03_Chapter 03. Routine Core Analysis.pdf

Chapter Outline
Chapter Outline
▪ Introduction
▪ Fluid Saturation Measurements
▪ Porosity Measurements
▪ Permeability Measurements

Introduction
 Core analysis provides data input for several reservoir
engineering calculations
 Typical routine core analysis (RCA) involves the
measurements of fluid saturation, porosity, air and
Klinkenberg permeability
 Test plugs used for RCA are typically 1’’ or 1.5’’ diameter
1/11/2025 78
Dr. Mai Cao Lân, Faculty of Geology & Petroleum Engineering, HCMUT, Vietnam

Fluid Saturation Measurements
Two principal methods for fluid saturation measurement:
 The retort distillation method
 The solvent extraction method
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Retort Distillation Method
 Principle of the measurement:
Distillation
 Fluid inside the rock sample is
heated and vaporizes out of the
rock sample
 The cooling water make the
vaporized fluid condenses through
the condenser and finally sink
down in the graduated receiving
tube
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Water from pores is recovered first
 If flattening of curve is not
apparent then estimating water
recovered from pore space can
be inaccurate
Later, water of crystallization is
recovered at higher temperatures
 This destroys the core sample

Retort Distillation Method Calculations
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Measured data include
 𝑉 : Volume of mercury injected
 𝑉𝑏 : The bulk volume of the rock sample
 𝑉𝑏 : The bulk volume of the retort sample
 𝑉 , 𝑉 : The corrected volumes of oil and water

Retort Distillation Method Calculations
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Fraction Bulk Volume Calculations:
𝑆 =
𝑉
𝑉𝑏
; 𝑆 =
𝑉
𝑉𝑏
; 𝑆 =
𝑉
𝑉𝑏
Fluid Saturation Calculations:
𝑆 =
𝑆 ( )
𝑆 ( ) + 𝑆 ( ) + 𝑆 ( )
𝑆 =
𝑆 ( )
𝑆 ( ) + 𝑆 ( ) + 𝑆 ( )
𝑆 =
𝑆 ( )
𝑆 ( ) + 𝑆 ( ) + 𝑆 ( )

Advantages
 Rapid (less than one hour)
 Direct measurement of both oil and water
volumes recovered
 Adequate accuracy
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Disadvantages
 High temperatures (1,000 - 1,100 F):
• destroys core sample
• water of crystallization in clays may vaporize – and
must be accounted for
• cracking and coking of oil
 cracking is breaking of long chain hydrocarbons
into smaller chain hydrocarbons, which may not
be recondensed
 coke is impure carbon residue formed from oil
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Solvent Extraction Method
 This method is recommended for core saturation
determination as it is not a destructive method.
 Water content is measured directly while oil content is
indirectly measured from the change in weight
 The sample is placed in chamber (3) connected with
flask (2) containing solvent (1). The water is
condensed in (5) and collected via the receiving tube
(8) and finally falls into collection vessel (10).
 Toluene or xylene can be used as the solvent to extract
water from the sample. Since xylene has higher boiling
points, it is more suitable for those formation brines
that contain large amount of salts, especially CaCO3.
 Heating rate must be high enough to ensure that
condensation of water occurs in the condenser, so that
water falls into the trap
 Solvent leaches hydrocarbons from the pore space and
oil remains dissolved in the solvent
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Solvent Extraction Calculations
1/11/2025 87
 
o
w
w
dry
i
o
ρ
ρ
V
W
W
V



p
w
w
V
V
S 
p
o
o
V
V
S 
Wi = Initial core weight, gm
Wdry = Core weight after leaching, gm
Vw = Volume of water collected, cm3
Vo = Volume of oil, cm3
Vp = Pore volume, cm3
w = Density of water, gm/cm3
o = Density of oil, gm/cm3
g = Density of gas is assumed negligible
o
w
g S
S
1
S 



1/11/2025 88
Advantages
 accurate determination of water saturation
 non-destructive to core samples
• determination of water saturation by solvent
extraction can be made part of the core sample
cleaning process for small incremental cost

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Disadvantages
 slow (can take several days)
 oil volume can not be directly measured
• oil remains dissolved in solvent

