SAGD Operations in the Presence of Overlying Gas Cap and Water Layer-Effect of Shale Layers
Authors: Pooladi-Darvish M. , Mattar L. 
Published: March 2010

Abstract
The technical and commercial success of SAGD projects over the past decade has opened the door to the development of a large number of bitumen reservoirs in Canada, previously thought uneconomical to produce. Some of these reservoirs have overlying gas caps and/or water zones. Some studies have suggested that gas-cap production might "sterilize" the underlying bitumen. Many such studies however, assumed rather thick continuous pays with high permeability, and considered an infinite gas-cap.
In this work, a simulation study was conducted to examine the feasibility of bitumen production from a certain project area in Alberta, using the SAGD process, and to study the effect of production from the gas cap. A decision needed to be made as to whether gas production should be delayed until after bitumen production. The large well spacing did not allow a detailed description of the connectivity of the shale layers. The uncertainty was compounded by the geological setting of the study area, a system of channel sands cut through the original marine sand and shale deposits. Since the actual shale connectivity and thickness was unknown, a methodology was developed to incorporate different geological descriptions using the available core and log data. Five reservoir models were developed. Bitumen recovery, average oil production rate, and cumulative steam-oil ratio (SOR) obtained from thermal simulation were the three main parameters used for evaluation of the attractiveness of bitumen recovery operations. These numbers were compared with some of the corresponding values reported and/or forecast for economically feasible operations such as the UTF and Christina Lake projects. The effect of pressure reduction (caused by gas-cap production) on production rate and SOR was also investigated.
The results indicated that for conditions considered in this study the effect of gas production on bitumen recovery was minor, and appeared as a small deceleration of the recovery and a small increase in SOR.

Introduction
Many bituminous reservoirs in Alberta contain overlying gas. The gas owners would like to produce the gas. However, there is concern as to whether or not gas production might adversely affect any possible bitumen recovery process in the future. The primary candidate recovery method for such bitumen formations is the Steam-Assisted Gravity Drainage (SAGD)(1) process. The objective of this study is to examine the feasibility of bitumen recovery from the underlying sands, and to determine the effect of "prior" gas production on the process.
The effects of overlying gas and/or water sands on the SAGD process are presented first. This is followed by a discussion of the effect of continuous and discontinuous shale layers, and their incorporation in the numerical model, along with the determination of rock properties. Thermal simulation results and conclusions follow.

Background
The UTF project demonstrated the viability of the SAGD process for production of some of the bituminous reservoirs of Alberta(2, 3), where more conventional thermal processes are less successful due to immobility of the bitumen at reservoir conditions.