Page 402 - Petrophysics 2E

370 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

The surfaces of carbonate rocks, on the other hand, are basic in
character and, consequently, they react readily with the acid compounds
in crude oils and exhibit neutral to oil-wet characteristics [30]. McCaffery
and Mungan showed that stearic acid is strongly attached to calcite [34].
Lowe et al. also showed that acid compounds in crude oils become
attached to the basic surfaces of carbonates, forming chemisorbed
films 1351.
The basic characteristics of carbonates may be due to Arrhenius-Oswalt
calcium hydroxide-type bases or to Lewis bases because of the electron
pairs available in the exposed oxygens of the -COS carbonate groups.
If the characteristics are due to calcium hydroxide groups, the reaction is:

0 0
1 I (6.11)
-CaOH+ HOC-R +. -Ca-OC-R + H20

If the basic characteristics of the carbonates are due to Lewis-type
bases, then the reaction is probably:

CaC02-O' + A- + CaC02-0: A (6.12)

Inasmuch as the carbonate surfaces are positively charged and
consequently behave like weak bases, they are strongly affected by
acidic components in crude oils, which are carboxylic acids, phenolic
compounds, and ring structures containing sulfur and oxygen [35-411.
Apparently, acidic compounds are more prevalent in crude oils than basic
compounds, which may account for the fact that carbonate rocks exhibit
a range of wettability from neutral to strongly oil-wet.
Due to the acid-base interactions between rock surfaces and crude
oils, the chemistry and pH of the brine associated with the crude oil is
very important. If the pH is greater than 7, the dissociation of hydrogen
ions is repressed and the surface will adsorb acidic organic compounds.
If multivalent metallic cations such as Ca++, Baff, Cu++, Fes++, and
Al+++ are present in the brine, or added to it, these ions will adsorb
on the negative silica surface. The multivalent cations then provide
positively charged sites that permit the adsorption of acidic compounds
on the silica rock [42, 431. Carbonate surfaces are positively charged at
pH ranges less than 7-8, but become negatively charged at pH ranges
greater than 8. Therefore, carbonate surfaces will adsorb positively
charged (basic) organic compounds if the pH of the brine is greater
than 8.