AN INNOVATIVE METHOD FOR QUANTITATIVELY MEASURING MIXED-WETTABILITY STATES OF UNCONVENTIONAL RESERVOIRS BASED ON ENVIRONMENTAL SCANNING ELECTON MICROSCOPE

Reservoir rocks are generally of mixed wetting in nature, because of the heterogeneous nature of pore walls. The mixed-wettability states of unconventional reservoirs may have significant impacts on oil migration, waterflood behaviour and residual oil saturation during petroleum development. Many researchers focused on the characterization of mixed-wettability states, however, effective methods for acquiring quantitative mixed-wettability states of reservoir rocks remain elusive. To solve this challenge, contact angles and Amott-Harvey index were measured, respectively, for homogeneous synthetic material and unconventional reservoir rock samples. Condensation characteristics of water on both types of samples were determined by using an environmental scanning electron microscope (ESEM). Through a series of experiments and comparison of contact angles and condensation features, an optimal experimental condition and discrimination criteria for mixed-wettability of rocks were determined. Based on the condensation features from ESEM under the established experimental condition, the mixed-wettability of reservoir rocks was characterized. It has been found that: (1) The optimal experimental condition for condensation observation by ESEM is 5℃ and about 8.1Torr, the observation duration needs to be as short as possible; (2) There are four typical condensation features on pore walls, including ones without condensate water and ones covered with film-shaped water, low dome-shaped water or droplet-shaped water. Pore walls without condensate water or with droplet-shaped water are oil-wet; and those with film-shaped water or low dome-shaped water are water-wet; (3) The area proportions of oil-wet pore walls and water-wet pore walls of reservoir rocks can be regarded as quantitative indexes to evaluate the mixed-wettability states. Compared with the classical Amott-Harvey index, the ESEM-based method is capable of distinguishing subtle changes of wettability for reservoir rocks at the range of weak water-wet, neutral and weak oil-wet.