Henderson  Petrophysics

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,
Rmfeq	is equivalent resistivity (activity) of mud filtrate at formation temperature,
Rweq	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  Schlumberger chart Gen-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  Schlumberger chart SP-2 to determine R_mfeq .

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 Rmf  :	Warthog - 1A Olivier 0.268 Ohmm at 84.2 oF
Depth (m)	SSP (MV)	Temp (o F)	K c	Rmf (Ohmm)	Rmfeq (Ohmm)	Rweq (Ohmm)	Rw (Ohmm)	Salinity (NaCl Eq.)
940	-16	93	73.4	0.244	0.220	0.133	0.150	~36000

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