Page 90 - Petrophysics

64 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

dissolved salts (TDS). If the reservoir pressure is 3,300 psi, estimate the
solubility of hydrocarbon gas in the oilfield brine at reservoir conditions.

Assume a geothermal gradient of 1°F/lOO feet of depth and use
Equation 2.11 to estimate the reservoir temperature (Tf):
Tf = 70 + 1.0(7,000/100) = 140°F

Use Figure 2.14 to obtain the solubility of gas in pure water (Rwp =
16 ft3/bbl). Then, extrapolate the salinity correction factor (X) to 140'F
using Table 2.4 (X = 55).
RB = 16[1 - 55(80,000 x lo-')] = 8.96 SCF/bbl

Viscosity
All fluids resist a change of form, and many solids exhibit a gradual yield
in response to an applied force. The force acting on a fluid between two
surfaces is called a shearing force because it tends to deform the fluid. The
shearing force per unit area is the shear stress (2). The absolute viscosity
is defined by:
z = -p(dv/dx) (2.14)

where: z = shear stress.
p = absolute viscosity.
v = fluid velocity.
x = distance.

Viscosity is reported in terms of several different units: Poise (c.g.s unit
of absolute viscosity) = g/cm.s = 14.88 lbm/ft-s; Centipoise = 0.01 Poise;
Stoke (c.g.s. kinematic viscosity) = g/[(cm-s)(g/cm3)]; Centistoke =
0.01 Stoke; and Pascal-seconds (SI units) = 0.1 Poise.
Figure 2.15 may be used to estimate the viscosity of oilfield waters
as a function of salinity, temperature, and pressure. A separate chart
(insert on Figure 2.15) is used to obtain a factor relating the viscosity to
pressure.

EXAMPLE

Estimate the viscosity of a brine containing 12% salts that was obtained
from a reservoir with a fluid pressure of 6,000 psi and temperature
of 180°F.
Obtain the pressure correction factor from the chart on Figure 2.15
(pressure correction factor = 1.018).