Page 121 - Basic Well Log Analysis for Geologist

YR RRTEES <

LOG INTERPRETATION

a

(Schlumberger, 1975) This chapter discussed several log interpretation
techniques. These techniques are based on many of the
Van Ay } ; formulas already presented in the text. What the formulas
Vv
Ra .
Sy =~ he YL. _ are, and where they are found is summarized on the log
Ra
interpretation flow chart included at the end of this chapter.
0.4% Ry, x 1.0-V
Where:
Sy = water saturation uninvaded zone corrected for Review - Chapter VI
volume of shale
1. The Archie equation is used to calculate a formation’s
Ry = formation water resistivity at formation
water saturation in both the invaded (S,,.) and uninvaded
temperature
(S,,) zones.
R, = true formation resistivity
2. The Ratio Method for determining water saturation
) = porosity corrected for volume of shale
(S,,) does not require a value for porosity (@).
Vo, 7 volume of shale
3. The ratio between the water saturations in the invaded
Ry, = resistivity of adjacent shale
(S,,) and uninvaded (S,,) zones (i.e. S,/S,,) can be used as
A major problem encountered in shaly sand analysis is an index for the degree of hydrocarbon moveability.
determining a resistivity value for shale in a formation. The 4. Bulk volume water (BVW) is important because it
percentage of shale is not the critical factor, rather, it is indicates when a reservior is at irreducible water saturation
clay’s cation exchange capacity (Hilchie, 1978), because (Sy in):
cation exchange capacity greatly affects resistivity of the 5. Quick look methods are important because they
clay. Kaolinite and chlorite have extremely low cation provide “flags” which indicate zones of potential interest.
exchanges: illite and montmorillonite have high cation 6. Hingle and Pickett crossplot techniques are simple
exchanges. Therefore, montmorillonite and illite lower and rapid methods for determining: (1) a formation’s
resistivity inuch more than kaolinite and chlorite. In shaly matrix; (2) a formation’s water resistivity (R,,); (3) a
sand analysis, a geologist must make an assumption that formation’s water saturation (S,,); and (4) a formation’s
resistivity of an adjacent shale (Ry) is the same as cementation factor (m).

resistivity of shale in the formation. This assumption is not 7. Log derived data can be used to estimate permeability
always correct. of a formation.
Most shaly sand interpretation problems occur in 8. The flow chart included in this chapter will help you
formations with Ry values which are not too salty (NaCL review the steps used in log interpretation and will also
20,000 ppm, or Ry, = 0.3 at 80°; Hilchie, 1978). Where provide an index of where different pieces of information
formation water is very salty, shale has less effect on the are located.
formation’s resistivity. Therefore, calculated water
saturations, without correction for shale, are close to true
formation water saturation

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