Impact of well configuration on performance of steam-based gravity drainage processes in naturally fractured reservoirs

Abstract

In spite of the immense resources of heavy oil and bitumen laid in naturally fractured reservoirs (NFR), the study of steam-assisted gravity drainage (SAGD) in this type of reservoirs is very limited and there is no study examining the impact of well configuration on the process performance. This numerical study investigates various steam-based gravity drainage process well patterns in NFR including conventional SAGD, off-set SAGD, staggered SAGD and vertical-injection SAGD. Furthermore, to obtain the best well location, sensitivity analysis of well spacing was carried out for each well pattern and the impacts of oil viscosity, preheating and fracture permeability and water saturation were assessed as well. The best well location case for staggered SAGD represented the ultimate oil recovery factor of 59 % among all the cases with the SOR of 4.1, while the best off-set SAGD case showed a low SOR of 3.7 with a moderate recovery factor of 57 %. With the exception of off-set well configuration, all well patterns illustrated their most efficient performance while having 8 m vertical well spacing. In cases of off-set and staggered patterns, horizontal well spacing is a crucial factor for oil production initiation. Original fracture water saturation is the key parameter to establish early communication between the wells and gives rise to fast steam chamber expansion as well. We found that preheating is a necessary part of the process in the off-set pattern due to horizontal well spacing. However, preheating operation does not play a significant role in processes with no horizontal well spacing even in high vertical well intervals and high oil viscosities, making SAGD process in NFR different from that in sandstones. We attributed these behaviors to zero irreducible water saturation in fracture which makes initial water saturation mobile. At the end, the term (Formula presented.) (where Swf: fracture water saturation, Kf: fracture permeability, Lhw: Horizontal well spacing and µo: oil viscosity) was introduced as the governing index of the oil production threshold.