Field-Scale Numerical Simulation of SAGD Process With Top-Water Thief Zone
Authors: Law David H.-S. , Nasr Tawfik N. , Good William K. 
Published: December 2007

Abstract
There is a major concern that the existence of thief zones such as top water and/or gas cap overlying the oil sand deposit has a detrimental effect on the oil recovery in the application of the steam-assisted gravity drainage (SAGD) process The objective of this numerical study is to investigate the SAGD performance in the Athabasca oil sands in the presence of a top water zone.
The reservoir model, STARS, developed by the Computer Modelling Group (CMG) Ltd. has been previously validated based on a 3-D SAGD laboratory experiment with top water that was conducted at the Alberta Research Council (ARC). It is believed that the numerical simulation captured the major mechanism of oil movement from the pay zone into the top water zone as observed in the experiment.
In the field-scale simulation, SAGD performance in the presence of confined and non-confined top water zones was investigated. The operating strategies under the conditions of non-depleted top water/non-depleted pay zones and depleted top water/non-depleted pay zones were considered. Numerical findings indicated that:there is detrimental effect of top water zone on SAGD performance,plugging of top water zone with oil was not observed in this study for a top water thickness of 8 meters, andoperating conditions that lead to higher pressure difference between the steam chamber and the top water, either by depletion of the top water zone pressure or a higher steam injection pressure, results in more detrimental effect on the SAGD performance.

Introduction
There is a major concern by Alberta oil producers that the production of natural gas in association with oil sands would lower reservoir pressure, reduce oil recovery and may be prohibit economic oil recovery. Alberta Department of Energy (ADOE) and Alberta Energy and Utilities Board (AEUB) initiated a series of field-scale numerical modeling studies1,2 to assess the potential applicability of the steam-assisted gravity drainage (SAGD) oil recovery process under a variety of reservoir conditions such as reservoir thickness, reservoir depth, initial pressure, oil saturation, and the presence of top water zones and gas caps. It was found that top water zones and gas caps are thief zones to SAGD process. These thief zones have a detrimental effect on SAGD recovery performance especially when the pressure in the thief zones is reduced below optimum SAGD operating pressures due to natural gas production. Movement of oil into the top water zones and gas caps is simulated to occur. The volume of this oil seems to be generally proportional to the amount of outflow from the pattern due to thickness of the top water zones/gas caps and the pressure difference between the steam chamber and the top thief zones. SAGD process costs depend on the amount of steam that flows into the top water zones and gas caps, from which no oil is produced.
A case in point is the Gulf Surmont oil sands lease. The lease has a gas cap and a mobile water zone overlying the pay zone. An observation well indicates that gas cap pressure at the pilot site fallen from 1,327 kPa to 858 kPa over 3 years due to production of the gas. It is estimated that the pressure may fall to less than 300 kPa when the gas wells will be abandoned. Based on the geology and pressure measurements, there is communication between the gas cap and the pay zone. This indicates that the gas cap may be a thief zone to the SAGD process at the site. It is also believed that the mobile top water zone may extend the area of influence of the pressure-depleted gas caps.