Page 136 - Reservoir Formation Damage

118 Reservoir Formation Damage

6,0 -

O 2.0-

o.o
0.0 2.0 4,0 6,0
Clay Content. (%)
Figure 6-11. Cation exchange capacity of the various Ceuta field core
samples by Maraven S. A., Venezuela (Arcia and Civan, ©1992; reprinted
by permission of the Canadian Institute of Mining, Metallurgy and Petroleum).

the wettability of rocks is altered by the rock and fluid interactions and
variations of the reservoir fluid conditions, prediction of its effects on
formation damage is a highly complicated issue. Although mineral matters
forming the reservoir rocks are generally water-wet, deposition of heavy
organic matter, such as asphaltenes and paraffines, over a long reservoir
lifetime may render them mixed-wet or oil-wet, depending on the compo-
sition of the oil and reservoir conditions. Wettability may be expressed
by various means, including the Amott and USBM indices. (See Chapter
4.) During reservoir exploitation, wettability may vary by various reasons.
For example, Figure 6-14 by Burchfield and Bryant (1988) is an evidence
of the alteration of the wettability of a water-wet berea sandstone to a
stronger water-wet state in contact with microbial solutions. Madden and
Strycker (1988) determined that the wettability of the Berea sandstone
saturated with oils vary by their asphaltene and polar components content