petrophysics 1

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I ) BASIC DEFINITION

• It is the physical & chemical properties of rocks, which are related to the

distribution of pores & fluids, particularly as they are directly related to the

detection &evaluation of hydrocarbon-bearing layers.

• Rock: = minerals +fluids

• Detection: only recognition of the presence or absence of hydrocarbons in

rocks.The presence of hydrocarbons in rocks does not necessarily mean that they are

present in either commercial or producible quantities.

• Evaluation : a ) Determination of the hydrocarbon volume in place ,

b ) Estimation ( if possible ) of the initial rate of production .

II ) BASIC PARAMETERS FOR EVALUATING A RESERVOIR

)Indirect )( Indirect ) ( Direct&

Indirect(

)Indirect(

A) BASIC DEFINITIONS

Petro h sics-:

1(Petrophysical Parameters 2(Geometrical Parameters

Permeable bed thickness ( h(

a ) Porosity)Φ) (PHI(

Geometrical Parametersc) Fluid

saturations

b)

Permeability



a) Porosity (Φ) (PHI-: (

i ) General definition- :Porosity: - It is the pore volume (i.e., the volume of void spaces or pores within a rock or

formation) per unit volume of the rock or formation.

It is usually expressed as a percentage (%).

P.V.: Pore Volume B.V.: Bulk Volume (B.V. = P.V. + V ma)

V ma: Matrix volume

Ex.:- If 1/5 th of the total rock volume was pore space, So:

Φ = 1/5 x 100 % = 20% & V ma = 100% - 20% = 80%

ii ) The basic Petrophysical types of porosity-:

1 ) Total Porosity ( Φ total, Φ T , ΦND , Φ D , or Φ N ) :-

It is the ratio of the total pore volume to the bulk volume of the rock or formation.

It can be obtained from a density ( γ - γ ) log , neutron log , or both , since these logs

respond to total porosity , regardless of porosity type .

1(Petrophysical Parameters

P.V.

Φ = ------- x 100%

B.V.

1(Total

Porosity

(Φ total,

Φ T ,

ΦND ,ΦD , or

Φ N(

2(Primary

Porosity

)Sonic or

Acoustic

Porosity)

(ΦSonic,

or, ΦS(

3(Secondary

Porosity

-)index )

(Φ sec , or

4(

Effective

Porosity

(Φ eff , or

Φ e(

5(EPT

-

Porosity

(Φ EPT(

6(Weighted

–average

Porosity

7(

Crossplot

Porosity

(Φxp(



ΦD : Density porosity ( Porosity from a density log ) ,

Φ N: Neutron porosity (Porosity from a neutron log).

2(Primary Porosity (Sonic or Acoustic Porosity) (ΦSonic, or, ΦS: (

It is the ratio of the volume of the pores that have formed during the formation of the rock to

the bulk volume of the rock or formation.

It can be obtained indirectly from a sonic (an acoustic) log, since this log tend to largely

ignore vuggy porosity and fractures & respond mainly to intergranular porosity.

3(Secondary Porosity (Φ sec ) or Secondary Porosity index (SPI-:(

It is the ratio of the volume of the secondary pores (pores that have formed after the

formation of the rock due to the action of formation water, tectonic forces or stresses, etc.) to the

bulk volume of the rock or formation.

An estimate of the secondary porosity can be obtained by using a sonic log + Density &/or

Neutron log.

Examples of Secondary pores : Preferentially dissolved feldspar grains or fossils from

sandstones , fracture pores , Vuggy pores , etc.,

4(Effective Porosity (Φ eff , or Φ e _: (

It is the ratio of the volume of the interconnected pore spaces to the bulk volume of the rock or

formation.

It can be obtained by using a Gamma Ray ( GR ) or Spontaneous Potential ( SP ) log +

Density &/or Neutron log.

Φsh : Shale Porosity . Vsh : Shale Volume proportion .

5(EPT - Porosity (Φ EPT: (

It is the porosity that can be obtained from the EPT ( Electromagnetic Propagation Tool ) . It

is mainly water-filled Porosity , since the EPT measurement is affected primarily by the water-

filled porosity.

ΦD+ Φ N

Φ total = ΦND = ΦD , or Φ N , or ------------

2

Φ primary ≈ ΦSonic ( or Φs(

Φ Sec ( or SPI ) = Φ total - Φs

Φeff = Φ total - ΦshVsh



6 ) Weighted –average Porosity :-

Generally speaking , a reservoir rock may show large variations on porosity vertically ( across

the bedding planes ) but does not show very great variations in porosity parallel to the bedding

planes . However , a change in sedimentation or depositional conditions can cause the porosityin one portion of the reservoir to be greatly different from that in another area . In such case , a

so-called weighte-average porosity will be used to characterize the rock .

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petrophysics 1