Petrophysics compressibility

8/16/2019 Petrophysics compressibility

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Neutron Logs

The neutron log can be used to determine the hydrogen content of the logged interval by counting

captured gamma rays or neutrons counted at a detector

Fast neutrons emitted by a radioactive source in the neutron log are slowed by collisions to

thermal energies

The ensuing thermal neutrons are captured by nuclei, which then emit detectable gamma rays.

Hydrogen nuclei are especially effective because hydrogen has a relatively large capture cross-

section for thermal neutrons. The neutron log response indicates the concentration of hydrogen in

the fluid filled pore space.

Oil and water have about the same concentration of hydrogen, while gas has a relatively low

hydrogen concentration

A small response suggests a low hydrogen concentration which can be interpreted as gas

presence

Formation Evaluation


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Neutron log



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Neutron log



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Neutron log



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Compressibility, c is the fractional change in volume perunit change in pressure

Compressibility of porous rocks

p

V

V

p

V

V c

T

T

1


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Normally pressured reservoirs

Downward force Fo by the overburden must be balanced by

upward forces of the matrix Fm

and the fluid Ff



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Fo = Fm + Ff

po = pm + pf

po = 1.0 psi/ft

pf = 0.465 psi/ft



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As fluid is produced from a reservoir, the fluid pressure pf will

usually decrease:

The force on the matrix increases, causing a decrease in bulk

volume and a decrease in pore volume



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Type of compressibility



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Formation compressibility



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V o

l u m e e x p e l l e d

C f

Overburden pressure, psi


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Measurement of compressibility



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Measurement ofFormation

compressibility

LaboratoryEquipment



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Fluid saturation

Fluid Saturation

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Petrophysics compressibility