Abstract:

There is a lack of systematic experimental studies on the phase transformation processes of ultra-deep oil and gas and their controlling factors, as well as a lack of visual presentation of these processes.In this study, we used crude oil of the SHB7 well as a case study and conducted both online and offline visual observation experiments.The conclusions are summarized as follows: (1) Before the temperature of the thermal simulation experiment reaches the crude oil cracking point, the red-green quotient value (Q650/500) of the crude oil changes reversibly during the heating and cooling processes.This indicates that crude oil fluorescence is influenced not only by its composition and density, but also by temperature.(2) After the temperature reaches the cracking point, the Q650/500 of the crude oil changes irreversibly with temperature.This suggests that the oil’s composition has undergone irreversible alterations due to heating and cracking.The observed trend of decreasing liquid oil and the increasing solid or semi-solid asphaltene content indicates that temperature plays a crucial role in controlling crude oil cracking and phase transformation.(3) Comparing the fluorescence evolution of crude oil at the heating rates of 0.1, 0.7, and 5 ℃/min, the temperature required for the separation of residual oil components increases with the heating rate.This suggests that a lower heating rate is more conducive to oil cracking, implying that long-term and slow heating under geological conditions is unfavorable for the preservation of liquid hydrocarbons.(4) Comparing the experimental results of different samples with various oil filling ratios, we conclude that an increase in pressure promotes the thermal evolution of crude oil after reaching a thermal maturity of R_o(1.89%).This suggests that pressure does not necessarily inhibit the cracking of liquid oil but may accelerate the process.(5) Based on the results of the visualization experiment, the process can be divided into three stages.In the first stage, with an R_o range of 0.80%-1.24%, heavy saturated hydrocarbons crack preferentially, leading to a redshift in the fluorescence color of the liquid hydrocarbons.In the second stage, with an R_o range of 1.24%-1.55%, numerous aromatic hydrocarbons condense into solid or semi-solid asphaltenes that adhere to the tube wall after cooling.This results in a slight increase in the ratio of saturated to aromatic hydrocarbons in the residual oil and a blue shift in the fluorescence of the liquid hydrocarbons.In the third stage, with an R_o greater than 1.90%, aromatic hydrocarbons continue to condense, forming solid or semi-solid asphaltene.The hydrocarbons in capillary capsules exhibit distinct fluorescence differentiation at room temperature.The blue fluorescent hydrocarbons represent light oil, primarily composed of non-polar saturated hydrocarbons, while the orange fluorescent hydrocarbons indicate heavy oil, predominantly containing polar asphaltenes.This phenomenon may explain the coexistence of fluid inclusions with different fluorescence colors within the same microdomain.

Key words: normal oil, simulated thermal evolution, visualization observation, fluorescence evolution characteristics, oil cracking influence factor