Page 47 - Applied Petroleum Geomechanics

38 Applied Petroleum Geomechanics

Table 2.4 Bulk density and porosity for various rock types.
Matrix or rock type Dt m (ms/ft) x Reference
Silica 55.5 1.60 Raiga-Clemenceau et al. (1988)
Calcite 47.6 1.76 Raiga-Clemenceau et al. (1988)
Dolomite 43.5 2.00 Raiga-Clemenceau et al. (1988)
Mudstone 67.1 2.19 Issler (1992)
Mudstone 63.4 2.34 Issler (1992)
alternative fit
Mudstone and 52e70 2.19 Nelson and Bird (2005)
sandstone

where x is an exponent specific for the matrix lithology. This equation does
not account for the effect of the pore fluids on the formation transit time. In
this equation, Raiga-Clemenceau et al. (1988) used the following parame-
ters related to the matrix natures (Table 2.4):
Porosity can also be obtained from the resistivity log. Archie (1942)
found that the resistivity of a given core sample was always related to the
water resistivity by a constant factor F (he called it the formation factor),
which is a function of porosity. The general form of the Archie equation
can be written as follows:

n a R w
S ¼ m (2.12)
w
f R t
where S w is the water saturation; R w is the resistivity of formation water; R t
is the formation resistivity; the constants a, m, and n need to be determined
for a formation being evaluated.
The above equation can be approximately expressed in a simplified
form for a 100% water-saturated formation (S w ¼ 1):
r ffiffiffiffiffiffi
R w
f ¼ (2.13)
R t
where the formation resistivity R t can be obtained from the deep resistivity
log and again need to take off the oil and gas effects. The resistivity of for-
mation water R w should be verified in as many ways as possible, including
calculations from the spontaneous potential log, water catalog, calculations
from nearby water-bearing formation, and/or formation water sample
measurement.