Page 59 - Basic Well Log Analysis for Geologist

RESET LOGS

However, in enlarged boreholes, a shale zone can exhibit
Minor, positive separation. In order to detect zones of
erroneous positive separation, a microcaliper log 1s run in
square both sides:
track #1 (Fig. 24), so that borehole irregularities are
detected. Nonporous and impermeable zones have high R
—m
1LQ=Fx
resistivity values on both the micro normal and micro
Reo
Inverse curves (Fig. 24). Hilchie (1978) states that
solve for F:
resistivities of approximately ten times the resistivity of the
drilling mud (R,,) at formation temperature indicate an pa Re
impermeable zone.
Rint
The Microlog* does not work well in saltwater-based
remember F = a/f™
drilling muds (where R,yp = Ry) or gypsum-based muds.
because the mudeake may not be strong enough to keep the therefore:
pad away from the formation. Where the pad is in contact
with the formation, positive separation cannot occur. a _ Ro
bm Rut
Microlaterolog* and Proximity Log* solve for porosity (d)

The Microlaterolog (MLL)* (Fig. 21) and the Proximity
Log (PL)* (Fig. 25), like the Microspherically Focused Log PF ER,
(MSFL)*, are pad type focused electrode logs designed to
Where:
measure the resistivity in the flushed zone (R,,). Because
the Microlaterolog® ts strongly influenced by mudcake & = formation porosity
thicknesses greater than 1/4 inch (Hilchie, 1978), the Ring = resistivity of mud filtrate at formation
Microlaterolog* should be run only with saltwater-based temperature
drilling muds. The Proximity Log*, which is more strongly Sy) = water saturation of the flushed zone
a = constant
focused than the Microlaterolog™, is designed to investigate R,, = resistivity of flushed zone from Microlaterolog*.
deeper so it can be used with freshwater-based drilling muds Proximity Log*, Laterolog-8*, or
where mudceake ts thicker. Microspherically Focused Log* values

Resistivity Derived Porosity a = |.0 for carbonates
a = ().62 for unconsolidated sands
The minerals that make up the grains in the matrix of the a = 0.81 for consolidated sands
rock and the hydrocarbons in the pores are nonconductive. m = constant
Therefore. the ability of rock to transmit an electrical m = 2.0 for consolidated sands and carbonates
current is almost entirely the result of the water in the pore m 2.15 for unconsolidated sands
space. Thus resistivity measurements can be used to F = formation factor
determine porosity. Normally, measurements of a
In hydrocarbon-bearing zones, the shallow resistivity
formation’s resistivity close to the borehole (flushed zone,
(R,,) is affected by the unflushed residual hydrocarbons left
R,,. or invaded zone, Rj) are used to determine porosity.
by the invading mud filtrate. These residual hydrocarbons
Shallow resistivity devices, used to measure R,,, and R;.
will result in a value for shallow resistivity (R,,) which is
include the following: (1) Microlaterolog*; (2) Proximity
too high because hydrocarbons have a higher resistivity than
Log*: (3) Laterolog-8*: (4) Microspherically Focused
formation water. Therefore. the calculated resistivity
Log*: (5) short normal log; and (6) Spherically Focused

porosity tn hydrocarbon-bearing zones will be too low. To
Log*.
correct for residual hydrocarbons in the flushed zone, water
When a porous and permeable water-bearing formation ts
saturation of the flushed zone (S,,,) must be known or
invaded by drilling fluid, formation water is displaced by
estimated. Then, a formation’s shallow resistivity (R,,) can
mud filtrate. Porosity in a water-bearing formation can be
be related to porosity by the following: "
related to shallow resistivity (R,,,) by the following
equations: Seo = /FX R-
R
/ RR. xO
S os Fx mf
eV Ry now square both sides:
Where S,,, = 1.0 (100%) in water-bearing zones. remember: F = a/@™

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