The Effect of Temperature and Oil Viscosity Reduction on Water Imbibition of Diatomite

Abstract

Abstract Previous studies of imbibition into diatomite reservoir core as a function of temperature have revealed a systematic increase in spontaneous water imbibition leading to increased oil recovery. Forced water imbibition resulted in a trend of possibly decreasing residual oil saturation as temperature increased. 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. The objective of this work is to delineate 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 oil saturation. All tests were isothermal and carried out at temperatures ranging from 45 to 230 °C (113 °F - 446 °F). 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 synthetic formation brine. Core samples were subject to different pre-test cleaning procedures aimed to preserve or alter the initial wettability of the sample. A core with preserved wettability was used with crude oil as the non-wetting phase. The other core was cleaned and used with the corresponding matched viscosity mineral oil. Increasing temperature resulted in a trend of increased spontaneous and forced oil recovery and lower residual oil saturation for both samples. Wettability shifts with temperature were more pronounced and systematic in the preserved wettability system. The cleaned system displayed lesser changes in wettability at all test temperatures as verified by experimental measurements of the Amott index.