The general concept of the SAGD process is that a steam chamber first expands to the top of the reservoir and thereafter continues to grow by lateral expansion. Consistent with this concept is the expectation that the steam oil ratio (SOR) rises as the SAGD operation matures, driven by expanding contact area and heat losses between the steam chamber and the overburden.

However, analysis of a number of successful SAGD projects, with operating histories of 10 years or more indicates that there is clear evidence that observed performance can deviate significantly from the performance predicted by the above concept. One particularly interesting set of observations shows a declining or unchanged SOR at the mature stage. An examination of this SOR behavior and several other unexpected SAGD performance characteristics detected in the field are presented.

It is proposed that the above field performance characteristics are consistent with the interpretation that the steam chamber does not rise to the top of the reservoir prior to expanding laterally. In these cases oil production is achieved by two different mechanisms: one is expansion of the steam chamber and the other is drainage of oil from the layer above the steam chamber. A simulation method and the results of a number of history matching studies are presented to explain the oil and gas flow in the layer above the steam chamber and its contribution to observed SAGD performance.

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