PNGE 450 EXAM 1 SP Log

Question 1

SP Log introduction
(Spontaneous Potential Log)

Answer 1

 SP is a passive measurement and no energy is provided
by the logging tool.
 SP log is a record of direct current (DC) voltage
differences between the naturally occurring potential of a
moveable electrode in the well bore, and the potential of
a fixed electrode located at the surface.
 Electric potentials are created by chemically induced
electric current.
 This electric potential is measured in millivolts.

Question 2

Applications

Answer 2

 Indicating permeable zones
 Detecting permeable zone boundary
 Indicating shaliness (shale volume) of a formation
 Determining Rw and hence of formation water salinity
 Indicating depositional environment changes
 Correlation

Question 3

• Liquid Junction Potential (Elj)

Answer 3

 Diffusion of ions with higher mobility (Cl-) from concentrated solution to the dilute solution.

 Happens from virgin zone to the invaded zone

Question 4

• Membrane Potential (Em)

Answer 4

 Selective diffusion of ions (Na+) from concentrated solution to the dilute solution.
 Happens from virgin zone to the borehole.
 Dominant source of SP

Question 5

Liquid Junction Potential

Answer 5

If two sodium chloride solutions of different concentrations
are separated by a permeable membrane, then ions from
the most concentrated solution will tend to migrate into the
less concentrated solution.
 Na+ and Cl- ions do not move with the same alacrity. Cl

ions (anions) move faster than Na+ ions (cations). The net
effect of more anions (negative ions) in the weak solution
and more cations (positive ions) in the more concentrated
solution creates a potential difference. This is known as a
liquid junction effect.
 In terms of the solutions present in a formation, mud filtrate can be substituted for the weak solution and formation water for the more concentrated solution. The potential is referred
to as the liquid-junction potential (Elj)

Question 6

Membrane Potential

Answer 6

Shales are permeable to Na+ ions, but not so
permeable to Cl- ions. A shale, therefore, acts as
an ionic sieve. This phenomenon occurs because
of the crystalline structure of clay minerals, which
have exterior surfaces with exchange sites to
which cations can temporarily cling.
 Because Na+ ions effectively manage to
penetrate the shale bed from the saline formation
water to the less saline mud column, a so-called
membrane potential (Em) is created.
 ET= Elj +Em =SP

Question 7

Study Reference

Answer 7

slide 7 and 8 SP LOG slides

Question 8

Reverse SP

Answer 8

A. Rw&raquo_space; Rmf Large positive (right) deflection

B. Rw > Rmf Positive (right deflection)

C. Rw = Rmf No deflection

Question 9

Normal SP

Answer 9

D. Rw < Rmf Negative (left) deflection

E. Rw &laquo_space;Rmf Large negative (left) deflection

Question 10

Static SP (SSP)

Answer 10

In the terminology used here, SP refers to observed SP deflection on the log and SSP
(static SP) to the value it would have had if all disturbing influences were removed. These
disturbing influences result in a smaller recorded SP deflection than SSP deflection.

Question 11

Conditions where SP measurement is equal to SSP:

Answer 11

Thick formations
 Water-bearing zones
Clean (no shale) zones
High permeable formation

Question 12

Factors Affecting SP

Answer 12

 No SP in oil-based muds: absence of electrical path in the
mud column
 Shaly formation will reduce the measured SP.
 Hydrocarbon saturation reduces SP deflection.
 Thin beds avoid SP to fully develop.
 SP amplitude deflection is also affected by following
parameters:
* The formation true resistivity, Rt .
* The invaded zone diameter, Di
.
* The invaded zone resistivity, RXO.
* The surrounding formations resistivity, Rs
* The mud resistivity, Rm.
* The diameter of the borehole, dh.

Question 13

Summary 1

Answer 13

• SP log deflection is the result of electric potential exists between the wellbore and formation due to
  the different salinity/resistivity between mud filtrate and formation water:
   Negative deflection (normal SP) when Rmf > Rw
   Positive deflection (Reverse SP) when Rmf < Rw
   No deflection when Rmf = Rw

Question 14

Summary 2

Answer 14

SP log can be used to:
 Detect permeable formations
 Detect boundary of permeable formation
 Calculate Shale volume, Vsh
 Calculate formation water resistivity, Rw