Page 20 - Basic Well Log Analysis for Geologist

BASIC RELATIONSHIPS OF WELL LOG INTERPRETATION

a

(i.e. sandstone, limestone, or dolomite). Porosity logs The formation temperature is also calculated (Asquith,
require a lithclogy or a matrix constant before a zone’s 1980) by using the linear regression equation:
porosity (cb) can be calculated. And the formation factor
y=mxre
(F), a variable used in the Archie water saturation equation

(iS, = VEX R,/R). varies with lithology. Asa Where:
consequence, water saturations change as F changes. Table x = depth
| is alist of the different methods for calculating formation y = temperature
factor, and tlustrates how hthology affects the formation m = slope-—in this example it is the geothermal gradient
factor. ¢ = aconstant—in this example it is the surface
Temperature of Formation—Formation temperature (T;) temperature
ts also important in log analysis because the resistivities of
An example of how to calculate formation temperature is
the drilling mud (R,,). the mud filtrate (R,yf), and the
illustrated here:
formation water (Ry) vary with temperature. The
temperature of a formation is determined by knowing: (1)
Temperature Gradient Calculation
formation depth; (2) bottom hole temperature (BHT); (3)
Assume:
total depth of the well (TD); and (4) surface temperature.
vy = bottom hole temperature (BHT) = 250°F
You can determine a reasonable value for the formation
x = total depth (TD) = 15,000 ft
temperature by using these data and by assuming a linear
© = surface temperature = 70°F
geothermal gradient (Fig. 8).
Solve for m (i.e. slope or temperature gradient)
Table 1. Different Coefficients and Exponents Used to
a
Calculate Formation Factor (F). (Modified after m= >
x
Asquith, 1980).
Therefore:
F = a/b general relationship
Where: 250° = 70°
m= seo :
a = tortuosity factor’ 15.000 ft
m = cementation exponent m = 0.012°/ft or 1.2°/ L100 ft
ch = porosity
Formation Temperature Calculation
“ER = [ide for carbonates
Assume:
EK = O81 cb? for consolidated sandstones
m = temperature gradient = 0.012°/ft
“EK = 0.62215 Humble formula for unconsolidated
x = formation depth = 8,000 ft
sands
c = surface temperature = 70°
F= 1.45’! 54 for average sands (after Carothers,
Remember:
1958)
F= 1.656) 33 for shaly sands (after Carothers. yrmxte
1958)
Therefore:
F= 1.45/61-70 for caleareous sands (after Carothers,
y = (0.012) x (8,000) + 70°
1958)
y = 166° formation temperature at 8,000 ft
Most commoniy used. chart is closely approximated by the Arp’s formula:
for carbonates (after Carothers.
F = 0.85: 2-14
After a formation’s temperature is determined either by
1958)
chart (Fig. 8) or by calculation, the resistivities of the
for Pliocene sands, Southern
Fo= 2.45, pl.08
different fluids (R,,. Riap or Ry) can be corrected to
California (after Carothers and
formation temperature. Figure 9 is a chart that is used for
Porter, 1970)
correcting fluid resistivities to formation temperature. This
F = 197.6129
tor Miocene sands, Texas-Louisiana
Gulf Coast (after Carothers and
Rep = Renp * (lemp + 6.77)(T; + 6.77)
Porter, 1970)
Where:
F = [.0/¢p2.95-4)
for clean granular formations (after
Sethi, 1979)
= resistivity at formation temperature
Rrp

“Tortuosity is a function of the complexity of the path the fluid -
= Tesistivity at a temperature other than formation
Riemp
must travel through the rock.
temperature