Wettability Alteration of Carbonate Rocks at Reservoir Conditions By Novel Poly(ethylene oxide) Gemini Surfactants

Carbonate reservoirs contain a significant fraction of the world’s remaining oil. Carbonate, however, in most cases tend to be oil-wet as a result of long-time contact with oil. Wettability alteration of carbonate has been proven to increase oil recovery significantly. Many surfactants are able to modify the wetting state of oil-wet carbonate rock, among these cationic surfactants perform the best. Gemini surfactants are surfactants with more than one hydrophobic tail group and a hydrophilic head group linked by a spacer. The main objective of this work was to evaluate the wettability alteration of carbonate rock at reservoir conditions using newly synthesized gemini surfactants.

A series of cationic gemini surfactants were synthesized and used to alter the wettability of carbonate. Carbonate samples were aged by crude oil under high pressure and at 102 °C for two weeks. The contact angle was measured as a function of time using drop shape analyzer (DSA 100 from Kruss) by placing a drop of surfactant at high temperature and pressure conditions. Scanning electron microscope (SEM) was used to scan the surface of rock samples at three different times: before oil-aging, after oil-aging, and after soaking in the surfactant solution.

High temperature and high pressure contact angle measurements before and after aging showed that carbonate surface changed to oil-wet. SEM images show a smoother rock surface after aging, indicating that the rock surface is coated by organic materials. A drop of surfactant solution induces a gradual change in contact angle with time, starting with a sharp decline, followed by a slower decrease until it stabilizes. Contact measurement at reservoir conditions showed the significant wettability alteration using synthesized surfactant. SEM images after soaking show relatively clear rock surface, confirming that at relatively low concentration, the novel cationic surfactant is able to significantly alter the oil-wet carbonate towards water-wet.

The newly synthesized poly(ethylene oxide) gemini surfactants proved to be effective in wettability alteration of carbonate rocks. This type of surfactants has never been reported in the literature for wettability alteration. Also, a relationship was noted between SEM and contact angle measurement.