WELLOG                         SHALE CORRECTION

Revised 8-11-2007

© 2007 WELLOG

All Rights Reserved

AFFECT OF SHALE:

The presence of shale in formations can influence the predictability of hydrocarbon analysis. In order to correct for shale, certain corrections are applied in log analysis. Calculations are based on shale content. Learn more about determination of shale content.

SHALE CORRECTIONS:

Correction for the effect of shale may be applied as follows:

Q          = (FV – FD)/ FV [chart]

FN       = F Vsh C                    where: C = Empirical reduction factor

FV       = F+ Vsh ((Dtsh – Dtma)/( Dtf – Dtma)) = Fa/2-a

In disseminated clays Dtsh may = Dtf and FV = F + Vsh

FD       = F + ((rma-rsh)/( rma-rf))

Vsh      = (rb (GR-Bo))/( rsh(GRmax – Bo))

% shale           =         1 – PSP/SSP = 1 - a

USING TWO POROSITY LOGS:

Two porosity logs can be used to determine shale volume (Vsh) as follows:

DENSITY-NEUTRON:

Vsh = (FN-FD)/( FNsh-FDsh)

VELOCITY-NEUTRON:

Vsh = (FN-FV)/( FNsh-FVsh)

DENSITY-VELOCITY: (CROSSPLOT)

Vsh = (FV-FD)/(FVsh-FDsh)

SHALE AND/OR GAS:

When shale and/or gas are present in a well, effective porosity (Fe) can be used to more accurately determine water saturation (Sw).

SHALE: (no gas)

Laminated:

Fe = FN – (Vsh x FNsh)

Fe = FV – (Vsh x FVsh)

Fe = FD – (Vsh x FDsh)

Dispersed:

Fe = Fa – Vsh

Structural:

Fe = Fa

GAS: (no shale)

Fe = FN/((Sxo x IHw + (1-Sxo) x IHg)

SHALE & GAS:

Fe = (FN-(Vsh x FNsh))/(Sxo + (1-Sx0) x IHg)

Where:

Fa = apparent porosity (as read from log)

Sxo = water saturation of the invaded zone

IHw = Hydrogen index of water (1.00 oil and water)

IHg = Hydrogen index of gas (fraction)

rag = apparent gas density (gm/cc)

SHALY SAND NOMOGRAPH:

WELLOG has published a shaly sand nomograph to assist in shaly sand log interpretation.

Reference: Gearhart-Owen Industries, Inc., Log Interpretation Reference Data Handbook, 1972

If you need more information on shale correction, contact info@wellog.com