Page 182 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Formation Evaluation 151

Poupon, Loy, and Tixier [204] found that for “average” residual oil saturations:

s, = sy (5-1 00)

Substituting Equation 5-100 into Equation 5-99 and rearranging terms:

[ 2;:: ly’
s, = - (5-101)

With this equation, water saturation can be found without knowing $. Note, how-
ever, that this interpretation method is based on the assumption that Sm = SA’5.
This relation is for “average” granular rocks and may vary considerably in other
rock types. Figure 5-83 is a chart [199] that solves Equation 5-101.
When two or more resistivity logs with different depths of investigation are
combined, permeable zones can be identified. In a permeable zone, the area
closest to the borehole will be flushed of its original fluids; mud filtrate fills
the pores. If the mud filtrate has a different resistivity than the original
formation fluids (connate water), the shallowest-reading resistivity tool will have
a different value than the deepest-reading tool (Figure 5-84). Many times this
difference is significant. The separation of the resistivity curves that result is
diagnostic of permeable zones.
Care should be taken not to overlook zones in which curves do not separate.
Curve separation may not occur if:

1. The mud filtrate and original formation fluids (i.e., connate water and
hydrocarbons) have the same resistivity; both shallow and deep tools will
read the same value This is usually not a problem in oil or gas-saturated rocks.
2. Invasion of mud filtrate is very deep, both shallow and deep tools may
read invaded-zone resistivity This occurs when a long period of time elapses
between drilling and logging or in a mud system with uncontrolled water loss.

Microresistivity TOOIS. Microresistivity tools are used to measure the resistivity
of the flushed zone. This measurement is necessary to calculate flushed zone
saturation and correct deep-reading resistivity tools for invasion. Microresistivity
tools are pad devices on hydraulically operated arms. The microlog and proximity
log are the two main fresh-mud microresistivity tools, while the micro SFL and
microlaterolog are the two main salt-mud microresistivity tools.
Figure 5-85 [58,200,205] shows the electrode pads and current paths for the
Microlog (5-85a), Micro SFL. (5-85b), and proximity log (5-85c), and Micro-
laterolog. Figure 5-85d shows the Micro SFL. sonde.
Theory. The microlog makes two shallow nonfocused resistivity measurements,
each at different depths. The two measurements are presented simultaneously
on the log as the micronormal and microinverse curves. Positive separation
(micronormal reading higher than the microinverse) indicates permeability. Rx,
values can be found by using Figure 5-86 [199]. To enter Figure 5-86, Rmc must
be corrected to formation temperature, and mudcake thickness (hJ must be
found. To find hmc subtract the caliper reading (presented in track 1) from the
borehole size and divide by two. In washed out or enlarged boreholes, hmc must

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