AL-AYEN UNIVRSITY
COLLEGE OF ENGINEERING
RESERVOIR ENGINEERING I
Reservoir Rock Properties
Porosity
2021-2022
AIED CHWAIED
NASIR ATALLAH
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Porosity
Outline
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Porosity definition
Classification of porosity
Ranges of porosity
Factors affecting porosity
Measurement of porosity
Significance of porosity
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Porosity
Reservoir engineer concerned with : The quantities of fluids contained
 productivity of reservoir flow unit
Two essential characteristics should be available in any reservoir rocks
• capacity for storage of fluids :- need void space within the rock (i.e porosity)
• Transmissibility capability of fluids: needs continuity of the void spaces
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Porosity: Definition
The porosity of a rock is a measure of the storage
capacity (pore volume) that is capable of holding
fluids.
Pores
Two Essential Characteristics for a Commercial Reservoir of
Hydrocarbons:
• A capacity for storage (requires void spaces within the rock)
• Transmissibility to the fluid (requires that there should be
continuity of those void spaces)
 Symbol, 
 Ratio of pore volume to bulk volume
=
𝑽𝒑
𝑽𝒃
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Porosity: Definition
Bulk volume, Pore volume & Grain volume
Vb=Vp+Vm
Vp=Vb-Vm
=
𝑽𝒑
𝑽𝒃
=
𝑽𝒃−𝑽𝒎
𝑽𝒃
 = Porosity, fraction
Vb=bulk volume of reservoir rock
Vp= Pore volume of the rock
Vm= Matrix or grain volume of the rock
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Porosity
 Classified according to origin mode to:
Primary porosity : a porosity in a rock due to sedimentation process.
Secondary porosity: a porosity in a rock which happen after sedimentation process
- fracturing
- re-crystallization
 Classified according to Pores connectivity
Total Porosity,  t : all connected and isolated pores in a rock
Effective porosity,  e : only interconnected pores in a rock
𝑻𝒐𝒕𝒂𝒍 𝒑𝒐𝒓𝒆 𝒔𝒑𝒂𝒄𝒆𝒔
 t=
𝒃𝒖𝒍𝒌 𝒗𝒐𝒍𝒖𝒎𝒆
𝑰𝒏𝒕𝒆𝒓𝒄𝒐𝒏𝒏𝒆𝒄𝒕𝒆𝒅 𝒑𝒐𝒓𝒆 𝒔𝒑𝒂𝒄𝒆𝒔
e =
𝒃𝒖𝒍𝒌 𝒗𝒐𝒍𝒖𝒎𝒆



Very clean sandstones : t = e
Poorly to moderately well-cemented intergranular materials: t  e
Highly cemented materials & most carbonates: e < t
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Porosity
Porosity ranges
 Maximum value ( =47.6 %)
Cubic packing of uniform spheres.
Cubes are formed by spheres centres
r = sand grain radius

Intermediate value ( =26 %)

Minimum value ( =0 %)
Compact formations
Vb= (2r)3=8 r3
Vm= 8 (1/8 sphere) = Vol. of 1 sphere
= (4/3) πr3
Φ=
𝑽𝒑
𝑽𝒃
𝑽𝒃−𝑽𝒎
= 𝑽𝒃
=
𝟖𝒓𝟑−
𝟒
𝟑
𝟖𝒓𝟑
𝝅𝒓𝟑
= 𝟎. 𝟒𝟕𝟔
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Porosity
Factors affecting porosity
PRIMARY
Particle sphericity and angularity
Packing
Sorting (variable grain sizes)
SECONDARY (diagenetic)
Cementing materials
Overburden stress (compaction)
Vugs, dissolution, and fractures
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Porosity
Factors affecting porosity: Particle sphericity and angularity
Well rounded particles have greater porosity than angular
Factors affecting porosity: Packing
The more closely packed the particles the lower the porosity
UNPACKED
PACKED
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Porosity
Factors affecting porosity: Sorting
 If all particles are the same size they are sorted
 If the particles are different sizes they are unsorted (poorly
sorted)
 Well sorted sediments have higher porosities than poorly
sorted sediments
 If a sediment is a range of particle sizes then the smaller
particles may fill in the voids between the larger particles
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Porosity
Factors affecting porosity
Particle Size alone does NOT affect
porosity!!!!! (Shaping, Packing, Sorting does)
Same porosity
in both counts,
Φ=47.6%
Diagenesis
 Diagenesis is the Post-Depositional Chemical and Mechanical Changes
that Occur in Sedimentary Rocks.
 Some Diagenetic Effects Include:
Overburden stress (compaction)
Precipitation of Cement
Grains and Cement
Vugs
Dissolution
Fractures
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Porosity
Laboratory measurement of porosity
 Porosity is determined though Routine Core analysis
 Three of the followings have to be measured:
1. Bulk volume, Vb
2. Pore volume, Vp
3. Matrix (Grain volume), Vm
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Porosity: Bulk volume calculations
Two procedure for Vb calculations:
A- Direct calculations from dimensions

