The False Lucre of Low-Pressure SAGD

Abstract

Abstract The recent trend in SAGD is towards low-pressure steam injection. The intention is to be more thermally efficient: steam at lower pressures has a greater proportion of its heat as latent heat, which is the dominant source of heat released to the cold reservoir. The oil sand then warms up to the steam temperature, thus mobilizing the bitumen. The SAGD process must be thermally efficient for optimal economic viability. However, a more rigorous examination of the SAGD process presented here, inclusive of surface processes, reveals that there are fewer thermal benefits in operating at lower pressures. In addition, SAGD will be hindered by low-pressure injection due to higher viscosities and the inhibited dilation of the unconsolidated sandstone reservoir. This paper demonstrates that a complete analysis of the SAGD process favours operation at high pressures, and that SAGD at low pressures will be less effective. Introduction Steam assisted gravity drainage (SAGD) has been successfully applied to the in-situ thermal recovery of bitumen beginning with AOSTRA's Underground Test Facility laboratory-scale pilot project, Phase A (1987 – 1991)(1), and the subsequent commercialscale pilot, Phase B (1991 – present)(2). Since then, a large number of commercial SAGD projects have emulated their success. The current trend in operating philosophy is towards low-pressure SAGD: LPSAGD. This is based on the fact that at lower pressures, a larger percentage of steam's total heat is latent heat. Attractiveness of Low-Pressure SAGD SAGD Process In SAGD, most of the heat transferred to the cold oil sands formation is by the condensation of steam into the periphery of the steam chamber. The latent heat released from the steam is transferred to the colder formation mainly by conduction. Therefore, along the slopes of the steam chamber, the predominant flow of condensed steam (i.e. hot water) and mobilized hot bitumen is perpendicular to the direction of conductive heat flow. Steam quality is the mass fraction of water converted from liquid to steam. In SAGD, the injection of less than 100% quality steam is counterproductive, since the injected liquid water fraction just falls from the injector well to the producer well under gravitational forces within the isobaric steam chamber. This adds to the water recycling costs while contributing neither to the release of energy to the formation nor to bitumen recovery. Steam Properties The energy in steam at constant pressure consists of sensible heat and latent heat. The sensible heat is the energy required to raise the temperature of the water from an initial source temperature to the steam temperature; the latent heat is the energy required for the phase change from that hot water to steam. It is this latent heat that provides the dominant source of heat for the SAGD process. While the enthalpy of steam over the pressure range of 1,000 to 3,500 kPa is relatively uniform (see Figures 1 and 2), at lower pressures the proportion of heat as latent heat is higher. Since latent heat is the dominant form of heat transfer to the formation, one can see the attraction of low-pressure injection, solely from a steam energy viewpoint.