In 2007, work was published on the predictability of hydrogen sulphide production in Athabasca SAGD projects. It was possible to predict the hydrogen sulphide production per unit volume of bitumen produced against steam zone temperature, by assuming pseudo-zero order kinetics, using the Arrhenius energy published by Strausz and his co-workers at the University of Alberta.

An examination of field results for carbon dioxide production in Athabasca projects shows that the predictability of carbon dioxide is less simple. The carbon dioxide production is not erratic but goes through a distinct temperature minimum, an important result in view of the dependence of silica production in SAGD and scaling behaviour in facilities on carbon dioxide. Again, as for hydrogen sulphide production, it is possible to estimate carbon dioxide production by a simple graphical technique, which is shown.

Partial explanations for the observed temperature minimum are offered.


The production of acid gases in SAGD projects is a matter of some importance. A number of operational characteristics depend on the presence of carbon dioxide as produced via dissolution in produced water. It has been shown previously1 that silica production is strongly dependent on dissolved carbon dioxide. There is likewise evidence that aggressive degassing of produced water has caused unwanted scales of silicate minerals, such as talc, chrysotile and tremolite, to be deposited in various parts of the facilities2.

A simple production diagram for hydrogen sulphide in SAGD was recently reported by Thimm3. The hydrogen sulphide produced per unit bitumen could be described as a simple Arrhenius-type relationship with steam zone temperature, using an activation energy of 56220–65920 kJ/mole (13450 to 15770 kcal/mole). The assumption of pseudo-zero order kinetics yielded a smooth curve which fitted the field data for most SAGD projects in Athabasca satisfactorily.

The behaviour for carbon dioxide was previously expected to be similar, with an Arrhenius activation energy of 17520 kJ/mole (4192 kcal/mole)4.

Field Results

The results from field measurements of carbon dioxide generation in Athabasca SAGD projects show a very different situation from that which might have been expected from the Strausz value of a single activation energy. Figure 1 shows an upper curve, which, owing to the low activation energy reported by Strausz from laboratory measurements, is almost straight. This curve was the line expected from use of the data by Strausz. The curve actually obtained in field measurements between 180 and 215 degree steam zone temperatures is the lower curve, with field data shown in bold triangles.

Surprisingly, the true production curve appears to go through a distinct minimum at approximately 195 degrees Celsius. As with the corresponding hydrogen sulphide curve in Figure 2, a first estimate of anticipated carbon dioxide production per m3 bitumen may be read off the graph. Since the true production curve seems to rejoin the Strausz curve (upper line), production values above 215 ° C may probably be read off the extrapolation of the Strausz curve. However, the reader is cautioned that few field data in this higher temperature range have been published.

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