(150c) Phase Behaviour of Heavy Oil and Butane Mixtures

Solvent-based methods are a potential alternative to thermal methods for the recovery of heavy oils because they are less water and energy intensive. Many proposed solvent-based recovery processes are designed to operate near the saturation pressure to maximize the solubility of the solvent in the oil while avoiding the formation of a second liquid phase. One solvent of interest for heavy oil recovery processes is n-butane because it has a saturation pressure at typical operating temperatures that is near the reservoir pressure for many heavy oil reservoirs. n-Butane is also a relatively low cost solvent. The injection of a low molecular weight paraffinic solvent such as n-butane into the reservoir can lead to complex phase behaviour including the formation of liquid-liquid, vapor-liquid, and vapor-liquid-liquid regions. The second liquid phase may be a liquid or a dispersed glassy asphaltene-rich phase depending on the conditions. Therefore, an accurate model of the phase behaviour and phase properties is required for simulation of solvent-based recovery methods.

Most commercial simulators use a cubic equation of state (CEoS) to model phase behaviour that is beyond the range of a black oil model. CEoS can match the boundaries of vapor-liquid, liquid-liquid, and vapor-liquid-liquid regions but generally fail to predict phase compositions and masses (including asphaltene yields) accurately [1]. Recently, composition dependent mixing rules were implemented in the Peng Robinson (APR) EoS to model the phase behaviour for a Western Canadian bitumen diluted with n-pentane [1] and propane [2]. The APR EoS is the Peng Robinson EoS implemented in VMGSim^TM with volume translation [3]. One drawback of this method is that binary interaction parameters must be tuned for each solvent and therefore phase behaviour data are required for the tuning. However, phase behaviour data for mixtures of heavy oil and n-butane are sparse. Therefore, the objectives of this study are to: 1) collect phase behaviour data from a bitumen diluted with n-butane, and; 2) tune the APR EoS with compositional dependent mixing rules to match the data.

Saturation pressures, liquid-liquid boundaries, phase masses, and phase compositions were measured for mixtures of n-butane and a Western Canadian bitumen. Saturation pressures were measured with the constant composition expansion method in a blind cell apparatus at temperatures from 50 to 230°C and solvent contents from 3 to 50 wt%. The liquid-liquid boundary was determined optically from titrations through a high pressure microscope (HPM) at temperatures from 50 to 180°C and pressures up to 10 MPa. The morphology of the second liquid phase was also examined. Phase compositions were determined in both a blind cell apparatus and a PVT cell at temperatures from 20 to180°C and pressures up to 10 MPa. The composition of each phase, defined as mass content of maltenes, asphaltenes and n-butane, was determined gravimetrically from samples taken from the light (butane-rich) and heavy (asphaltene-rich) phases. The mass of each phase was determined directly in the PVT cell experiments or from a mass balance in the blind cell experiments.

The data were used to generate pressure-composition (P-X) phase diagrams. At low n-butane contents, the mixtures of n-butane and bitumen formed a vapor-liquid (VL₁) phase at low pressure and a single liquid phase (L₁) above the saturation pressure. The saturation pressure increased monotonically with increasing n-butane content until a n-butane content of approximately 40 wt%. Above this n-butane content, the mixtures formed two liquid phases (L₁L₂) at pressures above the saturation pressure and a vapor-liquid (VL₁) region below the saturation pressure. The L₁/L₁L₂ boundary was insensitive to temperature and only slightly sensitive to pressure (a vertical line on the P-X diagram). The L₁L₂/V L₁L₂ boundary was approximately equal to the vapor pressure of n-butane (a horizontal line on the P-X diagram). In general, approximately half of the bitumen partitioned to the heavy liquid phase and the n-butane content in the heavy phase ranged from 8 to 22 wt%.

The APR EoS was tuned to model the data. The EoS parameters for the mixture were calculated using the classic Van der Waals mixing rules. The binary interaction parameters for the mixing rules were estimated first with “traditional” temperature dependent parameters and then with composition dependent parameters. The APR EoS with the temperature dependent interaction parameters generally fit the saturation pressures and onsets within the experimental error; however, larger deviations were found at temperatures higher than the critical temperature of the solvent (152°C). This approach significantly under-predicted the C5-asphaltene and maltene contents in the heavy phase. The APR EoS with composition dependent interaction parameters fit not only saturation pressure and onsets but also the phase compositions generally to within the experimental error. Higher deviations occurred near the critical point of the solvent. The results confirm that the APR EoS with composition dependent interaction parameters is capable of modeling the phase behaviour of heavy oil diluted with paraffinic solvents.

References

[1]. Johnston, K. A., Schoeggl, F. F., Satyro, M. A., Taylor, S. D., Yarranton, H. W. Phase behaviour of bitumen and n-pentane. Fluid Phase Equilibria, 2017, 442, 1-19.

[2]. Mancilla-Polanco, A., Schoeggl, F. F., Johnston, K., Richardson, W. D. L., Yarranton, H. W., Taylor, S. D. The Phase Behaviour of Heavy Oil and Propane Mixtures. SPE Canada Heavy Oil Technical Conference, Calgary, February 2017.

[3]. Virtual Materials Group Inc. VMG 2011, VMGSim Version 9.5, VMG Sim User's Manual, Calgary, Canada.