Recovery of moderately viscous oils (below 2000 mPaS) from reservoirs which are unsuitable for thermal treatment may be possible by non-thermal methods. This paper addresses this problem within the framework of field and laboratory experience. Many heavy oil reservoirs in Canada fit this category: they are too thin, deep, or oil saturation is low, or have other undesirable features for a thermal recovery method to be successful.
The non-thermal recovery methods considered are: improved waterfloods, polymer flooding, surfactant, caustic and other chemical floods, immiscible carbon dioxide flooding, solvent flooding, etc. Ranges in oil recovery as based upon field experience and from laboratory experiments are indicated. It is shown that non-thermal methods may be successful for heavy oil recovery under rather restrictive conditions. Based upon the data from some 113 field tests, it seems that immiscible carbon dioxide floods are more likely to be successful. In nearly all cases, the oil recovery is much lower than that attainable by thermal methods. This paper concludes with an evaluation of non-thermal recovery methods.
Introduction
Heavy oil deposits in Canada, Venezuela and the United States comprise several trillion barrels. Thermal methods, viz. steam injection and in situ combustion are best for recovering some of this oil, provided that the reservoir conditions are favourable. However, over half of the oil occurs in reservoirs that are not suitable for thermal methods. This is the case when the formations are thin (< 10 m), depths are large (> 1000m), formation permeability is too low to permit high enough injection rates, or the oil saturation is low, perhaps combined with a low porosity. Under such conditions, a non-thermal recovery method may be employed, which may be further modified for a viscous oil. Such methods would be expected to be applicable to moderately viscous oils (viscosity less than a thousand mPas), if the remaining conditions are favourable. This paper reviews alkaline flooding, polymer flooding, immiscible carbon dioxide flooding in detail, and discusses other non-thermal methods briefly. Results of laboratory investigations are discussed alongside field tests. Comprehensive tables list the laboratory data for all processes, as well as field data for each process tested. It is believed that the data presented would help in assessing non-thermal recovery methods for a specific reservoir. Furthermore, the laboratory data summarized provide an idea of the range of variability of results, and a comparison with the field recovery values underscores the problems associated with unscaled experiments.
Problems in Viscous Oil Recovery
The principal difficulty in the recovery of heavy oils is the high oil viscosity, leading to low oil mobility, which makes displacement by a cheap fluid, such as water or gas, inefficient on account of the "unfavourable" mobility ratio (i.e. mobility of the displacing fluid is greater than the mobility of the displaced oil). Under suitable reservoir conditions this may be corrected by the use of heat. However, this may be impractical for a variety of reasons, some of which were mentioned above.
