the target interval as a function of buoyancy is a

critical factor.• High in the column there is a greater probability that the oil is

charging the occluded pore systems.• Near Pc=0 only the large pore systems will contain oil.

– Near the contact low drawdown is necessary to prevent coning.• Horizontal wells that will have small drainage radii.• Consider using MRC wells to effectively increase drainage area.• Minimize porpoising. Sumps cut down the effective flowing cross

section, especially in low influx wells.• Strongly consider OBM drilling fluids combined with UBD and/or

CTD. Also pre-consider the deployment of ICDs.

2828

Full Cycle Impact of Pore GeometryFull Cycle Impact of Pore Geometry

• Drilling the well– Over balance drilling with non-wetting fluid introduces a relative perm dominated

skin which can be significant in low perm rock.– Over balance drilling conveys cuttings into the large pore system further reducing

to eliminating perm by reducing or blocking pore throats.– OBM allows you to easily detect if water is the mobile phase.

• Completions– Acid is conveyed as ions in water suspension and will only react if they come in

contact with the rock surface. Occluded pore volumes are connected by small pore throats and are most likely effected. Significant volumes of “trapped” water can be mobilized.

– Acidization dramatically alters the Kv/Kh in the near wellbore region.– Acid rinds in perforated completions can never be isolated by conventional

squeezes. The first continuous filament of water entering is conveyable through the entire continuous perforation length.

– If required in perforated production wells, consider short perforation intervals and numerous blanks to facilitate conformance at the well.

2929

Full Cycle Impact of Pore GeometryFull Cycle Impact of Pore Geometry

• Formation Evaluation– Computing total porosity is straight forward but what of efficient

porosity?– Do you trust your Sw computations?

• Archie exponents should be a function of RRT• Induction tools are conductivity seeking devices• Does the Archie Sw favorably match capillary response?• How do you measure drainage RI in oil wet rocks?• Is there a better way to obtain Sw using logs? Limitations?

– Do you understand your flow SCAL?• Which volume of porosity are you flowing through in the lab?• What would your Rel-k curves look like if the occluded pore volume was

eliminated (low buoyancy model)? • Is the hydrocarbon in the right pore geometry when you execute the test?• Do you do Deane-Stark on results to verify flood out ROS?• Have you made experiments that cover the entire column height range?

3030

Full Cycle Impact of Pore GeometryFull Cycle Impact of Pore Geometry

• Recovery Process– Do you understand the relative distributions of

hydrocarbons in the efficient and occluded pore geometries?

– At low sweep speeds, displacement should be more piston-like within the efficient pore system.

– Caution that water is often more mobile than the oil.– ROS will most probably be contained within the

occluded pore volume. The higher the column, the greater that volume.

• What mechanisms facilitate mobilizing hydrocarbons locked in occluded pore volumes?

3131

backupsbackups

3232

Heterogeneity at the Plug Level

Hi K

Low K

PoreBody

Plug Scale

Pore Throat

PoreBodyPore

Body

PoreBody

PoreBody

3333

Sample Preparation for RRTSample Preparation for RRT1 ½ x 3 inch host plug (Routine & SCAL)

1 inch RHC

P&P @ 800 psi

FF

Kbrine, miniperm

MICP

1 inch RHC

P&P @ 800 psi

Miniperm

TS

MICP spare

1 inch RHC

P&P @ 800 psi

Miniperm

TS spare

MICP spare

1 ½ inch RHC

P&P @ 800 psi

P&P @ elevated PSI

FF

Kbrine, miniperm