Since the late 1980's there has been significant primary development of thin heavy oil and bitumen pools with horizontal wells in Western Canada. These pools were previously not developable as they did not have high enough transmissivity for economic vertical well development. Horizontal wells permitted primary development within this resource base by providing an alternative production geometry from which low transmissivity reservoirs could be economically produced. As development proceeds in this resource it is apparent that the producing Gas Oil Ratio (GOR) characteristic of such wells is not typical of vertical wells in conventional oils. For saturated heavy oil and bitumen reservoirs producing on solution gas drive the producing GOR's tend to be much higher than the initial solution GOR (Rs;) for the heavy oil or bitumen produced. In the event the wellbore intersects primary or secondary gas caps, the producing GOR can be dramatically higher than the solution GOR (Rs). This paper derives an analytic approach that demonstrates the potential impact on producing GOR of a small gas saturation intersected by a horizontal well in a heavy oil or bitumen.

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