Numerical Simulation of ES-SAGD Process in Athabasca Oil Sands with Top Water and Gas Thief Zones
Authors: Bao Xia , Chen Zhangxin John , Deng Hui , Guo Ziqiang , Yu Song 
Published: March 2015

Abstract
Overlying top water and gas thief zones have a detrimental effect on the Steam-Assisted Gravity Drainage (SAGD) recovery process since steam penetrates into these zones which results in great heat loss. Due to the presence of these top thief zones, recovering bitumen by the SAGD process has become challenging for the Athabasca oil sands.
Numerical simulations, laboratory experiments and field production data have demonstrated that oil production tends to decrease as the depletion of top gas occurs; also, heat loss to the overlying thief zone will be more significant when a top water zone is present. Indeed, SAGD is a coupled geomechanical, thermal and fluid flow problem because continuous steam injection changes reservoir pore pressure and temperature, which can alter the effective stress in-situ. Therefore, to represent the physics of thermal flow and soil geomechanics, a coupled geomechanical simulation that solves the flow and stress equation simultaneously in the reservoir is crucial for modeling the SAGD process.
The objective of this paper is to construct a 3D geostatistical model for the Surmont pilot and implement coupled geomechanical modeling for the SAGD process aiming at investigating the impact of dilation and thermal expansion on the surface subsidence and bitumen recovery. Reasonable history match of oil and water rates has been achieved and steam chamber profiles have been conformed to the field data from the observation wells. An Expanding Solvent Steam-Assisted Gravity Drainage (ES-SAGD) process has been investigated on a full field-based heterogeneous simulation model using an optimal solvent mixture. Finally, geomechanical effects on the ES-SAGD process are investigated through an iterative coupling approach.

Introduction
The negative impacts of top water and gas cap on SAGD performance have been previously presented and discussed in the literature (Good et al. 1997; Nasr et al. 2000; Law et al. 2000). Both experimental and simulation approaches were conducted to determine SAGD steam chamber growth in the presence of a top thief zone. It was observed that the overlying top water and gas thief zones have a detrimental effect on the Steam Assisted Gravity Drainage (SAGD) recovery process since steam penetrates into these zones and results in great heat loss (AEUB, 1998). Thermal SAGD simulations models have been constructed and successfully matched using the computer assisted history-matching approach. The optimization strategies have been proposed in terms of the operating pressure and subcool control. Furthermore, ES-SAGD possibilities have been investigated using different solvent mixture co-injected with steam. Through a sensitivity study, an optimal solvent mixture has been proposed for this case with a top thief zone.
SAGD is a coupled geomechanical, thermal and fluid flow problem because continuous steam injection changes reservoir pore pressure and temperature, which can alter the effective stress in-situ. Dilation behavior associated with volumetric strains is triggered by the continuous steam injection, which causes the increase of porosity and permeability. Therefore, the fluid flow behavior must be coupled to the geomechanical behavior of the oil sands. Investigating the interaction between the cap rock integrity, dilation and thermal expansion under the continuous steam injection is the motivation of this study.

ES-SAGD Investigation
ES-SAGD is the co-injection of small amount solvent additive with steam in the SAGD process. Solvent will condensate at the boundary of the steam chamber and diffuse into bitumen, which will reduce the oil viscosity, yield the higher oil drainage and reduce the amount of steam required.