Despite successful experiences of Steam Assisted Gravity Drainage (SAGD) in sandstone and some studies of this process in fractured reservoirs, the impact of heterogeneity and its relative solutions have not been assessed yet. In this paper, SAGD in fractured reservoirs was studied and the impact of reservoir heterogeneity of fracture permeability, fracture spacing and matrix permeability and solutions to overcome negative impacts were investigated. In contrast to sandstone, it is found that due to the drainage of fracture initial mobile water, steam is injected into reservoir in large amount in early time resulting in fast rising of steam while the lateral expansion of steam is not considerable. Higher fracture permeability at top part of the reservoir showed lower oil recovery than the reversed condition. The impacts of matrix permeability and fracture spacing were combined and expressed in a term H m /K m (H m : matrix height, K m : matrix permeability). It was observed that higher value of this term for the top part of the reservoir deteriorates oil recovery more than the reversed condition. The negative impact of heterogeneity can be overcome by locating injector in more fracture permeable zone or the layer having lower quantity of the mentioned term.

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(where S wf : fracture water saturation, K f :fracture permeability, L hw : Horizontal well spacing and l o : oil viscosity) was introduced as the governing index of the oil production threshold.

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Low permeability, shale barriers and low thickness are the main issues making significant portion of the immense heavy oil and bitumen resource uneconomical to produce. Two main troublesome cases were investigated in this study to address by applying appropriate solutions in SAGD process; firstly reservoirs with shale barriers and low permeability and secondly thin reservoirs.In cases of low vertical permeability due to shale inclusion in the reservoirs, the effect of induced vertical fracture resulted in faster upward steam chamber expansion and increased oil recovery rate. Sensitivity analysis showed higher well spacing is beneficial to the process while applying the induced vertical fracture. In thin reservoirs, steam chamber reaches overburden faster and increases cumulative steam-oil ratio (cSOR), hence making recovery processes uneconomical. Appropriate placement of Induced Horizontal-Fractures (IHF) and off-set vertical wells with the later being in halfway of two adjacent horizontal well pairs in SAGD and acting as a steam injector was applied. The results showed such applications reduce cSOR.Found from the sensitivity analysis, Induced Horizontal-Fractures positioned in the injector and the producer place improves oil recovery in thin reservoirs. When applying the fracture just in producer location, the oil recovery result is superior to the former case. In fact, IHF provide a path facilitating the oil drainage to producer that leads to faster oil transportation. The Off-set vertical well in the thin reservoir sweeps a part of the reservoir located beyond the chamber edges of two adjacent well pairs, hence reducing recovery time and cSOR. Based on sensitivity analysis, the most promising result of the process is achieved when initiating steam injection in the vertical injector from the beginning of the process.

Conductive fractures as a reservoir heterogeneity can impact on steam assisted gravity drainage (SAGD) mechanism. Numerical simulation of Athabasca field scale SAGD with and without vertical, horizontal, and oriented fractures and sensitivity analyses of fracture properties is conducted. Contrary to previous studies that explain change of oil recovery of conductive fractured reservoirs due to change in steam chamber expansion, we explained it as result of change in steam chamber expansion and fluid transfer pattern. Furthermore, we found, depending on location of fractures; three types of fractures can enhance or reduce oil production rate, but do not have influence on ultimate oil recovery.