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Calculating Formation Water Resistivity From the SP Log

The standard SP response equation can be expressed as;

(1)

Re-arranging Equation 1 and solving for R_weq;

where;

SSP is Static SP (mv) measured from a shale baseline,

R_mfeq is equivalent resistivity (activity) of mud filtrate at formation temperature,

R_weq is equivalent resistivity (activity) of formation water,

or,

Analysis Procedure

Measure SP deflection from the shale baseline, preferably from a thick, clean water sand.

If mud filtrate resistivity (R_mf ) at 75^oF is higher than 0.1, Correct R_mf to formation temperature using;
Rmf = Rmf at surface temperature x (Surface temperature + 6.77) / (Bottom hole temperature + 6.77)

.. Or use a chart such as Schlumberger's General Chart 9 and use R_mfeq = 0.85R_mf.

If R_mf at 75^oF is lower than 0.1 then correct R_mf to formation temperature and use a chart such as Schlumberger's Sp-2.
Note:Schlumberger charts are available for download at www.schlumberger.com

Use equation (1) to calculate R_weq .

Estimate formation water resistivity (Rw) using Chart SP-2.

Estimate water salinity using chart Gen-9.

Notes:

Drilling fluids rarely have the same salinity as the drilled formations. Therefore, when a well is drilled the ionic equilibrium established over geological time is disturbed resulting in a movement of ions and development of an electrical potential between the wellbore and the formation.

SP deflection to the left from the shale baseline ("normal SP") indicates that formation water is more saline than the mud filtrate.

SP deflection to the right from the shale baseline ("reverse SP") indicates that the formation water is less saline than the mud filtrate.

Where possible, SSP should be measured in the thickest and "cleanest" sands. However, service companies have charts to correct for thin beds.

Sometimes the SP log baseline will drift or shift with depth. Baseline drift must be removed from the SSP measurement.

Sometimes hydrocarbons can supress the SP response.

SP response will be supressed by shale laminations, dispersed clay and thins beds.

Sometimes a "streaming potential" can affect the SP log. This is not a funtion of salinity.

Example Calculations

Well:
Formation:
Surface R_mf: Warthog - 1A
Olivier
0.268 Ohmm at 84.2 ^oF

Depth
(m) SSP
(MV) Temp
(^o F) K _c R_mf
(Ohmm) R_mfeq
(Ohmm) R_weq
(Ohmm) R_w
(Ohmm) Salinity
(NaCl Eq.)

940 -16 93 73.4 0.244 0.220 0.133 0.150 ~36000

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SSP	is Static SP (mv) measured from a shale baseline,
R_mfeq	is equivalent resistivity (activity) of mud filtrate at formation temperature,
R_weq	is equivalent resistivity (activity) of formation water,
	or,

Well: Formation: Surface R_mf:	Warthog - 1A Olivier 0.268 Ohmm at 84.2 ^oF
Depth (m)	SSP (MV)	Temp (^o F)	K _c	R_mf (Ohmm)	R_mfeq (Ohmm)	R_weq (Ohmm)	R_w (Ohmm)	Salinity (NaCl Eq.)
940	-16	93	73.4	0.244	0.220	0.133	0.150	~36000