To petroleum engineers' ears, attuned to the recovery factor that one can expect from a water flood (typically 30% to 40%, and only occasionally over 60%) the recovery factors that steam flooding can deliver, somewhere between 50% and 80%, seem inordinately high. A pinch of salt seems the appropriate accompaniment to the kind of claims that are routinely made for steam flood projects.
But Chevron is one of the acknowledged leaders in the application of steam flooding, and their assessment is that a typical steam flood should recover between 50% and 80% of the original oil in place, see slide 7 in this 2006 presentation. Their view is based on practical experience in fields which have been steam flooded for many years, giant fields such as Kern River and Duri. It turns out that there are very good reasons behind the significantly higher recovery factors achievable with steam flooding, as well as a solid track record of actual project outcomes.
Recovery factor is simply a product of the microscopic displacement efficiency (how much of the oil you can get out of the pores in the laboratory) and the macroscopic displacement efficiency (how much of the reservoir you can actually sweep in situ). Compared to a cold seawater flood, steam flooding typically improves both these parameters, the former quite dramatically.
Another measure of microscopic displacement efficiency is the residual oil saturation, that is the amount of oil that just can't be swept out of the rock no matter how much water, or gas, or steam is pumped through it. Core studies have shown that the residual oil saturation depends on both the fluid doing the sweeping and the temperature it is done at.
Experimental results have shown that residual oil saturation falls both as temperature rises and as salinity falls. The chart below shows the effect of temperature on residual oil saturation for a range of initial oil viscosities. At very high temperatures the residual oil saturation to steam can fall as low as 5%.
This decline in residual oil, and thus the increase in the recoverable oil, needs some explanation. It is not just one simple effect but a number of factors working together, here they are:
- the viscosity of the oil declines dramatically with temperature making the oil flow much more easily (see the animated chart on this page);
- the increased temperature alters the wettability of the rock making it more water wet (an oil wet rock is one where oil clings to the grains of sand; it will have a higher residual oil saturation than a water wet rock which prefers water to cling to it);
- the reduction in salinity also tends to make the rock more water wet (this effect is the reason that BP is implementing a low salinity water flood in the Clair field);
- at high temperatures some of the lighter ends in the residual oil boil off (in fact in a steam flood the produced oil is slightly lighter than the original oil in place); and finally
- as the phase of the fluid sweeping the reservoir changes from liquid (water) to gas (steam) the connate water also boils off which disrupts and releases some more of the remaining trapped residual oil.
The macroscopic displacement efficiency for a steam flood also tends to be high for two main reasons. Firstly, steam floods have very high well densities which helps ensure more of the reservoir is well swept; and secondly, the fingering that one would expect because of the high mobility ratio between steam and oil, is mitigated by the tendency of fingers of steam to cool and then condense rather than extend further away from the flood front. This makes a steam flood front much more stable than an equivalent gas flood front.
So what is the field experience. The table below details actual and expected recovery factors for a number of steam flood projects. The table is not exhaustive nor does it necessarily contain the most up to date estimate of ultimate recovery from these projects, just the data that was publicly available, but the data support Chevron's claim of a 50% to 80% range for steam floods. The only real outliers in the table are, a fractured carbonate steam flood in Oman (Qarn Alam) and another carbonate project in the Neutral Zone (Wafra). Qarn Alam is still a major success as the expected recovery factor of 30% there is a dramatic improvement on the initial primary recovery factor of just 3%.
Steam flooding really can deliver high recovery factors; the challenge is to do that economically.
Update 28th March 2015: Included all the entries from Farouq Ali's 1979 paper and calculated the average recovery factor for multi-darcy reservoirs to be 60% and for reservoirs with one darcy or less to be 50%.
Update 6th May 2015: Included details for the Wafra project.
Update 11th & 20th June 2015: Included a mention of the Emlichheim field (the German part of the Schoonebeek field) & minor edits.