Insights Into Some Key Issues With Solvent Aided Process
Authors: Gupta S. , Gittins S. , Picherack P. 
Published: March 2010

Abstract
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&apos; 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.