Pentane as a Potential Candidate for ES-SAGD and Hybrid Processes: Its Phase Behaviour with Bitumen

Steam-assisted gravity drainage (SAGD) is a promising method for producing oil sand in Alberta; however, it has been suffering from high energy and water consumption and significant greenhouse gas (GHG) emissions. Solvent-based processes represent an alternative to steam-based processes but they have their own drawbacks. The main problem with solvent-based processes is a relatively low production rate because they rely on diffusion and dispersion mechanisms, which are very slow processes. To overcome the limitations of steam and solvent processes, hybrid processes have been proposed in which steam and solvent are co-injected. As a result, several recovery techniques, such as expanding-solvent SAGD (ES-SAGD) and steam-alternating solvent (SAS), were proposed and patented. However, there is a fundamental lack of basic data and mechanistic knowledge relevant to solvent/heat-assisted-recovery processes. Even the most fundamental experimental data, such as the solubility of commonly used solvents in bitumen, is not available. The quantitative effects of solvent on bitumen viscosity and density, and phase behaviour are also not well understood.

Pentane as a potential additive to steam-based processes has saturation pressure close to steam at in situ thermal process; thus, pentane and its mixtures with other hydrocarbons can be considered for bitumen and heavy oil recovery. In this study, the phase behavior of pentane / Athabasca bitumen mixtures was experimentally studied. The measurements include the properties of phases such as solvent solubility, number of phases, and composition, density, and viscosity of each phase as well as the possibility of asphaltene precipitation. The experiments were conducted over wide range of temperatures (23–200°C) and at pressures (1–10 MPa) which cover single liquid, liquid-liquid and vapor-liquid equilibrium conditions.

The results showed that the solubility of pentane in bitumen was relatively high even at high temperatures leading to significant density and viscosity reductions. The asphaltene was separated as a second liquid phase rather than a solid phase, and there was a transition zone between single liquid and liquid-liquid regions in which the asphaltene was instable in the system. A significant reduction in density and viscosity of bitumen was observed with the dissolution of pentane over studied temperature condition.