Porosity Measurements
Two lab methods for porosity measurements, in fact for rock
volume measurements
 Direct Method
 Fluid Displacement (Archimedes) Method
Notes:
 Porosity can be determined from any two measurements of
bulk volume 𝑉 , pore volume 𝑉 or matrix/grain volume 𝑉
𝜙 =
𝑉
𝑉
=
𝑉 − 𝑉
𝑉
=
𝑉
𝑉 + 𝑉
 Porosity can be estimated using openhole wireline logs if no
core is available
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• Applicable for regularly shaped cores or core plugs
• Calculate from core dimensions
• For example, volume of right circular cylinder:
𝑉 =
𝜋𝑑 𝐿
4
Bulk Volume by Direct Measurement

Wsat Wsub
Wdry
Archimedes (Fluid Displacement) Method
𝑊 : Weight of dry sample
𝑊 : Weight of saturated sample
𝑊 : Weight of buoyant sample

Wsat Wsub
Wdry
Bulk Volume by Archimedes (Fluid
Displacement) Method
𝑉 = ; 𝑉 = ; 𝑉 =
𝜙 =
𝑉
𝑉
=
𝑊 − 𝑊
𝑊 − 𝑊

Quiz
A core sample fully coated with paraffin is immersed in a
container of liquid displaced 10.9 cm3 of the liquid. The weight
of the dry core sample was 20.0 g, while the weight of the dry
sample saturated with paraffin was 20.9 g. Assume the density
of paraffin is 0.65 g/cm3.
Calculate the bulk volume of the rock sample.

• Assume rock density based on lithology and
measure dry mass
• Boyle’s Law
Rock Matrix Volume Measurements

Given rock matrix density, one has:
Density
Matrix
Sample
Dry
of
Mass
Vm 
Rock Matrix Volume from Rock Density

• Involves compression of gas into pores
• Uses Boyle’s law
𝑝 × 𝑉 = 𝑐𝑜𝑛𝑠𝑡.
Gas Expansion Method for Matrix
Volume

Valve
closed
Cell 1
Cell 2
Initial conditions
V1
P1
Core
Gas Expansion Method for Matrix Volume

Valve
open
Final conditions
P2
Core
Cell 1 Cell 2
P1
Gas Expansion Method for Rock Matrix
Volume

Quiz
Given the measurement data from Boyle’s method:
𝑉 = 𝑉 = 100𝑐𝑐
𝑝 = 75 𝑝𝑠𝑖; 𝑝 = 52 𝑝𝑠𝑖
Calculate the matrix volume of the rock sample.

Quiz
Establish the pore volume formula following the figure below:

 Archimedes
𝑽𝒑 =
𝑾𝒔𝒂𝒕 − 𝑾𝒅𝒓𝒚
𝝆𝒇𝒍𝒖𝒊𝒅
 Boyle’s Law:
𝒑 × 𝑽 = 𝒄𝒐𝒏𝒔𝒕.
Lab Methods for Pore Volume

Using the gravimetric method with the following data:
Dry weight of sample, Wdry = 427.3 g
Weight of sample saturated with water, Wsat = 448.6 g
Density of water (f ) = 1.0 g/cm3
Weight of saturated sample submerged in water, Wsub = 269.6 g
Calculate the pore and bulk volumes and the porosity
Quiz

• Involves compression of gas into pores
• Uses Boyle’s law
𝑝 × 𝑉 = 𝑐𝑜𝑛𝑠𝑡.
Gas Expansion Method for Pore Volume

Core
V1
Valve
closed
Cell 1 Cell 2
P1
Initial conditions

Final conditions
Valve
open
Core
Cell 1 Cell 2
P1
P2

Measurements of Absolute Permeability
A. Introduction
 Core samples for permeability measurements
 Lab procedure
 Factors affecting permeability measurements
 Samples of core analysis report
B. Measurement Methods
 Steady-State Method
 Unsteady Method
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Heterogeneous
Whole Core
Full Diameter
Heterogeneous
Most Common
Plug
Slab taken for
• Photography
• Description
• Archival
1” or 1 1/2”
Typical Rock Core Samples

 Uses selected pieces from the full or whole core
– Core sizes 2 1/2 to 5 1/2 inches in diameter
– Several inches to several feet long
 Most applicable approach for very heterogeneous
formations.
 Additional expense limits the practical number of tests
Whole Core Samples

• Most commonly used
• Small cylindrical core samples
– 3/4 inch to 1 1/2 inch diameter
– 1 to a few inches long
• May not be used for heterogeneous formations
Core Plug Samples

