Page 21 - Basic Well Log Analysis for Geologist

BASIC RELATIONSHIPS OF WELL LOG INTERPRETATION

Temp = temperature at which resistivity was measured Bulk Volume Water:
Ty = formation temperature BYVW =¢ xX Sy

Using a tormation temperature of 166° and assuming an Permeability
R,, of 0.04 measured at 70°, the Ry at 166° will be: K. = [250 x (b4/Sy oil
in)
|?
K. = permeability in millidarcies
Ryieo = 0.04 * (70+ 6.77)/(166 + 6.77)
Ry too = 0.018 K. = [79 x (9/Sy inn) ]? gas
Sw in = irreducible water saturation
Resistivity values of the drilling mud (R,,). mud filtrate
(Ryyp). mudeake (R,,,). and the temperatures at which they tn = saturation exponent which varies from 1.8 to 2.5 but most
are measured, are recorded on a log’s header (Fig. 2). The often equals 2.0
resistivity of a formation’s water (R,) is obtained by
Review - Chapter I
analysis of water samples from a drill stem test, a water
|. The four most fundamental rock properties used in
producing well, or from a catalog of water resistivity
petrophysical logging are (1) porosity: (2) permeability: (3)
values. Formation water resistivity (R,,) is also determined
water saturation; and (4) resistivity.
from the spontaneous potential log (discussed in Chapter I)
2. The Archie equation for water saturation is:
or can be calculated in water zones (i.c., S,, = 100%) by
the apparent water resistivity (R,,) method (see Chapter
VI). FX R,\
Sw = R
Fundamental Equations
Table 2 is a list of fundaniental equations that are used for Where:
the log evaluation of potential hydrocarbon reservoirs.
Sy = water saturation of uninvaded zone
These formulas are discussed in detail in subsequent
F = formation faetor
chapters.
Ry = formation water resistivity
Table 2. Fundamental Equations of Well Log R, = formation resistivity (uninvaded zone)
Interpretation.
3. Where a porous and permeable formation is
Porosity: At~ At penetrated by the drill bit, the drilling mud invades the
Sonic Log bsontc = ya formation as mud filtrate (Ry p).
ma 4. The invasion of the porous and permeable formation
. a o7 p by mud filtrate creates invasion zones (R,, and R,) and an
Density Log pen = fm
uninvaded zone (R,). Shallow, medium, and deep reading
ma f
resistivity logging tools provide information about the
Neutron-Density Log dup =
invaded and uninvaded zones and about the depth of
invasion.
Formation Factor:
5. The lithology of a formation must be known because:
F = a/cb™ General (1) porosity logs require a matrix value—sandstone,
= 1.0/2 Carbonates limestone, or dolomite—in order to determine porosity; (2)
F = 0.81/2 Consolidated Sandstones the formation factor varies with lithology; (3) the variation
F = 0.62/2.!5 Unconsolidated Sands in formation factor causes changes in water saturation
Rind Ry with changes in temperature. Consequently, a formation’s
Formation Water Resistivity: values.
6. The four fluids that affect logging measurements are:
SSP = —K X log (RyRy) (1) drilling mud, R,,; (2) mud filtrate, Rug (3) formation
>
Ry
Rye water, R,,; and (4) hydrocarbons.
R
R, = 7. The resistivities of the drilling mud (R,,). mudcake
vo F (Rye), mud filtrate (Rp) and formation water (R,,) all vary
Water Saturations:
Syt= F x (R,/R,) water saturation uninvaded temperature (T;) must be determined and all resistivities
corrected to Ty.
zone
Sxot= FX (Rin/Ryo) water saturation flushed zone
. R./R, \ 0.625 ; ;
Sy = (ae walter saturation ratio method