Gravity drainage processes, such as SAGD and VAPEX, aim at exploiting viscosity reduction of the target oil, either through thermal diffusion or dilution. Thermal diffusion being much faster than molecular diffusion, production rates from a steam process are expected to be higher than a solvent-alone process. Despite this apparent drawback, solvent-alone processes promise to be attractive owing to lower heat losses and energy requirements, lesser impact to the environment, possible downhole upgrading, etc. One is naturally led to thinking of combining the benefits of SAGD with that of the solvent-alone process. The resultant process, aimed to improve the performance of SAGD by introducing hydrocarbon solvent additives to the injected steam, is the subject matter of this paper and is called the " Solvent Aided Process" (SAP). With the combined potential benefits come the combined challenges of the two processes. Although substantial understanding has been developed around SAGD in recent years, a number of unknowns associated with solvent processes exist. This paper, drawing heavily from the authors' extensive investigation of SAP, dwells on the estimated benefits of SAP over SAGD, discussing possible mechanisms at work, optimal design of injectant (involving lighter alkane additives), and operational aspects and issues such as solvent retention, etc. Introduction The concept of adding solvents to the injected steam for improving the performance of steam-based recovery processes is not new. Various authors(1–8) have described and analyzed the benefits of adding hydrocarbon solvents to primarily steam flood processes. Their work mainly focused on the oil-recovery enhancements that solvent addition brings to steam-flood(1–3, 5–8) or steam stimulation(4). With the advent of SAGD(9, 10), exploitation of the vast heavy oil and bitumen resources of the Western Canadian Sedimentary Basin is now feasible. However, owing to a highly energy-intensive process and the nature of the target product (heavy oil), the economics of SAGD is very susceptible to fuel prices and heavy oil market forces. Use of solvents in place of steam in gravity drainage for heavy oil recovery(11–14) promises to be a more energy-efficient process but suffers from poor (estimated) rates of recovery(12,13). This is because molecular diffusion, the mechanism responsible for the dilution of heavy oil which reduces its viscosity in a solvent-alone process, is much slower than its counterpart, thermal diffusion, in the case of SAGD. Among other things, modification of the drainage geometry(15) has been suggested for providing a large contact area to compensate for a low rate of diffusion and to raise the rates of production. Given earlier efforts(1–8) to improve the steam-flood process with the use of solvents, the combination of solvents and steam in conjunction with the concept of gravity drainage seems to be a natural progression from SAGD and VAPEX. A few investigators, ncluding Butler and Yee(16) and Palmgren and Edmunds(17), have suggested the combination of thermal effects and solvent dilution. Viscosity reduction, as pointed out by Butler et al.(11), has a direct impact on the rate of production from a gravity drainage process.

The authors have previously described a Solvent Aided Process (SAP) that aims to combine the benefits of SAGD and VAPEX. In SAP, a small amount of hydrocarbon solvent is introduced as an additive to the injected steam during SAGD. While steam is intended to be the main heat-carrying agent, the solvent will dilute the oil to reduce its viscosity over and above what is accomplished by heating alone. The overall effect should be an improved oil to steam ratio (or reduced energy intensity). Although promising based on the authors' calculations, the process has not been previously applied or tested on a field scale. This paper describes the implementation of a SAP pilot at Encana's Senlac Thermal Facility. In addition to dwelling on some of the important parameters of a SAP test, it discusses the design considerations for the field pilot and the necessary modifications to an existing SAGD plant, specifically in the area of boiler operations controls. Although the design calls for an assessment of reservoir performance results on a longer-term basis, initial results from this pilot look very encouraging. The oil rates have shown a substantial increase, and the steam-oil ratio has shown a corresponding decrease. This paper also discusses directional economics with SAP and its beneficial impact on the environment. Introduction Just as steam tackles the viscosity reduction of in situ oil in SAGD(1,2) by heating it, solvents(3–7) do this by diluting the oil. Although employment of both steam and solvent together has been discussed in the literature(8–15), these discussions have mostly focused on enhancement of steam-flood or steam-stimulation. In their discussion on the subject, Gupta et al.(16) described SAP as a process enhancement to SAGD where a small amount of a light alkane solvent, namely propane, butane, pentane, etc., or a mixture thereof, is added to the injected steam. They also suggested, with the help of lab experiments and numerical modelling, that SAP has the potential to substantially improve the performance of SAGD. Expected SAP Advantages Figure 1 shows a comparison of an expected numerically obtained oil rate profile from a SAGD application vs. one obtained similarly with the application of SAP in the same reservoir. It is assumed that SAP would start after the expiry of a certain initial period in the life cycle of SAGD to allow for the initial development of the chamber with steam. The units from the rate and time axes are omitted to emphasize the general nature of these profiles. The comparison of the rate profiles is provided in order to suggest that the bulk of the oil that would have been produced in the later period with SAGD can be produced sooner with SAP. The acceleration of production and the corresponding cash flow could lead to improved economics for the project. Apart from the improved economics as a consequence of production rate acceleration, the other expected advantages of SAP include reduced environmental impact, possible down-hole upgrading of the heavy oil, and a small increase in the ultimate recovery.