The unprecedented commercial success of in situ bitumen recovery by cyclic steam stimulation (CSS) at Cold Lake has relied on progressive reservoir interventions that have increased bitumen recovery levels to 25% of OBIP. These interventions were the consequences of continual improvement in understanding of how the CSS recovery process works. A mechanistic model has now been developed describing the recent progress in describing CSS process physics.
The CSS mechanistic model relates how the recovery mechanisms including compaction drive, solution gas drive, steam flashing, gravity drainage and the more unconventional bitumen mobilization processes of foaming and emulsification interact, particularly in the mature stages of depletion. This model provides a basis to explain a wide suite of operational behaviors and field observations including the stable WOR and GOR production patterns.
The model attributes much of the effectiveness of the CSS process to the formation during production cycles of an active "foamy" impedance zone cushioned between cold and heated reservoir zones. The impedance or transition zone allows efficient exploitation of native solution gas drive to displace live bitumen at below steaming temperatures for a considerable portion of the production cycle. The heated zone closer to the wellbore acts as a thermally stimulated conduit for the displacement of bitumen into the wellbore. Cold reservoir supplies the compaction and solution gas drive energy. It is concluded that Cold Lake CSS is a surprisingly effective solution gas drive process. The improved understanding of CSS may provide insights to heavy oil primary production mechanisms.
Conventional expectations of primary production, suggest that recovery levels from reservoirs containing high viscosity bitumens (50,000 – 100,000 mPa.s) such as at Imperial Oil's Cold Lake operations should be on the order of approximately 1% OBIP. In contrast, the application of cyclic steam stimulation (CSS) at Cold Lake is expected to provide substantially more with ultimate recoveries in the range of 25–35 % OBIP. CSS is a process in which steam is injected into a well to heat the adjacent bitumen, followed by extended periods of hot production from the same well. Because steam in not injected to displace the oil, CSS acts essentially like a primary bitumen recovery process. Thus, the high recoveries from CSS are that much more surprising.
Other approaches, such as continuous steam drive, which has been successfully applied elsewhere has not been successful in numerous attempts at Cold Lake. Only recently, with a pressure cycling approach that more closely imitates the mechanisms of CSS, is "steam drive" beginning to provide performance comparable to CSS.
A number of attempts have tried to explain the key mechanisms that control CSS. The starting point of such analyses has been to study production and observation well data. As observed previously, the production data within a cycle usually show extended periods of stable water-oil ratio and gas-oil ratios. A typical example of a single CSS production cycle is provided in Figure 1. After the initial steam condensate flow back period is over, water rates remain more or less proportional to the bitumen rates except when there are occasional water "communication" events caused by steaming in nearby wells. Similarly, gas rates also remain closely linked to bitumen rates in spite of the very unfavorable mobility ratios. Therefore, it continues to be hypothesized, consistent with the previous studies, that foams and/or emulsions are playing a key role in controlling gas-oil ratio (GOR) and/or water-oil ratio (WOR).