WELLOG POROSITY CORRECTION & CROSS PLOT

PART II, PAGE 4

COMBINATION POROSITY LOGS:

Compensated Neutron porosity (apparent – limestone) and Acoustic velocity (Dt) combination:

Corrections are applied to correct the compensated neutron log for other matrices using information from the

Acoustic log.

Dolomite - Limestone:

A two step procedure is performed;

Step 1: The calculation for f(corr) in Dolomite is performed using apparent neutron porosity, f(CN).

Assuming: Dt (f) = 189 usec/ft.

((Dt – Dt(lms)) f(dol) – f(CN) (Dt(dol) – Dt(lms))

f(corr) = ---------------------------------------------------

((D(f) – Dt(lms)) f(dol) – 100 (Dt(dol) – Dt(lms))

Step 2: The calculation for matrix porosity is made using the corrected porosity f(corr) found in step 1.

When f(corr) < 4 percent, the porosity in dolomite is calculated from the equation:

f (dol) = 100 - (101.64 – (f /.0311 + 6.97))^1/2 /(100 - <f) x 100

When f(corr) > or = to 4 percent, the porosity in dolomite is calculated from the equation:

f (dol) = 100 - ( 94 – f) / (100 - f ) x 100

Sandstone - Limestone:

A two step procedure is performed;

Step 1: The calculation for f(corr) in sandstone is performed using apparent neutron porosity f(CN).

Assuming: Dt (f) = 189 usec/ft.

(Dt – Dt (lms)) x f(sd) – f(CN) x (Dt (sd) – Dt (lms))

f (corr) = -------------------------------------------------------

(Dt (f) – Dt (lms)) x f(sd) – 100 x (Dt (sd) – Dt (lms))

Step 2: The calculation for matrix porosity is made using the corrected porosity found in step 1.

f (sd) = (104 – f) / (100 – f) x 100

CROSS PLOT:

Cross plots were given early consideration in “Openhole crossplot Concepts – a powerful technique in Well Log Analysis” in the Journal of Petroleum Technology, Vol. 33, March, 1981 pages 535-549, By Walter Fertl.

For information on cross plot techniques, go to cross plots.

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