An Experimental Study of Steam-Assisted Gravity Drainage

Abstract

Summary In this paper we present a large-scale experimental study of the compositional effect on produced bitumen properties in steam-assisted gravity drainage (SAGD). The SAGD experiment used a sandpack in the cylindrical pressure vessel that was 1.22 m in length and 0.425 m in internal diameter. The pore volume of the sandpack was 58 L, and the porosity and permeability were 0.33 and 5.5 darcy, respectively. The sandpack was initially saturated with 93% bitumen and 7% deionized water. The SAGD experiment after preheating was operated mostly at a steam injection rate of 35 cm3/min (cold-water equivalent) at 3,600 kPa (244°C). The produced fluids (gas, oil, and water) were analyzed; for example, 10 oil samples were analyzed in terms of carbon number distribution (CND), the asphaltene content, density, and viscosity to investigate the compositional change of the produced bitumen. After the experiment, the sandpack was excavated, and samples were taken for analysis of solid, water, oil, asphaltene, and sulfur contents. Experimental data (e.g., propagation of a steam chamber and production of oil and water) were history matched using a numerical reservoir simulator. The produced bitumen was lighter and contained 1- to 5-wt% less asphaltenes than the original bitumen. Also, the remaining oil inside the steam chamber contained 6-wt% more asphaltenes. As a result, the produced bitumen was 1- to 6-kg/m3 less dense than the original bitumen. This is an increase in API gravity from the original 7.9° to 8.6°. In the actual operations, bitumen is diluted with condensate to decrease the oil viscosity for pipeline shipping. This decrease in bitumen density corresponds to a decrease of the diluent cost by 5 to 10%. The produced bitumen became less dense with increasing steam-chamber volume. Results were history matched with a simulation model that considers capillary pressures to properly model the mixed flow regimes of oil/water countercurrent and cocurrent flow with an expanding steam chamber. The history-matched simulation indicated that the progressively decreasing density of the produced bitumen can be attributed to the vaporization of the relatively volatile components in the remaining oil and condensation of those components near the chamber edge.