RESERVOIR PETROPHYSICS

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RESERVOIR PETROPHYSICS
LABORATORY DETERMINATIONOF POROSITY
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POROSITY DEFINITION

• Porosity The fraction of a rock that is
  occupied by pores

• Porosity is a static property it can be
  measured in the absence of flow
• Determining effective porosity requires fluid
  flow to determine if pores are interconnected

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ROCK MATRIX AND PORE SPACE
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MEASUREMENT OF POROSITY

• Core samples (measure two of Vb, Vp, or Vm)
• Openhole wireline logs

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LABORATORY DETERMINATIONOF POROSITY

• Most methods use small samples (core plugs)
• multiple samples must be analyzed to get
  statistically representative results
• sampling technique is important
• often all samples are taken from sweet spots
  skewing analysis
• To determine porosity, measure 2 of 3
• volumetric parameters
• Bulk volume, Vb
• Matrix volume, Vm (also called grain volume)
• Pore volume, Vp

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Volume is an extensive property
Fraction of volume consisting of pores or voids
Fraction of volume consisting of matrix
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MATRIX DENSITIES (?m) OF TYPICAL PURE COMPONENTS
OF RESERVOIR ROCK
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LABORATORY METHODS OF POROSITY DETERMINATION

• Bulk volume determinations
• Direct calculation
• Fluid displacement methods
• Gravimetric
• Volumetric mercury pycnometer ( a precisely
  calibrated bottle)

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BULK VOLUME BY DIRECT MEASUREMENT

• Applicable for regularly shaped cores or core
  plugs
• Calculate from core dimensions
• For example volume of right circular cylinder

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LABORATORY METHODS OF POROSITY DETERMINATION

• Bulk volume determinations
• Direct calculation
• Fluid displacement methods
• Gravimetric (Archimedes) methods
• Volumetric in pycnometer

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ARCHIMEDES METHOD
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EXAMPLE 1 Bulk Volume Calculated by Displacement
A core sample coated with paraffin 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 coated
with paraffin was 20.9 g. Assume the density of
the solid paraffin is 0.9 g/cm3. Calculate the
bulk volume of the sample.
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SOLUTION - Example 1
Weight of paraffin coating, Wparaffin Weight of
dry core sample coated with paraffin - Weight
of dry core sample Wparaffin 20.9 g 20.0 g
0.9 g Volume of paraffin coating Weight of
paraffin / density of paraffin Vparaffin 0.9
g / 0.9 g/cm3 1.0 cm3 Bulk volume of core
sample (Bulk volume of core coated with
paraffin) (volume of paraffin) Vb 10.9
cm3 1.0 cm3 9.9 cm3
(V m/?)
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LABORATORY METHODS OF POROSITY DETERMINATION

• To determine porosity, measure 2 of 3
• basic parameters
• Bulk volume
• Matrix volume
• Assume matrix (grain) density
• Displacement method
• Boyles Law
• Pore volume

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LABORATORY METHODS OF POROSITY DETERMINATION

• Matrix (Vm)
• Assume rock density based on lithology and
  measure dry mass
• Displacement methods
• volumetric
• gravimetric (see previous description)
• Boyles Law

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MATRIX VOLUME FROM MATRIX DENSITY

• Known or assumed matrix density

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APPLICABILITY AND ACCURACY OF MATRIX MEASUREMENT
TECHNIQUES

• Known or assumed matrix density
• Accurate only if matrix density is known and not
  assumed
• Core samples are often mixtures of several
  components with varying matrix densities, so
  density must be measured

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LABORATORY METHODS OF POROSITY DETERMINATION

• To determine porosity, measure 2 of 3
• basic parameters
• Bulk volume
• 2. Matrix volume
• Assumed matrix (grain) density
• Displacement method
• Boyles Law
• 3. Pore volume

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MATRIX VOLUME FROM DISPLACEMENT METHOD

• Reduce sample to particle size
• Measure matrix volume of particles by
• Volumetric method
• Archimedes method (gravimetric measurement)

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EXAMPLE 2SOLUTIONCalculating the Matrix Volume
and Porosity of a Core Sample Using the
Displacement Method
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SOLUTION - Example 2Calculate the Porosity of a
Core Sample Using the Displacement Method and
Matrix Volume
The core sample from Example 1 was stripped of
the paraffin coat, crushed to grain size, and
immersed in a container with liquid. The volume
of liquid displaced by the grains was 7.7
cm3. Calculate the matrix volume and the core
porosity. Is this effective porosity or total
porosity? (It is total porosity)
Bulk Volume, Vb 9.9 cm3 Matrix Volume, Vma
7.7 cm3
0.22 or 22
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LABORATORY METHODS OF POROSITY DETERMINATION

