Well Log Interpretation SP Log

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Well Log InterpretationSP Log
Earth Environmental Science University of
Texas at Arlington
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Spontaneous Potential Log
The SP log is the oldest type of log and is still
common. It measures the D.C. voltage difference
between surface and borehole electrodes
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Spontaneous Potential Log
The millivoltages are generated by differences in
salinity between the formation waters and the mud
filtrate i.e. differences between Rw and
Rmf Cant be used with oil based mud.
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Spontaneous Potential Log

• SP Log Used to
• Detect porous permeable beds
• Calculate Rw (used in Archie Eq.)
• Estimate shale

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Spontaneous Potential Log

• Factors that affect SP response
• Thicker beds increases
• Higher Rt reduces
• More shale reduces
• Higher porosity/permeability increases if Rw ltgt
  Rmf

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Spontaneous Potential Log

• Some Definitions
• SSP Static SP Maximum SP response under ideal
  conditions (thick, permeable, porous layer) with
  assumed Rmf/Rw
• Shale Base Line Minimum SP response produced by
  thick impermeable, nonporous shale
• Hydrocarbon Suppression reduced SP response
  because petroleum has no salinity.

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Spontaneous Potential LogCalculation of Rw
WARNING Calculation of important parameters from
well log data is messy because none of the
equations are derived from basic
physical/chemical equations. All the equations
are determined experimentally, are approximations
and involve many steps.
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Spontaneous Potential LogCalculation of Rw
Steps to calculate Rw are shown in Table 2.1 of
text. Resistivities are temperature
dependent. The first step is to find the
formation temperature at the depth Rw is
required. This is just a linear interpolation
between surface and bottom hole temperature. The
petroleum industry usually uses oF, not
Celsius. Surface Temp (Ts), Formation Depth (FD),
Bottom Hole Temp (BHT), and Total Depth (TD) are
usually given on well header.
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Spontaneous Potential LogCalculation of Rw
The 2nd step is to convert Rm Rmf measured at
the surface to Rmf at Tf, the formation temp.
This is NOT a linear interpolation (see Fig 1.11
in text) The equation below is an approximation
to the graphical solution in Fig 1.11. The
constant 6.77 is 0F. For Celsius it is 21.5
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Spontaneous Potential LogCalculation of Rw
The 3nd step is to convert find the ideal
potential (SSP), correcting for bed thickness,
using the Fig. 2.3 Bed thickness is determined
from the SP log by measuring the distance between
inflection points of the SP curve (8 ft) To use
Fig. 2.3, Must use Ri, from the SN (short reading
resistivity) log next to the SP log in Fig 2.2
(33 Ohm-M). Calculate Ri/Rm (make sure you use
the temperature corrected Rm), use Fig 2.3 to get
the SP correction factor, then read off SSP from
nomogram (71)
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Spontaneous Potential LogCalculation of Rw
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Spontaneous Potential LogCalculation of Rw
The 4th step is to find the ideal formation water
resistivity, Rwe, using the following equation
(remember Rmf is at formation temperature)
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Spontaneous Potential LogCalculation of Rw
The final step is to find the actual water
resistivity, Rw, using the following equation
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Spontaneous Potential LogCalculation of Sw
Once Rw is calculated for the reservoir, use the
Archie equation to calculate Sw.
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Spontaneous Potential LogAssignment

• Create a spreadsheet using the data in Fig. 2.2
  and the formulas in Table 2.1 to calculate Rw,
  formation water resistivity, in the stratigraphic
  unit centered at 7446 in Fig 2.2. Email the
  results by next class period.
• You will need to use the graph in Fig 2.3 to
  estimate SSP.
• Some corrections to formulas in Table 2.1
• SP in third equation should be SSP
• BHT in 3rd 4th equations should be Tf,
  formation temperature.