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In this work, phase behavior was analyzed for recombined mixtures of a Brazilian pre-salt crude oil with carbon dioxide and methane. This oil was sampled at stock tank conditions during a well-test, and mixtures with gas were prepared with a gas molar composition ranging from 20.0 to 86.0 mol %. Their phase behavior was investigated from 293.15 to 378.15 K and up to 100.0 MPa. Direct phase transition detections were achieved by using a full visibility PVT cell, coupled with a short-wave infrared camera. At this wavelength, crude oil has a lower absorbance, and it has been noted that macroscopic phase transitions can be easily detected by visual inspection. For the pseudo-binary system crude oil + carbon dioxide systems, liquid−liquid phase transitions were observed when the gas content is higher than 70.0 mol %. In addition, at the higher carbon dioxide composition, an asphaltic phase was formed at high pressures together with liquid−liquid phases. Similarly, crude oil and methane systems presented a liquid−liquid immiscibility region at methane composition above 60.0 mol % in all temperature domain studied. The Peng−Robinson equation of state was used for modeling purposes, and liquid−liquid and liquid−liquid−vapor transitions were qualitatively described. Additionally, it was observed that liquid−liquid equilibria behavior was highly dependent on the crude oil heavy fraction immiscibility because of the increasing system asymmetry by increasing mixture gas content.

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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 (Experimental Study of the Phase Behavior of Methane and Crude Oil MixturesRomero YanesJ. F.FeitosaF. X.FlemingF. P.de Sant’AnaH. B. Romero Yanes, J. F. Feitosa, F. X. Fleming, F. P. de Sant’Ana, H. B. Fuel2019255115850), 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.