Page 44 - Basic Well Log Analysis for Geologist

THE SPONTANEOUS POTENTIAL LOG

ee eee ee ee scree ence cece neeeeec eee eee en

is corrected to static SP (SSP). SSP represents the The value of Ry. is then corrected to Ry, using the chart
maximum SP a formation can have if unaffected by bed illustrated in Figure 14, for average deviation from sodium
thickness. Figure |2 is a chart used to correct SP to SSP. chloride solutions, and for the influence of formation
The data necessary to use this chart are: (1) bed thickness, temperature. A careful examination of Figures 11-14 should
(2) resistivity from the shallow-reading resistivity tool (Rj), help you gain an understanding of the Ry from SP
and (3) the resistivity of the drilling mud (R,,) at formation procedure. But, rather than using charts in the procedure,
temperature, you might prefer using the mathematical formulas listed in
Once the value of SSP is determined, it is used on the Table 3.
chart illustrated in Figure 13 to obtain a value for the Itis important to remember that normally the SP curve
Ray Ry, ratio. Equivalent resistivity (Ry.) is obtained by has less deflection in hydrocarbon-bearing zones: this is
dividing R,,- by the RyRy, value from the chart (Fig. 13). called Avdrocarbon suppression, and results in too high a

Table 3.0 Mathematical Calculation of Ry from’ SSP (modified after Bateman & Konen, 1977).

Instead of charts, some individuals may preter using these formulas. especially if they want to computerize the procedure.
Rig At 75°F = Rasy emp X (temp + 6.77)/81.77
Correction of R,,; to 75°
K = 60 + (0.133 « Tp)
R Rye = LO-SSPK f
mie’ we
R (146 x Runt — 5)(337 x Ran
mie ~~ + 77)
Rae formula if Ry at 7S°F < 0.1

Rinte = 0.85 x Rung
Rune Formula if Ryyp at 75° > 0.1

Ry. = Rintef Rinte’ Rye)
Ry at 75°F = (77 X Ry. + SyUl46 — 377 & Ry.)
Ry at 75° formula if Ry. < 0.12
R, at 75°F __ [0.58 _ [O10 O9xRwe-O aa

Ry at 75° formula if Ry. = 0.12
R,, at formation temperature = Ry at 75° « 81.771; + 6.77)

"Riatremp = Rye ata temperature other than 75°F
“The e subscript (i.e. Rye) stands for equivalent resistivity.

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