Clay mineral properties are irreversibly altered by adsorption of petroleum heavy ends. As a result of this alteration, rock sensitivity to injected fluids is reduced; rock wettability is changed from "water wet" to "neutral"; adsorption of surfactants is reduced; rock property measurements are changed; and cores can be irreversibly altered. All of these changes can affect enhanced recovery processes and related laboratory studies.

The extent of this interaction has been found to depend upon the type of clay minerals present in the rock, the composition of the petroleum heavy ends, and the interaction environment. After the adsorption occurs, a clay-organic complex is formed which is hydrophobic and very stable. Expansion of swelling clays is reduced and clay surface area and cation exchange capacity are reduced.

Practical implications of these observations are demonstrated in Berea sandstone cores and in friable California sand cores when the clays were stabilized against dispersion and subsequent migration. Berea sandstone permeability is rendered insensitive to fresh water and the friable sand, in which clays are the primary cementing material, is stabilized against failure under increased flow rates. In other studies, surfactant adsorption on montmorillonite was found to be three to ten times lower when the clay was pretreated with petroleum heavy ends. Surfactant retention in clean Nevada sand is also reduced when a surface layer of heavy ends is present on the sand grains.

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