Page 112 - Basic Well Log Analysis for Geologist

LOG INTERPRETATION

Rur= resistivity of the mud filtrate at formation The following results:
temperature
(Su g ys — Rul,
R,, = resistivity of the formation water at formation
temperature (Sy 4/5 . Rin Rw
Therefore:
When S,, is divided by S,,. the formation factor
(F = a/™) is cancelled out of the equation because ~ f RAR, 98
formation factor is used to calculate both S, and S,, (Table
Sr 7 Ra/Ry
2). This can be very helpful in log analysis because, from
the ratio of (R,,/R,)/(RyR,,). the geologist can determine a or S = RAR, U.025
value for both the moveable hydrocarbon index (S,,/S,,) and
“ RawRy
walter saturation by the Ratio Method without knowing
Where
porosity. Therefore, a geologist can still derive useful
Sy, = water saturation uninvaded zone, Ratio Method
formation evaluation log parameters even though porosity
= shallow resistivity from Laterolog-8*,
logs are unavailable.
Microspherically Focused Log*, or
Formulas for calculating the moveable hydrocarbon
Microlaterolog*
index and water saturation by the Ratio Method are:
R, = true resistivity (Rqy or Ryyg corrected for
invasion)
{ Ra/R, \'"
Su
Microspherically Focused Log*, or temperature
Rag = resistivity of mud filtrate at formation temperature
Where Sxo RadRw Ry = resistivity of formation water at formation
S/S, = moveable hydrocarbon index
Ry, = shallow resistivity from Laterolog-8*, uninvaded zone by both the Archie and Ratio methods, he
After the geologist has calculated water saturation of the
Microlaterolog* should compare the two values using the following
R, = true resistivity (Ripg or Rppg corrected for observations:
invasion)
1. IfS, (Archie) = S, (Ratio), the assumption of a
Rip = Tesistivity of mud filtrate at formation
step-contact invasion profile ts indicated to be correet, and
lemperature
all values determined (S,,. Ry. Ry. and dj) are correct.
Ry = resistivity of formation water at formation
2. IfS, (Archie) > S, (Ratio) then the value for Ry/R,
temperature
is too low. Ry, 1s too low because invasion is very shallow,
or R, is too high because invasion is very deep. Also. a
If the ratio S,/S,, 1s equal to 1.0 or greater, then
transition type invasion profile may be indicated and S,,
hydrocarbons were not moved during invasion. This is true
(Archie) is considered a good value for Sy.
regardless of whether or not a formation contains
3. If Sy (Archie) < S,, (Ratio) then the value for R,,/R,
hydrocarbons. Whenever the ratio of S,/S,,, is less than 0.7
is too high. R,, 1s too high because of the effect of adjacent,
for sandstones or less than 0.6 for carbonates, moveable
high resistivity beds. or Ripg (Ry is too low because Ry, is
hydrocarbons are indicated (Schlumberger, 1972).
less than R,. Also, an annulus type invasion profile may be
To determine water saturation (S,,) by the Ratio Method,
indicated and/or S,,, < Sy’. In this case a more accurate
you must know the flushed zone’s water saturation. In the XO
Sxo (SWcor = corrected water saturation of the uninvaded
flushed zone of formations with moderate invasion and value for water saturation can be estimated using the
“average” residual hydrocarbon saturation, the following following equation (from Schlumberger, 1977):
relationship is normally true: (Sweor = Swa < (: Swa 0.2: ”
5
wr
Where: Sxo = (Syl Where:
Sy» = water saturation of the flushed zone
zone
S, = water saturation of the uninvaded zone Swa = water saturation of the uninvaded zone
However, by substituting the above equation in the (Archie Method)
relationship: S, \2 Ryo/Ry Swr = wuter saturation of the uninvaded zone (Ratio
Method)
4. IfS, (Archie) << Sy (Ratio). the reservoir may be a
Rint Ry

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