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.