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 (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.

Measurement of Fluid Phase Equilibria for High Gas Ratio Mixtures of Carbon Dioxide, Methane, and Brazilian Presalt Crude Oil

“…This result is expected for mixtures of medium crude oil with high gas content. 15 For this reason, two different techniques to confirm saturation pressure should be used as follows: (i) visual identification of gas phase in the cell content from sapphire windows and (ii) NIR transmittance measurements. It is important to point out that at 41.3 MPa, a few KPa above bubble point pressure, a small change in the SDS signal is observed.…”

“…However, phase transitions present the same morphological characteristics than those observed when pure methane is used as solvent, that is, a high and fine dispersed phase that does not aggregate to form bigger flocks. 14,15 Even considering pressures of almost 22.9 MPa below the AOP, the phase remains as a fine powdery and nonsticking material. That could suggest a minimal enrichment of the asphaltenic phase on other classes of heavy oil compounds.…”

“…It is interesting to mention that mixture homogenization was also done in the PVT-cell. Details of the PVT cell are given elsewhere. , Briefly, the apparatus is based on a thermal controlled volume variable cell, embedded with a high-pressure pump that controls the system pressure (accuracy of 0.1%), and volume measurements (accuracy of 0.001 cm 3 ) of the cell content. Maximum operational conditions are 100 MPa, 200 °C, and 1000 cm 3 .…”

“…For these complex reservoirs, diverse multiphasic conditions can be encountered during crude oil production. For instance, liquid–vapor equilibria (LVE), liquid–liquid equilibria (LLE), liquid–liquid–vapor equilibria (LLVE), and solid–liquid equilibria (LSE) have been reported and studied. − To properly assess these phase transitions, recent research has been done regarding analytical techniques for complex fluid evaluation, , along with experimental procedures for complex phase transition detection. − Because of the complex fluid phase behavior of these oils, experimental evaluation requires different approaches than the typical PVT measurements.…”

“…In our past works, the phase behavior of presalt crude oil and pure methane mixtures was investigated by coupling visual high-pressure microscopy (HPM) test and near-infrared (NIR) transmittance measurements, aiming to better understand the complex fluid phase equilibria observed in the synthetic live oil. ,, In this regard, it was possible to identify nontypical phase transitions when crude oil is mixed with methane above 72.5 mol % . Specifically, stable and low asphaltenic crude oils presented an uncommon highly dispersed asphaltic phase when mixed with a high content of methane, that is, asphaltenes with no fractal morphology and rapid redissolution with pressure increase.…”