Porosity - Webs

AZMAN IKHSAN/SKM2313

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RESERVOIR ROCK PROPERTIES

Porosity

• Theory on porosity

pores in rocks, pore types, porosity

• Determination of porosity

Determination of porosity in laboratory


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Porosity• Theory on porosity


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Porosity• Theory on porosity

Reservoir rock has interconnected pores filled with fluid (oil, gas, water) that

can flow through the rock. There are also pores that are not interconnected

(isolated pores).

Porosity is the fraction of void space in a rock to its bulk volume .

The unit for porosity in the Eq 1 is in percent. The unit can also be in fraction.

1

rock of ebulk volum

space pore of volume

100

b

p

b

p

V

V

V

V

Generally;


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Porosity

Porosity can be classified into;

1. Original porosity

2. Induced porosity

The original porosity is developed during the deposition of the rock material.

intergrannular porosity in sandstone

intercrystalline and oolitic porosity in limestones

Induced porosity is developed by geologic process subsequent to the

deposition of the rock.

fractures

vug or solution cavities commonly found in limestones

Theory on porosity


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Porosity

Sand grain

Effective porosity/interconnecte (25%)

Cementing material

Isolated porosity (5%)

Total porosity

(30%)

Theory on porosity


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Porosity

3 types of porosity

1. Effective porosity

2. Isolated porosity

3. Total porosity

Effective porosity only consider interconnected pores.

Isolated porosity refers to the isolated pores.

Total porosity includes both interconnected and isolated pores.

Which type of porosity is important in reservoir engineering?

Theory on porosity


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Porosity

Porosity is influenced especially by:

1. Arrangement and shape of the grains

2. Mixture of various shapes and sizes of the grains

3. Amount of cementing material

Large sphere

Small sphere

Cubic packing

48%

Rhombohedral

packing

26%

Varied sizes of spheres affect the

type and volume of the pore.Effect of cementing

material

Porosity

36%

Porosity

20%

Theory on porosity


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Porosity• Porosity measurement

ebulk volum

pore of volume

100

b

p

b

p

V

V

V

V

Measurement made in lab:

Vp , volume of pore can be directly or indirectly measured

Vb , bulk volume can be measured directly or indirectly


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A

L

Porosity

Direct measurement, Vb

Regular shape samples (cylindrical , cube etc): Measure the dimensions

and calculate the bulk volume


• Porosity measurement


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Porosity

Indirect measurement,, Vb

Regular and irregular shape samples


A

L


Two methods will be discussed:

- volume of displaced mercury

- gravity method


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Porosity


volume of displaced mercury

Hg



Additional volume of mercury after the

sample is pushed into the mercury is equal

to bulk volume of the sample.


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Balance

Porosity

GravityWtd = weight of dry sample

Wthg = weight of mercury

Wtb = balance reading due to the sample

being immersed into the mercury

A

B

BA

Weight of displaced mercury = (Wtb - (Wtk + Wthg))

Volume of

displaced

mercury

=Weight of displaced mercury

Mercury density

Bulk volume

of sample = Volume of displaced

mercury

Balance

Hg





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H2O

Porosity

Vp , pore volume can be measured directly or indirectly

Direct measurement, Vp

Saturating sample with liquid

Wtd = weight of dry sample

Wtt = weight of saturated sample

Wtf = weight of liquid in the sample

= Wtt - Wtk

Vp = (Wtt - Wtk)/rf



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H2O

Porosity


Direct measurement, Vp

Saturating sample with liquid


Vb 35 cc

Wtd 20 g

Wtt 25 g

Wf 5 g

rhof 1.25 g/cc

Vp 4 cc

Porosity 0.114

11.4


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Porosity

Indirect measurement, Vp

Grains volume by using Boyle’s Law

Helium tank

P1 V1 T1

P2 V2 T1

Sample

cylinder

Stainless

steel discs

P1 V1 = P2 V2

V2 = V1 + Vl + Vs -Vc

line



P2 = ?


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Helium tank

P1 V1 T1

Sample

cylinder

Stainless

steel discs

line

Porosity

P3 V3 T1

P1 V1 = P3 V3

V3 = V1 + Vl + Vs -Vc - Vg

sample





P3 = ?


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Porosity

P1 V1 = P2 V2 V2 = V1 + Vl + Vs - Vc

V1 + Vl + Vs - Vc = V2 = (P1 V1 )/ P2

Vg = V1 + Vl + Vs - Vc - V3

P1 V1 = P3 V3 V3 = V1 + Vl + Vs - Vc - Vg

Vg = (P1 V1 )/ P2 - (P1 V1 )/ P3

Vp = Vb - Vg

= Vp / Vb = (Vb - Vg)/ Vb





Vl = volume of tubing connecting

helium tank to the sample

cylinder

V1 = reference volume of helium tank

Vs = volume of empty sample cylinder

Vc = volume of stanless steel discs

Vg = grains volume of the core

sample


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Helium tank

P1 V1 T1

P2 = ?

P3 = ?

Vb 35 cc

P1 100 psia

V1 100 cc

P2 68 psia

P3 85 psia

Vg 29.41 cc

Vp 5.59

porosity 0.160

16.0 %

Vg = (P1 V1 )/ P2 - (P1 V1 )/ P3


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Porosity• Average porosity


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Porosity - Webs