THE EFFECT OF TEMPERATURE AND OIL VISCOSITY REDUCTION ON WATER IMBIBITION OF DIATOMITE

Previous studies of diatomite reservoir core have revealed a systematic increase in spontaneous water imbibition with temperature leading to increased oil recovery. Forced water imbibition resulted in a trend of possibly decreasing residual oil saturation with temperature. These tests, however, were not conducted in a fashion that allows a simple quantification of the fraction of recovery associated with wettability change and that associated with oil and water phase viscosity reduction. This work delineates the relative impacts on core‑scale oil recovery due to oil viscosity reduction as compared to wettability evolution during thermal recovery of light‑oil from diatomite. Water imbibition tests were conducted with core samples from the exact same depth within a diatomaceous reservoir. The experiments included spontaneous counter‑current water imbibition followed by forced co‑current water imbibition to residual saturation. All tests were isothermal and carried out at temperatures ranging from 45 to 230 oC. Non‑wetting phases were a light diatomite crude oil and mineral oils viscosity‑matched to the crude oil for each test temperature. The wetting phase was a synthetic formation brine. Core samples were subject to different pre‑test cleaning procedures aimed to preserve or alter the initial wettability of the sample. Mixed wettability cores were used with crude oil as the non‑wetting phase. The other core was water‑wet and used with the corresponding matched viscosity mineral oil. Increasing temperature resulted in a trend of increased oil recovery and lower residual oil saturation for both samples; whereas wettability shifts with temperature were only found in mixed wettability systems, due to the expected effects of temperature on wettability and fines detachment. Water‑wet systems displayed minimum changes in wettability at all test temperatures; as verified by experimental measurements of the Amott index in every test carried out.