LA SEGUNDA MITAD DEL SIGLO XX (ENTRE GUERRAS)

Quicklook techniques can be the next step after scanning the logs, or can be conducted during the scanning process. Two of several techniques, Neutron-Density and Apparent Water Resistivity, are discussed here.

NEUTRON-DENSITY QUICKLOOK

Openhole Interpretation

The Neutron-Density quicklook technique is a quick way of determining formation lithology. The most important aspect of the technique is determining the relative positions of the neutron and density curves (with respect to each other). While the positions of the curves on the log will vary with changing porosity, the relative positions of the curves will remain fairly constant with lithology.

The photoelectric effect curve (Pe) is not required for the technique, but may be useful to resolve some ambiguities which occur with some lithologic mixtures.

The following conditions must be met for the technique to work well:

• The Neutron porosity curve is recorded on a limestone matrix.

• The Density porosity is calculated with a limestone matrix (matrix density = 2.71 g/cm3 or 2710 Kg/m3). Alternately, the bulk density curve can be used if it is scaled to closely approximate the scale of the neutron porosity curve (as shown in this example).

• The formations are assumed to be clean (no clays/shales).

• The formation fluids are assumed to be liquid-filled (water or oil only; no gas present).

The porosity of the formation can be estimated by taking the average of the neutron porosity and density porosity readings. In most cases, this will provide a porosity within one porosity unit of that derived from neutron-density crossplot porosity techniques.

The descriptions in the table below correspond to the lithologies in the example on the previous page. The responses listed in the table are general responses for the listed lithology types.

Lithology Porosity Neutron-Density response Pe response Shale -- Neutron greater than Density by some variable amount

depending on the shale composition and depth.

Variable, but about 3.

Limestone 0.05 Neutron and Density values overlay. About 5.

Limestone 0.15 Neutron and Density values overlay. About 5.

Dolomite 0.10 Neutron values greater than Density by 12 to 14 porosity units (0.12 to 0.14).

About 3.

Shale -- As described in the Shale section above. As above.

Sandstone 0.26 Neutron values less than Density (“crossover”) by 6 to 8 porosity units.

2 or slightly less.

Sandstone 0.05 Neutron values less than Density (“crossover”) by 6 to 8 porosity units.

2 or slightly less.

Anhydrite -- Neutron porosity greater than Density by 14 porosity About 5.

Coal -- Responses variable depending on coal composition.

High Neutron and Density porosities (low bulk density).

Less than 1.

units or more. Neutron porosity near zero.

Shale -- As described in the Shale section above. As above.

Salt -- Neutron porosity slightly negative. Density porosity >40 porosity units (bulk density near 2.0). Check the caliper for bad hole and bad density data.

About 4.7.

Shale -- As described in the Shale section above. As above.

Openhole Interpretation

Lithology Porosity Neutron-Density response Pe response Shale -- As described in the Shale section above. As above.

Limy Dolomite

0.10 Variable response with lithologic mix, but Neutron generally greater than Density.

3 to 5.

Sandy

LImestone generally less than Density.

0.10 Variable response with lithologic mix, but Neutron 2 to 3.

Dolomitic 0.10 Highly variable, with Neutron greate Sand

r or less than Density, depending on the lithologic mix.

2 to 5.

Shale -- As described in the Shale section above. As above.

The wa ater resistivity between

inte ls ones, or within the same

zon a st

alue of Rwa is the closest approximation to the true formation water resistivity, Rw, and that values of R greater than the minimum value are indicative of the presence of hydrocarbons. A

alculate an “apparent” water resistivity, Rwa, from the porosity and uninvaded zone resistivity measurements.

s of Rwa calculated in the other zones.

If desired, an Archie water saturation can be calculated from the Rwa values in the compared

The patterns to observe a

ith est ter-bearing, and the wa

the l val

valu Rwa ave some

hydrocarbon saturation.

n p

The Rw values in the zones that are compared are assumed to be the same.

ity (les actual Rw value.

The basis for the technique:

from Arc ’s equa

OH.01 and

APPARENT WATER RESISTIVITY, Rwa, QUICKLOOK R technique relies on the comparison of calculated values of w rva in a well. This comparison can be made between different z

e if water-hydrocarbon contact is suspected in that zone. The assumption is that this lowe v

wa

water saturation can also be calculated from the values of Rwa. The technique is:

C

Look for the lowest value of Rwa in a porous and permeable zone and compare it to the value

zones.

re:

The zone w the low value of R is the most likely to be wawa

ue of Rw in the formation.

value of R is closest to actua

Zones with es of greater than the minimum observed are likely to h

Interpretatio itfalls:

In low poros zones s than about 10 percent porosity), the Rwa value will be lower than the

Recall hie tion that

Openhole Interpretation

Combining equations OH.01 and OH.02, and solving for Rw yields:

R

⋅ φ

R

uation abo

From eq OH.03 ve, define “apparent” water resistivity, Rwa, as:

R

⋅ φ

R

r-bearin ones (S Rt = Ro and Rwa = Rw

hydrocarbon-bearing zones (Sw < 1.0):

en , the following values can be a = 1.0, m = 2.0. The Deep Induction or Deep Laterolog is used as Rt, usually

h ng, the neutron-density crossplot porosity should be used for the best estimate of

SCANNING AND QUICKLOOK TECHNIQUES: SUMMARY

he purpose of scanning and quicklook techniques is to identify potential zones of interest (both er-bearing) from the bulk of the drilled interval which usually has no

he era of computer-aided data processing, where the difference in n the zones in the well with the most

By comparing a number of zones (or different depths in the same zone, where a water-hydrocarbon contact is suspected), and assuming the zone with the lowest value of Rwa is wet, that minimum value of Rwa can be used as an estimate for the value of Rw in all the zones being considered. If the zone with the minimum Rwa value actually contains some hydrocarbons, th the other zones will be even more hydrocarbon bearing than anticipated.

In practice, especially when calculated and displayed as a curve used for simplicity:

without any environmental corrections. Porosity is usually derived from the sonic or density, wit the proper matrix and fluid parameters for the formations to be encountered. If available in real time during loggi

porosity.

T

hydrocarbon-bearing and wat production potential. Even in t

time of processing is trivial between the entire well and only interesting zones, these techniques are useful in helping the interpreter to quickly focus o

potential.

Openhole Interpretation