Immiscible carbon dioxide floods have been shown to hold promise for the recovery of moderately heavy oils of Alberta and Saskatchewan by both laboratory and field studies. This paper addresses the important question of the effect of carbon dioxide purity on the process efficiency.
A number of experiments was carried out In scaled linear and two-dimensional models, employing mixtures of carbon dioxide and nitrogen, with nitrogen concentrations of up to 30%, in a WAG (Water-Alternating-Gas) type process. Results were compared with previous findings tor pure carbon dioxide and pure nitrogen.
It was found that the oil recovery was affected only to the extent of 6 percentiles if impure (containing 15%, or less, nitrogen) carbon dioxide was utilized. Increasing the carbon dioxide partial pressure in the mixture resulted in an increase in oil recovery, while an increase in the partial volume showed the opposite trend. Other parameters studied include the effect of operating pressure and the injection rata. On the whole, it is shown that oil recoveries of up to 61 % are possible under optimal conditions, when impure carbon dioxide is employed in the water-alternating gas mode, using a slug size of 20% pore volume at the operating conditions.
The immiscible carbon dioxide WAG displacement process is being used increasingly as an enhanced oil recovery technique. Within the last decade, many studies have been conducted to examine the effectiveness of the process, as well as its mechanism. In the United States, many field studies and projects have been conducted with good results 1,2. In Hungary, this method has been in use for thirty years to recover oil, and excellent oil recoveries have been reported3. In Alberta, the start of the commercial immiscible carbon dioxide project in Retlaw, in January 1991, marked the success and effectiveness of this method in recovering medium gravity heavy oils. The cumulative production, as reported in June 1991, exceeded 120,000 standard cubic meters4. It has also been shown that, based on laboratory and field studies, this method works much better than any other thermal recovery method in moderately viscous oil reservoirs where the formations are thin (less than 10m) and the reservoir depths are large (greater than 1000 m).
In Alberta and Saskatchewan, there are many moderately viscous heavy oil reservoirs where the reservoir conditions fall in these regions. Moreover, many of them are underlain by bottom water zones. This makes the application of thermal recovery methods inefficient and uneconomical due to excessively high heat loss to the bottom water zone and the resulting high operating costs. An alternative is to employ the immiscible carbon dioxide WAG method, which involves the injection of small slugs of carbon dioxide alternating with water.
Laboratory studies on the application of the immiscible WAG process to the recovery of oil from such reservoirs conducted by the previous researchers5–12 showed that this process can recover a substantial volume of oil, about 10 to 30% incremental oil compared to a waterflood, when pure carbon dioxide is used.