Two characteristics of XSAGD that accelerate bitumen recovery and improve thermal efficiency are discussed in this simulation study.
First, it is well understood that the significant oil mobilization process during SAGD occurs at the periphery of the steam chamber where steam transfers heat to the reservoir rock and bitumen. However, once the SAGD steam chamber is well established, it tends to have a low surface area to volume ratio due to its generally cylindrical geometry. In contrast, XSAGD tends to have a higher ratio of surface area to volume once its multiple steam chambers are well established. This allows a given amount of heat injected as steam in XSAGD to contact bitumen faster than the same amount of heat injected in SAGD after the initial steam chamber formation period.
Second, fluids moving through the parallel horizontal wells in SAGD follow pathways that remain relatively stable in temperature throughout the life of the operation. Conversely, fluid flow through the perpendicular arrangement of wells in XSAGD exposes cooler portions of the reservoir to conduction heat transfer from hot steam flowing in the injectors or heated bitumen and steam condensate flowing in the producers. This heat transfer accelerates heating in the reservoir and reduces the heat that is produced back to the surface so that more of the injected heat is beneficially applied to the reservoir compared to SAGD. The heated areas close to the wells accelerate the development of lateral displacement pathways promoting more rapid spreading of the steam chambers in XSAGD.
The increased thermal efficiency and acceleration of recovery of XSAGD are more pronounced for thinner pay and for lower pressure operation compared to SAGD. However, XSAGD retains some economic advantage even as pay thickens and injection pressure increases.