Invert emulsion oil based drilling fluids rely on surfactants and emulsifiers to maintain the stability of the invert emulsion. An excess of oil wetting agents in the system is usually necessary in order to oil wet the drilled solids. This excess of surfactants causes wettability alteration in the drilled formation. This is recognised as one of the mechanisms of formation damage.

A study has been carried out to assess the influence of the surfactant concentration in oil based mud filtrates on the wettablility alteration of glass surfaces. Micromodel work has shown that oil based mud filtrate can cause emulsification and migration of connate water, thus reducing the water saturation as well as the alteration of the wettability of the pore walls. Results of contact angle studies suggest the surfactants in oil-based mud filtrates adsorb on to silica surfaces to build up a monolayer, Thus changing the initailly strongly water-wet surface to one of intermediate wetting.

This paper reports in detail the experiments carried out to demonstrate the wettability alteration caused by an oil based mud filtrate. A detailed investigation carried out to discover any correlation between the concentration of oil-wetting agent in the filtrate and the magnitude of the change in wettability produced on glass surfaces.

The studies of contact angle variation as a function of surfactant concencentration show that even very small concentrations of surfactants are potentially damaging to reservoir formations.

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