~1 ft
kH
kV kV
Suitable
Unacceptable
kH
?
Or
Full-
Diameter
Full
Diameter
`4” - 9”
kV
kH
Whole Core
Analysis
(2-3 ft)
Fracture
k and ?
Matrix
Only
III IV V
I IIa IIb
Different
lithologies
require careful
selection of
suitable core
sample types
for
permeability
measurements

• Cut core plugs from whole core or use sample from
whole core
• Clean core and extract reservoir fluids, then dry the
core
• Flow a fluid through core at several flow rates
• Record inlet and outlet pressures for each rate
Lab Procedure

Slope =
k

0
0 (p1 - p2)
L
q
A
Darcy Flow Non-Darcy Flow
Laboratory Determination of
Absolute Permeability, Liquid Flow

• Core Preparation
• Fluid-Rock Interactions
• Pressure Changes
• Rock Heterogeneities (Fractures)
• Gas Velocity Effects (Klinkenberg)
Factors Affecting Laboratory
Measurements of Absolute Permeability

• Core Handling
• Cleaning
• Drying (Clay Damage)
• Storage (Freezing)
• Sampling
Core Preparation Affects on
Permeability Measurement

• Fresh water may cause clay swelling, reducing
permeability
• Tests may cause fines migration, plugging pore
throats and reducing permeability
• Reservoir or synthetic reservoir fluids are
generally preferred
Rock-Fluid Interaction Affects
Measurements of Permeability

• Core alterations resulting from loss of
Confining Pressure during core recovery
• Core testing may be conducted by applying a
range of net overburden pressures
Pressure Affects Measurements of
Permeability

• Naturally-fractured reservoirs
– Core plugs represent matrix permeability
– Total system permeability (matrix + fractures)
is higher
• Core Mineralogy problems (Salts, Gypsum)
Core Heterogeneities Affect
Measurements of Permeability

Sample of Core Analysis Report

• Measure inlet and outlet pressures (p1 and p2) at
several different flow rates
• Graph ratio of flow rate to area (q/A) versus the
pressure function (p1 - p2)/L
• For laminar flow, data follow a straight line with
slope of k/
• At very high flow rates, turbulent flow is indicated
by a deviation from straight line through origin
RCA for Absolute Permeability

Methods for Permeability Measurement
1. Measurement Principles
 Darcy flow
 Non-Darcy flow
2. Measurement Methods
 The steady-state method
 The unsteady method
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Measurement Principles
Apply Darcy’s law to determine absolute permeability of a rock
when injecting a single-phase liquid through core sample
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Non-Darcy Flow – Klinkenberg’s Effect
Apply Klinkenberg’s method to determine the absolute permeability of
a rock when injecting a gas through the core sample.
At low pressure, gas moves through the rock surfaces with little friction
loss. At higher pressure, friction loss becomes significant. At infinitive
pressure, the gas permeability would be equivalent to the permeability
to a non-reactive liquid.
Since experiments with infinitive pressure is impossible, Klinkenberg
suggested measuring flow rates at some definite pressures and
performing an extrapolation to determine gas permeability at infinitive
pressure
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Non-Darcy Flow – Klinkenberg’s Effect
Klinkenberg’s relationship between gas permeability 𝑘 and the mean
pressure 𝑝 as follows:
𝑘 = 𝑘 1 +
𝑏
𝑃
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Exercise
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Given the following gas permeability values and mean
pressures:
𝑘 (mD)
𝑃 (atm)
34.3
1.4
29.2
1.9
25.4
2.6
22.8
3.3
Find the Klinkenberg-corrected equivalent liquid
permeability of the rock core (mD).

In-class exercises
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Exercise 1: A helium porosimeter is used to find the porosity of a
certain core sample. Both the chambers in the porosimeter have a
volume of 100 cm3, and the sample has a bulk volume of 16.2 cm3.
Initially, helium is contained in chamber 1, the sample is placed in
chamber 2 and the valve separating the two chambers is closed.
The initial pressure in chamber 1 is recorded to be 30 kPa, and the
pressure after the valve is opened is recorded to be 16 kPa.
Find the porosity of the core sample.

In-class exercises
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Exercise 2: A core sample containing only water (ρw = 1 g/cm3)
and oil (ρo = 0.87 g/cm3) has a 13.6% porosity, 3 inch length, and
1.5 inch diameter. Its saturated weight was measured to be 144.3
g, and its dry weight was measured to be 133.2 g.
Calculate the water and oil saturations.

03_Chapter 03. Routine Core Analysis.pdf

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