Density Log - Engineering Geology - Lecture Slides, Slides of Genetic Engineering

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THEORY OF MEASUREMENT

• A 1.5 Ci Cs137 chemical source bombards gamma rays at 662keV energy into the formation.
• The high-energy gamma rays interact with the electrons of the formation by way of Compton scattering and lose energy in the process.
• Other processes also occur namely photoelectric absorption and pair production, although pair production only becomes significant at energies above 1MeV.
• A low number of gamma rays detected through Compton scattering will indicate a high electron density.
• A spectral or litho density tool measures not only the bulk density but also a photoelectric absorption index PEF.

APPLICATION

• The density tool is used to determine formation density and estimate formation

porosity.

• Together with other tools like the Neutron, the lithology and formation fluid type

can also be determined.

• The density tool can distinguish between oil and gas in the pore space by virtue of

their different densities.

• Modern density tools also measure the photoelectric effect to help distinguish

between rock lithologies, recognize presence of heavy minerals, fracture

identification when barite is present and additional clay evaluation.

• In addition the density can be used to determine Vclay and to calculate reflection

coefficients to process synthetics.

Synthetic Processing

• The Density is used along with the Sonic velocity to compute Acoustic impedance (I) by:- I = Density(rho) * Velocity (V)
• The Reflection coefficient (R) at a bed boundary is then determined by:-

R = (I 2 - I 1 ) / (I 2 + I 1 )

• The Reflection coefficient is then used to generate synthetic processing curves that can match the exploration seismic

Using compatible limestone scales for density (1.95 RHOB 2.95) and neutron curves (45% NPHI –15%) then:-

• In a clean wet limestone RHOB and NPHI curves will overlay.
• In a shale RHOB plots right of NPHI depending on the amount of shale present.
• In a gas limestone RHOB plots > 3pu left of NPHI
• In a clean wet sand RHOB plots 3pu left of NPHI
• In a dolomite RHOB plots right of NPHI

Using compatible sandstone scales for density (1.90 RHOB 2.90) and neutron curves (45% NPHI –15%) then:-

• In a clean wet sand RHOB and NPHI curves will overlay.
• In a shale RHOB plots right of NPHI depending on the amount of shale present.
• In an oil sand RHOB plots 1-3pu left of NPHI
• In a gas sand RHOB plots > 3pu left of NPHI