The document discusses key properties of reservoir rocks including porosity, permeability, and fluid saturation. It defines porosity as the ratio of pore space volume to total rock volume and describes different types of porosity such as primary, secondary, and effective porosity. It also defines permeability as the ability of rocks to transmit fluids and discusses how it is affected by factors like grain size. Additionally, it explains concepts such as critical oil saturation and critical water saturation which refer to saturation levels above which fluids will flow.

1. Properties of reservoir rocks
Porosity
Permeability
Fluid saturation
Mahboob Ahmed

2. Porosity
 Ratio of the volume of space to the total volume of a rock.
 Porosity of a rock is a measure of its ability to hold a fluid.
 Porosity is expressed as a percentage of the total rock which is taken up by
pore space.
 a sandstone may have 8% porosity.
 This means 92 percent is solid rock and 8 percent is open space containing oil,
gas, or water
Total porosity
 the ratio of the entire pore space in a rock to its bulk volume.
Effective porosity
 percent of bulk volume occupied by interconnected pores spaces.
 A total porosity less the fraction of the pore space occupied by shale or
clay
 In very clean sands, total porosity is equal to effective porosity.

3. Primary porosity
 porosity of the rock that formed at the time of its deposition.
 primary porosity of a sediment or rock consists of the spaces between the
grains.
 Primary porosity decrease due compaction and packing of grains.
 Intergranular pores of clastics or carbonates.
 Primary porosity less than one percent in crystalline rocks like granite.
 more than 55% in some soils.
Secondary porosity
 develops after deposition of the rock.
 Vugular spaces in carbonate rocks created by the chemical process of
leaching.
 fracture spaces formed due to stress distortion in reservoirs rocks.

4. Porosity in sandstone
• Sandstone usually has regular
grains; and is referred to as a
grainstone.
• Porosity Determined mainly by
the packing and mixing of grains.
• Fractures may be present.

5. Porosity in Sandstone
 The porosity of a sandstone
depends on the packing
arrangement of its grains.

6. Grain-Size Sorting in Sandstone

7. SANDSTONES POROSITY TYPES
Intergranular (Primary)
Interstitial Void Space Between Framework Grains.
Micropores
Small Pores Mainly Between detrital Framework Grains or Cement.
Dissolution
Partial or Complete Dissolution of or Authigenic Grains (Can Also Occur
Within Grains)
Fractures
Breakage Due to Earth Stresses.

9. CARBONATES POROSITY TYPES
Interparticle porosity
Each grain is separated, giving a
similar pore space arrangement
as sandstone.
Intergranular porosity
Pore space is created inside the
individual grains which are
interconnected.
Intercrystalline porosity
Produced by spaces between
carbonate crystals.
Mouldic porosity
Pores created by the dissolution
of shells, etc.

10. CARBONATES POROSITY TYPES
Fractured porosity
Pore spacing created by the
cracking of the rock fabric.
Channel porosity
Similar to fracture porosity but
larger.
Vuggy porosity
Created by the dissolution of
fragments, but unconnected.

12. Permeability
 The rate of flow of a liquid through a formation depends on:
– The pressure drop.
– The viscosity of the fluid.
– The permeability.
 Permeability measures the capacity and ability of the formation to
transmit fluids.
 it controls the directional movement and the flow rate of the reservoir
fluids in the formation.
 The unit of measurement is the Darcy.
 Reservoir permeability is usually quoted in millidarcies, (md).

13. DARCY LAW
K = permeability, in Darcies.
L = length of the section of
rock, in centimetres.
Q = flow rate in centimetres /
sec.
P1, P2 = pressures in bars.
A = surface area, in cm2.
μ = viscocity in centipoise.

14. types permeability
Absolute Permeability
When the medium is completely saturated with one fluid, then the permeability
measurement is often referred to as specific or absolute permeability
Effective Permeability
When the rock pore spaces contain more than one fluid, then the permeability to a
particular fluid is called the effective permeability. Effective permeability is a measure
of the fluid conductance capacity of a porous medium to a particular fluid when the
medium is saturated with more than one fluid
Relative Permeability
Defined as the ratio of the effective permeability to a fluid at a given saturation to the
effective permeability to that fluid at 100% saturation.

15. PERMEABILITY AND ROCKS
In formations with large grains, the permeability is
high and the flow rate larger.
The permeability in the horizontal direction is controlled by the
large grains.

16.  In a rock with small grains the permeability is less and the flow lower.
The permeability in the vertical direction is controlled by the small grains
 Grain size has no bearing on porosity, but has a large effect on permeability.

17. CLASTIC RESERVOIRS
Permeability
• Determined mainly by grain size
and packing, connectivity and
shale content.

18. CARBONATE RESERVOIRS
Permeability
Determined by deposition
and post deposition events,
fractures.

20. FLUIDS IN A RESERVOIR
 A reservoir normally contains either water or hydrocarbon or a mixture.
 The hydrocarbon may be in the form of oil or gas.
 The specific hydrocarbon produced depends on the reservoir pressure and
temperature.
 The formation water may be fresh or salty.
 The amount and type of fluid produced depends on the initial reservoir
pressure, rock properties and the drive mechanism.
 Initially, pore space filled 100% with water
 Connate water saturation remains in hydrocarbon zone.

21. Critical oil saturation, Soc
For the oil phase to flow, the saturation of the oil must exceed a certain
value which is termed critical oil saturation. At this particular saturation,
the oil remains in the pores and, for all practical purposes, will not flow.
Movable oil saturation, Som
Movable oil saturation Som is another saturation of interest and is defined
as the fraction of pore volume occupied by movable oil as expressed by
the following equation:
Som = 1 - Swc - Soc
where
Swc = connate water saturation
Soc = critical oil saturation

22. Critical gas saturation, Sgc
As the reservoir pressure declines below the bubble-point pressure, gas
evolves from the oil phase and consequently the saturation of the gas
increases as the reservoir pressure declines. The gas phase remains
immobile until its saturation exceeds a certain saturation, called critical
gas saturation, above which gas begins to move.
Critical water saturation, Swc
The critical water saturation, connate water saturation, and
irreducible water saturation are extensively used interchangeably
to define the maximum water saturation at which the water phase
will remain immobile.