Design of Steam-Hexane Injection Wells For Gravity Drainage Systems
Authors: McCormack M.E. 
Published: March 2010

The steam-assisted gravity drainage (SAGD) process has been demonstrated to be commercially viable for the recovery of bitumen from the Athabasca Oil Sands region. One of the potential long-term concerns with the process is the relatively high use of energy. The majority of the projects underway have steam-oil ratios (SOR's) in the range of 2.5 to 3.5. This translates into combustion of approximately 20 – 28% of the energy contained in the oil produced. To reduce the current level of energy consumption, many studies have investigated the addition of solvents to the steam. A small number of field trials have been conducted, with apparently encouraging results. The use of hexane, or a similar commercial grade naphtha mixture, as a solvent appears promising, as it would closely match the vapour pressure curve of steam and would tend to flow and condense with the steam.

This study presents the results of numerical investigations regarding the implications of the addition of hexane to steam with respect to the design of SAGD solvent injection wells.

This study found that the SOR at full depletion of a steam-hexane SAGD, in the absence of a wind-down strategy, would be improved over that for conventional SAGD only by the degree that heat losses to the over- and underburden are reduced due to more rapid bitumen production rates. At short and intermediate times, a more significant SOR improvement will result primarily from the more efficient drainage of bitumen from the chamber walls and floor, reducing the thickness of the heated formation that is undrained.

It was also concluded that injection wells for steam-hexane SAGD will require the same diameter wellbores as for conventional SAGD injection wells. Existing SAGD injection wells can be directly converted to steam-hexane use as overall project economics dictate.

Introduction

The SAGD process has now been demonstrated to be a robust technique to produce bitumen from high quality reservoirs that are too deep to be mined economically from the surface. There are a significant number of commercial projects at the time of writing, with many more in the planning stages.

However, it is well-known that the SAGD process is very energy intensive. A typical high quality SAGD project exhibiting a wellhead SOR of 2.5 will require the consumption of ~20% of the bitumen produced to generate steam (based on 80% conversion efficiency in the boiler). There have been many authors(1–10) that have suggested that the addition of solvents to steam might be a method to improve the energy efficiency of the process and also to potentially increase the production rate. If successful, such steam-solvent processes would be economically beneficial.

A laboratory investigation by Nasr et al.(1) showed that drainage rates with the injection of a steam-hexane mixture with 1% mass fraction hexane produced average drainage rates that were approximately double those obtained for a steam only system. This same study showed that steam-hexane produced the highest drainage rates of the solvents tested (C1 through C8, plus a commercial diluent).