Phase Behavior for Crude Oil and Methane Mixtures: Crude Oil Property Comparison

Abstract

A wide variation in composition and thermodynamic properties is expected for different reservoir fluids, from the simplest one containing only methane and light compounds to those with high compositional complexity. In this work, phase behavior of six different reservoir fluids recombined with methane was compared and contrasted. From our past work (Romero Yanes, J. F.; Feitosa, F. X.; Fleming, F. P.; de Sant’Ana, H. B. Experimental Study of the Phase Behavior of Methane and Crude Oil Mixtures. Fuel 2019, 255, 115850, DOI: 10.1016/j.fuel.2019.115850), a low asphaltene crude oil [28.0° American Petroleum Institute (API) gravity, 0.68 wt % n-C7 asphaltenes], referred to here as BRO, has presented a complex phase behavior when mixed with methane, especially when the methane content is around 75.0 mol %. On the basis of these results, the phase behavior of six different crude oils mixed with 75.0 mol % methane was studied in this work. The main characteristics of samples can be summarized as follows: a light condensate fluid (P1, 44.0° API gravity, and no detected n-C7 asphaltenes), three medium crude oils [(P2, 30.8° API gravity, and 0.09 wt % n-C7 asphaltenes), (P3, 26.0° API gravity, and 0.54 wt % n-C7 asphaltenes), and (P4, 25.1° API gravity, and 0.12 wt % n-C7 asphaltenes)], and two heavier crude oils [(P5, 19.5° API gravity, and 2.69 wt % n-C7 asphaltenes) and (P6, 18.7° API gravity, and 2.09 wt % n-C7 asphaltenes)]. From these fluids, mixtures with methane were prepared and their phase behavior was evaluated using a constant composition expansion test, coupled with a near-infrared solid detection system and high-pressure microscopy. For P1 and P4 systems, a unique phase transition at the bubble point was detected. On the other hand, for P2 and P3 systems, non-typical multiphase equilibria were observed associated with asphaltene precipitation, similar to that described for the BRO mixture. Phase segregation with no fractal geometry was observed from the phase transition onset pressure to the bubble pressure for P2, BRO, and P3 crude oils. Additionally, BRO and P3 show minor aggregation at pressures above the bubble pressure, with rapid redissolution when gas evolves at saturation pressures. P5 and P6 systems have phase transitions at a higher pressure because of their high asphaltene content. Phase transitions and related characteristics were associated with crude oil solvency for heavy compounds and their stability.