Page 80 - Geology of Carbonate Reservoirs

SATURATION, WETTABILITY, AND CAPILLARITY 61

They concluded that the wide variations in data on formation resistivity factor
in carbonate reservoirs can be resolved by conducting detailed studies on rock and

pore characteristics. According to their findings, rocks with intergranular porosity
(depositional) and sucrosic dolomites (diagenetic) generally show m values of about
2. Wackestones, packstones, and rocks with only matrix porosity also show m values
of about 2. However, rocks with both matrix and vuggy or moldic porosity show m
values greater than 2 depending on the percentage of separate vugs or molds. For
example, moldic porosity in oolitic grainstones, such as the Arab D in the Middle
East or the Smackover of the Gulf Coast, show m values ranging between about 1.8
at 5% porosity to 5.4 at 30% porosity. Fractured and fissured rocks may have m

values less than 2 and theoretically could approach unity.
In carbonate reservoirs, the importance of looking at rocks cannot be overem-
phasized. Resistivity values from wireline logs substituted into the Archie equation
can give totally erroneous results in the absence of some qualifying information
about rock and pore characteristics. If S w is wrong, it is easy to see what an impact
the error can have on reserve estimates by changing S w in the following expression
for calculating original oil in place, or OOIP:

OOIP = 7758Ahφ( 1 − w S )
B
oi
Here, OOIP is original oil in place, 7758 is a conversion factor (the number of
stocktank barrels in a one acre - foot volume), A is the area of the reservoir rock in

acres, h is the thickness of the reservoir rock in feet, φ is porosity as a decimal, (1 −

S w ) is oil saturation as a decimal, and B oi is the formation volume factor for oil at
initial reservoir conditions. For estimates of OOIP without knowing the formation
volume factor, B oi can be set equal to one.
Archie concluded from his laboratory studies of electrical resistivity in saturated
and partially saturated rocks that a single power function relates the electrical
resistivity ratio R t / R o to saturation. His work showed that S w in the uninvaded zone
that contains water with some hydrocarbons can be determined from the following
expression:

) = F × R w
n
(S w
R t
and that

( S w ) −n = I ( resistivity index) = t R
R
o
so that

⎛ R w ⎞ 1/ n ⎛ a R w ⎞ 1/ n
S w = ⎜ ⎝ F × ⎟ ⎠ = ⎜ ⎝ φ m × ⎟ ⎠
R t R t

The choice of an appropriate value for n is important because large errors in
the value of S w can result when inappropriate n exponents are used. Laboratory