rock physics coures_lect#5

8/18/2019 Rock Physics Coures_Lect#5

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Rock Physics: Principles and Applications

Petroleum Geophysics

Lectures and practical sessionsPGH-214-GEOPH

Lecture #5


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Lecture outcome

During this lecture, we will learn:

1. Effect of hydrocarbon on the bulk density

2. Relationship between the bulk density and P-wave velocity

3. Relationship between the P-wave and S-wave velocities


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Seismic cross section

Do you see seismic amplitude anomalies?


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Low Vp High Vp

Well A

P-wave velocity section

Well B


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Effect of hydrocarbon presence on the bulk density


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Pore fluid effect on bulk density

Gas Saturation - Rho Plot

2.05

2.1

2.15

2.2

2.25

2.3

2.35

2.4

0 20 40 60 80 100

Gas Saturation (%)

R h o ( g / c c )

ρb

ρ b

• The bulk density of the rock decreases gradually with increasing in the hydrocarbon saturation.

• There is a significant difference in the bulk density between the live gas and residual gas saturated rocks.

• The bulk density could be used to estimate the hydrocarbon saturation.

• When seismic data inverted into bulk density, then it can be used to define the hydrocarbon accumulations

Water-bearing sand


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Marlin Field, Gulf of Mexico


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Low density(gas)

High density(water)

Marlin Field, Gulf of Mexico

GR Density


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Relationship between the bulk density and P-waveseismic velocity


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Relationship between P-wave velocity and bulk density

Gardner’s equation “relation” (1974)

Gardner’s equation is applicable only for

the water-bearing reservoirs. It is not

applicable in the hydrocarbon reservoirs.

where ρb is bulk density in g/cc,

Vp is the P wave velocity in km/s,and d and f are constants.


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According to the following equation, the P-wave velocity has reverse relationship

with the bulk density. How can the Gardner’s equation be valid ?!

The Gardner’s equation (proportional relationship between the P-wave velocity and

bulk density) is valid as the velocity is not only controlling by the bulk density of

the rock but it is mainly affecting by the bulk and shear moduli of the rock.


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Castagna et al modification (1993)


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Castagna et al (1993) Vp-Density relationship

Ahmed Hafez et al, 2013

The constants (d and f) of the Gardner’s equation are highly depending on

the lithology . Theses constants are vary from one sedimentary basin to

another due to subsidence history and compaction diagenesis.


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Relationship between the P-wave seismic velocity and S-wave seismic velocity


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Relationship between P-wave and S-wave velocities

Castagna et al (1985)

The mudrock line also called Castagna's equation or Castagna's relation, is

an empirical linear relation between seismic P-wave velocity and S-wave velocity in brine-

saturated clastic rocks (sands, sandstones and shales).

Water point:

Vp = 1.5 km/s

Vs= 0 km/s

Quartz point:

Vp = 6.03 km/s

Vs= 4.1 km/s

Mudrock line

Where Vp and Vs in km/s


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Ahmed Hafez et al, 2013

The mudrock line slope is varying from one basin to another. Therefore, it

is strongly recommended to build a mudrock line for each basin per each

stratigraphic level.


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1

2

3

4

5

6

0

0 1 2 3 4

Vs (km/s)

V p

( k m / s )

Mudrock line

(water sand and shale)

Gas sandVp/Vs~1.5

Gas sand points are deviated from the mudrock line because the gas sand reservoirs

have lower P-wave velocity than the shale and water sand.


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1

2

3

4

5

6

0

0 1 2 3 4

Vs (km/s)

V p

( k m / s )

Clean gas sand

Shaly gas sand

The increasing of the shale/clay content of the sand reservoirs are resulting in increasing in

the VpVs ratio and so that the deviation from the mudrock line will decrease

rock physics coures_lect#5

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