Page 435 - Petrophysics

PROBLEMS 403

develop microfractures that contribute to the measurement of porosity at
ambient conditions (in addition to the expanded matrix porosity). Thus,
measurements of the petrophysical properties of reservoir rocks must be
conducted at simulated reservoir conditions.

EXAMPLE

The areas under the water-displacing-oil (from Siwl to S,,), and the
oil-displacing-water (from S,,r to Siw2) capillary pressure curves are
56 and 30, respectively. Compute the USBM wettability index and the
energy required for the displacement of oil by water (in J/m3). Core
properties: 2.54 cm in diameter by 2.54 cm long, porosity = 2296,
Siw = 0.28, and Sw,r = 0.77.

SOLUTION

From Equation 6.14, the USBM wettability index is:

I, = log ($) = 0.271 (slightly water-wet)

Vp = 3.14 x (2.54/2)2 x 2.54 x 0.22 = 2.83 cm3

Volume of oil displaced = 2.83 x (0.77 - 0.28) = 1.39 cm3

Displacement energy = 30, 000N/m2 x 1.39(10-6) m3

= 0.0416 N-m/m3 = 0.0416 J/m3

PROBLEMS

1. If the contact angle of a drop of water placed on a smooth plate
submerged in a clear oil is 120°, what will be the value of the contact
angle for a drop of the oil on the plate if it is submerged in water (see
Figure 6.2)?
2. A 0.655 kg sample of rock is flooded with a tetravalent cation
solution which displaces the following quantities of cations:
30 mg Mg2+, 50 mg K+, 70 mg Ca2+, and 120 mg Na+. Calculate the
cation exchange capacity of the rock. Report the value as milli-
equivalents of cations per kilogram of rock.
3. (a) If a carbonate rock surface is in equilibrium with an acetic brine,
will the rock adsorb acetic polar organic compounds from an oil?
Explain your answer using chemical formulas.