Page 114 - Basic Well Log Analysis for Geologist

LOG INTERPRETATION

the measured value of SP from the spontaneous potential Where:
Jog in a wet zone. The presence of hydrocarbons results in Sy = water saturation of the uninvaded zone
R, values which are greater than R,. This means that when R, = resistivity of formation water at formation
SP is calculated from the R,, and R, values, it will be lower temperature
than the measured value of SP. Rya = apparent formation water resistivity from Ry.
On the log, the R,,/R, curve is plotted as a dashed line. curve
The dashed line tracks the SP curve in wet zones (see Fig. Note: When Ry = Ry, then Sy = 100%
Wit
6A, Chapter I), but deflects to the right, away from the SP Besides the use of Ry, as a quick look method for
curve, in hydrocarbon bearing zones (see Fig. 99, Chapter hydrocarbon detection, Ry, can also be applied as a
VII).
calculated value for formation water resistivity (R,,) in
Ryq Curve —The Ry, curve is presented in track #1 as an water-bearing zones. R,,, is a value for Ry whenever S,,
overlay to the SP curve, similar to the R,,/R, curve. In equals 100%. In hydrocarbon-bearing zones, the value of
water-beiring zones. the Archie equation for the uninvaded Ry, from a water-bearing zone can be used as Ry to
zone ean be rewritten as follows:
calculate water saturation (S,,) if both zones have a constant
formation water resistivity.
Sy = VF x (R,/Ry)
Conductivity Derived Porosity Curve—-The conductivity
Or: derived porosity curve is a Dresser Atlas (1975) quick look
il
1.0 = VE xX (R,/R,) curve, plotted in track #1 along with the SP curve. Chapter
(where S,, = 100% or 1.0) III discussed how resistivity (remember that resistivity =
i ,000/conductivity) assists in determining porosity. Here.
Next. square both sides:
resistivities of the uninvaded zone (R,), rather than flushed
Sy = water saturation of the uninvaded zone Water-bearing zones: ax Ry \l’™
1.0 = F x (R,/R,)
zone (R,,) resistivities, are applied to find resistivity
Now. solve for Ry: porosity. The formulas are as follows:
Rwa = R,/F
(remember: R, = R, when S, = 100%)
b=
{-
w.
R,
Where: Sy = L00% and R, = R,
R,, = resistivity of formation water at formation Hvdrocarbon-bearing zones:
temperature
R, = true formation resistivity (R, = R, when S, = : / lim
= eee
i00%)
Ww
F = formation factor (a/b™)
Sy < |.Oand R, > R,
F = 1/2 carbonates
Where
F = 0.81/c? consolidated sands
cb = resistivity (conductivity) derived porosity
F = 0.62/2.15 unconsolidated sands
Ry = resistivity of formation water at formation
Rya= apparent water resistivity (Ry, = Ry in
temperature
water-bearing zones)
R, = formation resistivity when S, = 100%
In water-bearing zones (S,, = 100%), the calculated Ry, R, = true formation resistivity (Ry or Ryepy)
gy
value is equal to Ry. However, if hydrocarbons are present, Remember: R, = R, when S, = 100%
R, will be greater than R,, and R,,, will be greater than R,, = water saturation of the uninvaded zone
(Fertl, 1978). The R,,, curve is plotted as a dashed line a = constant (Dresser Atlas uses 1.0 for carbonates and
along with the SP curve. Low Ry, values are recorded on 0.62 for sandstones)
the left-hand side of the log. The R,,, curve will deflect to m = constant (Dresser Atlas uses 2.0 for carbonates and
the left in wet zones and to the right in hydrocarbon-bearing 2.15 for sandstones)
zones (see Fig. 79, Chapter VII). This deflection is similar
The calculated resistivity porosity of water-bearing zones
to the behavior of the R,,/R, curve for hydrocarbon or wet
be less than true porosity. This apparent porosity loss results
zones. An advantage of an R,,,curve, rather than an R,,/R, (Sy. = 1.0) is close to true porosity. However, if hydro-
curve, is that Ry values can be converted to a quantitative carbons are present, the calculated resistivity porosity will
value for water saturation (S,,). The procedure is as follows: because hydrocarbons have greater resistivity than
Sy. =V Ry/Rwa formation water. The resistivity porosity formula for

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