Gas-Over-Bitumen Geometry and Its SAGD Performance Analysis With Coupled Reservoir Geomechanical Simulation
Authors: Li P. , Chalaturnyk R.J. 
Published: March 2010

Abstract
In Northeastern Alberta, gas pools and oil sands reservoirs exist in which the energy content of the bitumen (or oil sands) is estimated to be 600 times greater than the gas for this region. The sealing layer between the top gas pool and the bitumen reservoir, if it exists, plays a critical role in the optimization of SAGD projects. Gas pool pressures may or may not be in communication with oil sands reservoir pressures. So, although different cases of gas-over-bitumen geometries exist, this paper is focused on the gas-over-bitumen geometries with a mudstone layer between the top gas/water and the oil sands reservoir. The top water zone is assumed to be a thief layer. The SAGD process applies higher thermal stress and induces higher pore pressures inside the mudstone layer. Thus, the mudstone becomes weaker or even fails as a result of shear failure or tensile failure. As a result, the permeability of the mudstone layer may be increased significantly and more heat loss to the gas sands and top thief water zone can result in a poor SAGD production performance. This paper discusses the SAGD production performance of the gas-over-bitumen geometries with varying mudstone permeability. The effect of steam injection pressure is also studied. The research was conducted based on both a conventional reservoir simulation and a coupled reservoir geomechanical simulation. Prior to the simulation study, the geomechanical properties of oil sands and mudstones are briefly discussed.

Introduction
It is reported that almost one-third of the area of Athabasca oil sand deposits have both oil sands reservoirs and gas pools (see Figure 1)(1). The associated gas is in pressure communication with bitumen within a region of influence either directly or through a connecting water zone. The non-associated gas is not in pressure communication with bitumen. As a result of an exhaustive geological study conducted of the region(2), the relationship between gas and bitumen has been divided into seven cases. This paper is focused on one of those cases: an oil sand reservoir with non-associated gas pools. A mudstone layer separates the gas pool from the underlying bitumen reservoir.
For the gas-over-bitumen SAGD geometries, the effect of gas pool depressurization on the SAGD production performance has been discussed since the last decade and it is still an area of active research(3, 4). The role played by geomechanics has been discussed in terms of the process of gas pool depressurization, re-pressurization and wellbore stability analysis(3). However, to our knowledge, a detailed assessment of the mudstone geomechanical behaviour was not taken into account in the prediction of SAGD production performance.
In a previous work, Li et al.(5) provided a coupled reservoir geomechanical simulation methodology which involves both the reservoir simulator, EXOTHERM, and the geomechanical simulator, FLAC. This paper applies that coupled reservoir geomechanical simulation technique to study the effect of mudstone permeability and steam injection pressure on the SAGD production performance. Prior to the section on the simulation, the geomechanical properties of oil sands, the geomechanical behaviour of the mudstone layer and the permeability variations associated with geomechanics are discussed.