Page 111 - Basic Well Log Analysis for Geologist

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CHAPTER Th,

LOG INTERPRETATION

General

Once porosity and true resistvity of a potential zone are (volumetrically) there are economic, recoverable
determined. a geologist is ready to calculate and use log hydrocarbon reserves.
parameters. Log parameters can help evaluate a zone and Water saturation of a formation’s flushed zone (S,,) is
determine whether a well completion attempt is warranted. also based on the Archie equation, but two variables are
This section will cover some of the different methods which changed:
help establish these important parameters. Methods
S. _ a Rag in
discussed are: Sy. Sy. bulk volume water, Pickett and
, bo Ryo
Hingle crossplots, and “quick look” analysis. Also,
determination of log derived permeability (K.) shaly sand Where:
analysis will be discussed. S,) = water saturation of the flushed zone
As important as log parameters are. however, they should Rap = resistivity of the mud filtrate at formation
not be applied to the exclusion of other data. This statement temperature
is, perhaps obvious to the reader, but nevertheless, it can“t R,, = shallow resistivity from Laterolog-8*,
be over-emphasized. A geologist should always consider Micropherically Focused Log*. or
every item of relevant data. such as drill stem tests, sample Microlaterolog*
shows, mud log analysis, nearby production, ete.. before od = porosity
making a decision to “set pipe.” a = tortuosity factor (Table 1; Chapter I)
m = cementation exponent (Table I; Chapter 1)
Archie Equation S, and S,, n = saturation exponent which varies from 1.8 to 2.5,
but is normally equal to 2.0
Water saturation (S,) of a reservoir’s uninvaded zone is
calculated by the Archie (1942) formula. Water saturation of the flushed zone (S,,) can be used as
an indicator of hydrocarbon moveability. For example, if
a Ry, \ba
x the value of S,, is much larger than S,,, then hydrocarbons
in the flushed zone have probably been moved or flushed
out of the zone nearest the borehole by the invading drilling
Sy = water saturation of the uninvaded zone (Archie fluids (Rinp)-
method)
R,, = resistivity of formation water at formation Ratio Method
temperature
The Ratio Method identifies hydrocarbons from the
R, = true resistivity of formation (i.e. Ryg or Rug
difference between water saturations in the flushed zone
corrected for invasion)
Sxo RnR
ch = porosity (S,,) and the uninvaded zone (S,,). When water saturation
of the uninvaded zone (S,,) ts divided by water saturation of
I)
a = tortuosity factor (Table |: Chapter the flushed zone (S,,), the following results:
I)
m= = cementation exponent (Table |: Chapter
Nn = saturation exponent which varies from 1.8 to 2.5, Sw \7_ Ro/RL
but is normally equal to 2.0
The uninvaded zone’s water saturation (S,,). determined Where:
by the Archie equation, is the most fundamental parameter Sy = water saturation uninvaded zone
used in log evaluation. But, merely knowing a zone’s water Sy = water saturation flushed zone
saturation (S,,) will not provide enough information to R,o= formation’s shallow resistivity from Laterolog-8*,
completely evaluate a zone’s potential productivity. A Microspherically Focused Log*, or
geologist must also know whether: (1) water saturation 1s Microlaterolog*
low enough for a water-free completion, (2) hydrocarbons R, = formation’s true resistivity (Rag or Rppy corrected
are moveable, (3) the zone is permeable, and (4) whether for invasion)

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