When you are checking out the screening criteria for a successful steam flood, the fact that there is a depth limitation might seem a little surprising. You would expect to see the reservoir quality and thickness limits and we all know steam flooding is most effective with heavy oils; but why does depth feature as a criterion and how many reservoirs are shallower than 3,000' anyway?
Well, in the North Sea the answer to the last question is: "Not many." There are the Pilot and Harbour fields, and Chevron's Captain field and maybe a few more. But why does the reservoir have to be so shallow for a steam flood to be successful? Well, there are two conventional answers to that question, the first is heat loss, which turns out not to matter so much for highly productive offshore fields; but the second is all to do with thermodynamics and last I heard the laws of physics apply everywhere.
Heat loss seems like a pretty difficult hurdle to overcome, the deeper the reservoir the longer the wellbore and the more heat that is lost as the steam makes its way down to the reservoir. We cover this in a lot more detail in an earlier post (Update: and also in this excerpt from our presentation at DEVEX 2015).
The reason it is less important for offshore projects is because offshore wells are normally highly productive and we can plan for steam injection rates of over 10,000 bbls/day cwe (cwe is cold water equivalent). Now the rate at which heat is lost from the well is pretty much the same whether 1,000 bbls/day or 10,000 bbls/day is flowing down the well, but of course in proportion to the heat that is being injected into the reservoir it is ten times smaller in the 10,000 bcwe/day case. When we modelled Pilot steam injection wells we found that if we started with 95% quality steam at the wellhead we could easily have 90% quality steam at the sand face, no fancy vacuum insulated tubing required.
So in fact for highly productive offshore wells heat loss is not so big an issue. But that still leaves thermodynamics. I really need a chart to explain this properly, so here is the one I used to understand the issue myself. It is a plot of enthalpy, that is the energy in the water (or steam), versus pressure. Pressure is measured in bars and is on a log scale. There are also a number of isotherms plotted in red, isotherms are lines of constant temperature. So for water at a particular pressure with a certain amount of energy we can use this chart to read off both the state of the water (gas or liquid) and the temperature.
The thick black line is the phase envelope; to the left of the phase envelope water is liquid, to the right it is steam. To get our eye in on the chart let's go along the horizontal line at one bar of pressure and imagine boiling a kettle. At first the water is cold, as we add energy, the kettle warms and the temperature rises, soon we reach the left hand side of the phase envelope where the kettle begins to boil – you can see the 100ºC isotherm turns at right angles just here – as we add more and more energy the temperature doesn't change, but all the water turns into steam. Inside the phase envelope the dotted lines tell you the proportion of steam, or steam quality. When we are at the right hand side of the phase envelope we have 100% steam and the kettle has boiled dry. You can see it does take an awful lot of energy to turn water into steam. But once it is steam, if we keep adding heat, taking the temperature from 100ºC to 200ºC doesn't increase the energy in the steam by very much.
And that is the whole point of steam flooding. We inject steam to heat up the oil and to do that we have to heat the whole reservoir, all the oil in it and to some extent the rocks above and below as well. So we need a mechanism that delivers an awful lot of energy into the reservoir. It is the energy released as steam condenses and turns back into water which does that work.
As the pressure increases the enthalpy of condensation (illustrated on the chart by the distance between the two dark lines) reduces until, at about 200 bar, there is no difference between steam and water and no enthalpy of condensation at all. So at 200 bar steam is not as efficient at transferring energy and heat into the reservoir. In fact at 200 bar steam that has as much energy in it as steam at 1 bar and 100ºC, has a temperature of almost 400ºC.
The conventional limit for steam flooding is considered to be 3,000' which for a normally pressured reservoir is about 90 bar, just around the top of the green zone on the chart. At that pressure the steam temperature is about 300ºC (600ºF). Most conventional downhole components have a temperature limit of 150ºC (300ºF), it takes specially designed equipment to work at such high temperatures, and there are few components rated for much more than 300ºC. So, in the end, it is really the temperature of steam at reservoir pressure that is the main reason why steam flooding has a practical depth limitation of about 3,000'.