Design of the Steam and Solvent Injection Strategy in Expanding Solvent Steam-Assisted Gravity Drainage
Authors: Gates I.D. , Chakrabarty N. 
Published: March 2010

Abstract
Steam-Assisted Gravity Drainage (SAGD) is a commercial in situ recovery technology that is effective at recovering heavy oil and bitumen. However, generation of steam by combusting natural gas adversely impacts the economics of the process, especially when the natural gas price is high, as has been the case lately. It has been shown that solvent additives can improve oil production rates, or at least maintain similar oil production rates with reduced steam injection. This is the basis of the Expanding Solvent Steam-Assisted Gravity Drainage (ES-SAGD) process. The key idea is that steam plus solvent is better than steam alone to mobilize heavy oil in the reservoir. This implies that ES-SAGD can potentially use less water and require smaller water handling and treatment facilities than those in SAGD. One key capability of ES-SAGD is that the recovered solvent can be recycled and re-injected into the reservoir. However, if too much solvent is injected and too little is recovered, the process can be uneconomic because the solvent is often more valuable than the produced heavy oil. In this research, the solvent injection strategy is designed for a single wellpair ES-SAGD operation by optimizing the net energy injected to oil ratio in a detailed and realistic three-dimensional heavy oil reservoir. The process parameters for design include the operating pressure and relative amounts of steam and solvent in the injected stream. The results show that the operating pressure and injection strategy must be carefully controlled to ensure high energy efficiency and solvent recovery.

Introduction
At in situ native conditions, the viscosity of Athabasca bitumen is typically greater than one million centipoise; often ranging between two and six million centipoise. The key barrier to be overcome for producing bitumen from Athabasca reservoirs is to mobilize it in the reservoir, that is, lower its viscosity sufficiently so that it readily flows in the reservoir to a production wellbore. There are several means to do this: first, heat the bitumen to a sufficiently high temperature; second, dissolve solvent in the bitumen and dilute it; and third, induce a compositional change of the oil that leads to a mobile oil phase, e.g., asphaltene precipitation or in situ upgrading.
The effect of temperature on the viscosity of Athabasca bitumen is plotted in Figure 1(1) and is taken advantage of in the Steam-Assisted Gravity Drainage (SAGD) process(2). SAGD is now considered a commercial technology to produce Athabasca and Cold Lake bitumen reservoirs of Alberta(2–6). Typically, the original temperature of Athabasca reservoirs ranges from 7 to 11 ºC. The correlation displayed in Figure 1 shows that the viscosity falls by four orders of magnitude after the bitumen is heated by 100 ºC. Figure 1 reveals that the viscosity of Athabasca bitumen drops to less than 10 cP after its temperature exceeds about 196 ºC. This paper will consider this as the target oil phase viscosity to enable sufficient production from SAGD. At steam saturation conditions, this corresponds to a steam pressure equal to about 1,472 kPa.