• To determine porosity, measure 2 of 3
• basic parameters
• Bulk volume
• 2. Matrix volume
• Assumed matrix (grain) density
• Displacement method
• Boyles Law (Gas Expansion)
• 3. Pore volume

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MATRIX VOLUME FROM GAS EXPANSION METHOD

• Involves compression of gas into pores
• Uses Boyles law

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GAS EXPANSION METHOD TO CALCULATE Vma

• Initial conditions, with volumes of 2 cells known
• Place core in second cell, evacuate gas (air)
  from second cell
• Open valve

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GAS EXPANSION METHOD TO CALCULATE Vma

• Initial conditions

P1
Core
V1
Valve closed
Evacuate Cell 2
Cell 1
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GAS EXPANSION METHOD TO CALCULATE Vma
Final conditions
P1
P2
Core
Valve open
Cell 1
Cell 2
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GAS EXPANSION METHOD TO CALCULATE Vma

• Vf Volume of Cell 1 Volume of Cell 2 -
  Matrix Volume of Core
• Vt Volume of Cell 1 Volume of Cell 2
• Vm Vt - Vf

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APPLICABILITY AND ACCURACY OF MATRIX MEASUREMENT
TECHNIQUES

• Displacement method - Very accurate when core
  sample is crushed without destroying individual
  matrix grains
• Gas expansion method - Very accurate, especially
  for samples with low porosities
• Neither method requires a prior knowledge of core
  properties

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LABORATORY METHODS OF POROSITY DETERMINATION

• To determine porosity, measure 2 of 3
• basic parameters
• Bulk volume
• Matrix volume
• Pore volume

(Vb)
(Vm)
(Vp)
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LABORATORY METHODS OF POROSITY DETERMINATION

• Pore volume determination (Effective)
• Gravimetric (Archimedes)
• Wsat - Wdry
• ?fluid
• 2. Boyles Law
• (Gas expansion)

Vp
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PORE VOLUME FROM SATURATION METHOD

• Measures the difference between the weight of a
  core sample saturated with a single fluid and the
  dry weight of the core
• Pore volume,

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EXAMPLE 3 Archimedes Method of
CalculatingPorosity a Core Sample
Using the gravimetric method with the following
data, calculate the pore and bulk volumes and the
porosity. Is this porosity total or
effective? 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
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EXAMPLE 3Solution Archimedes Method of
CalculatingPorosity a Core Sample
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Applicability and Accuracy of Pore Volume
Measurement Techniques

• Saturation (Archimedes) method
• Accurate in better quality rocks if effective
  pore spaces can be completely saturated
• In poorer quality rocks, difficult to completely
  saturate sample
• Saturating fluid may react with minerals in the
  core (e.g., swelling clays)

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LABORATORY METHODS OF POROSITY DETERMINATION

• Pore volume determination (Effective)
• Gravimetric (Archimedes)
• Wsat - Wdry
• ?fluid
• 2. Boyles Law
• (Gas expansion)

Vp
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PORE VOLUME FROM GASEXPANSION METHOD

• Initial conditions

P1
Core
V1
Valve closed
Cell 1
Cell 2
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PORE VOLUME FROM GASEXPANSION METHOD
Final conditions
P1
P2
Core
Valve open
Cell 1
Cell 2
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PORE VOLUME FROM GAS EXPANSION METHOD

• Very accurate for both high-quality (high ?) and
  low-quality (low ?) core samples
• Should use low-molecular-weight inert gases
  (e.g., helium)
• Measures effective (connected) pore volume

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SUMMARY
To determine porosity, measure 2 of 3 basic
parameters

1. Bulk volume
2. Matrix volume
3. Pore volume

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CORES

• Allow direct measurement of reservoir properties
• Used to correlate indirect measurements, such as
  wireline/LWD logs
• Used to test compatibility of injection fluids
• Used to predict borehole stability
• Used to estimate probability of formation failure
  and sand production

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SOME KEY FORMULAS
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CROSS BEDDING, CARRIZO SANDSTONE