Regularly shaped cores

Bulk volume obtained from core dimensions

For example: Cylindrical core 𝑽𝒃 =
𝝅𝒅𝟐 𝑳
𝟒
B- Fluid displacement
Vernier

Dropping the sample into liquid

Observe the volume charge of liquid

Testing liquids must not enter pores space of the sample
by one of the followings:
(a) Coat with paraffin
Vb= displaced volume
(B) Use mercury as test liquid
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Porosity: Matrix volume calculations
1. Assume matrix density
 Assuming matrix density depending on the lithology
 Measure the dry sample weight
Weight 𝑜𝑓 𝑑𝑟𝑦 𝑠𝑎𝑚𝑝𝑙𝑒
𝑉𝑚 =
𝑀𝑎𝑡𝑟𝑖𝑥 𝑑𝑒𝑛𝑠𝑖𝑡𝑦
Lithology
Matrix density,
gm/cc
Sandstone
2.65
Limestone
2.71
Dolomite
2.87
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Porosity: Matrix volume calculations
2. Fluid displacement
 Sample is crushed ( Reduced to grains)
 Use gravitational method
Immerse in a container
measure the fluid displaced from the container
 Volume of displaced fluid= volume of matrix
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Porosity: Matrix volume calculations
3. Boyle's Law (Gas expansion)




Depend on Boyle's Law P1V1 = P2V2
Put core in Second chamber
Evacuate
Open valve
Vm =VT - V2
V2 = Vol. of first chamber & volume of second chamber-matrix volume of core ( calculated)
VT = Vol. of first chamber +vol. second chamber (known)
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Porosity: Pore volume calculations
Vp calculations by Fluid saturation method
 Measure the weight of dry sample in air
 Measure the weight of sample when saturated in water
 Difference in weight represents the weight of pores
𝑆𝑎𝑡𝑢𝑟𝑎𝑡𝑒𝑑 𝑐𝑜𝑟𝑒 𝑤𝑒𝑖𝑔𝑕𝑡 𝑖𝑛 𝑎𝑖𝑟 − 𝐷𝑟𝑦 𝑐𝑜𝑟𝑒 𝑤𝑒𝑖𝑔𝑕𝑡
𝑉𝑝 =
𝑊𝑎𝑡𝑒𝑟 𝑑𝑒𝑛𝑠𝑖𝑡𝑦
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Porosity
Significance of porosity
 Storage capacity of the reservoir
 Important parameter in volume calculations for OIIP
OIIP =
𝟕𝟕𝟓𝟖 𝑨 𝒉 𝜱 (𝟏−𝑺𝒘𝒊)
𝑩𝒐𝒊
Where:
OOIP = original oil in place, STB
7,758 = factor converting acre-feet to barrels
A = reservoir area, acres
h = average reservoir thickness, feet
Φ = average reservoir porosity, fraction bulk volume
Swi = average water saturation, fraction pore volume
Boi = oil formation volume factor, RB/STB
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Porosity
Averaging of porosity
The reservoir rock may generally show large variations in porosity vertically.
A change in sedimentation or depositional conditions cause this variation.
These averaging techniques:
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