Abstract
To extract viscous bitumen from oil sands reservoirs, steam is injected into the formation to lower the bitumen’s viscosity enabling sufficient mobility for its production to the surface. Steam-assisted gravity drainage (SAGD) is the preferred process for Athabasca oil sands reservoirs but its performance suffers in heterogeneous reservoirs leading to an elevated steam-to-oil ratio (SOR) above that which would be observed in a clean oil sands reservoir. This implies that the SOR could be used as a signature to understand the nature of heterogeneities or other features in reservoirs. In the research reported here, the use of the SOR as a signal to provide information on the heterogeneity of the reservoir is explored. The analysis conducted on prototypical reservoirs reveals that the instantaneous SOR (iSOR) can be used to identify reservoir features. The results show that the iSOR profile exhibits specific signatures that can be used to identify when the steam chamber reaches the top of the formation, a lean zone, a top gas zone, and shale layers.
Highlights
The instantaneous steam-to-oil ratio (iSOR) profile is relatively rough with an early stage plateau eventually reaching a minimum value where there is a distinct change of the slope after which, the iSOR climbs through time
The minimum value of the iSOR profile at Time C corresponds to the point of time where the ‘thermal momentum’ of the steam chamber has been established where the shape is wider, the steam rises up the middle of the chamber, and the condensate largely drains down the sides of the chamber
The results show that the iSOR reflects the presence of a thief zone and flow barrier
Summary
Examined the use of the iSOR as a setpoint variable in a feedback control strategy to reduce the overall SOR of a pad of SAGD well pairs They showed that using the iSOR in the control strategy could lower the cumulative SOR (cSOR) by up to 16% (the cSOR is the ratio of the cumulative steam injected up to a point of time and the cumulative oil produced up to that point of time). Despite the different time scales for steam injection and bitumen mobilization, drainage, and production, the iSOR will be used here because it is an established measure of process performance that SAGD operators monitor during the process. It has not been used before as a signal for decoding how the reservoir is responding to the injected steam
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