Performance of the SAGD Process in the Presence of a Water Sand- A Preliminary Investigation
Authors: Doan L.T. , Baird H. , Doan Q.T. , Ali S.M. Farouq 
Published: March 2010

Many heavy oil and oil sand reservoirs in Canada are in communication with a water sand, and in some cases, with a gas cap. Conventional thermal recovery methods using vertical wells in these thin reservoirs (5 - 10 m of pay) have been unsuccessful. The use of optimally placed horizontal wells has proved, in a few cases, to be economically feasible, due to a reduction of the heat loss to the cap rock and bottom water layer, as well as better overall heat distribution in the reservoir.

Steamflooding heavy oil/oil sand reservoirs with a contiguous water sand is risky due to the possibility of short circuiting the steam chamber. The success of the Steam-Assisted Gravity Drainage (SAGD) process at the Underground Test Facility (UTF) in Fort McMurray, Alberta has, in many ways, lent its application to these reservoirs. The success of subsequent projects depends largely on the efficient utilization of heat injected into the reservoir, among several aspects related to reservoir heterogeneities and operational strategy. This work studied how the SAGD process is affected by the presence of a water sand, and determined how heat is distributed in these reservoirs. Reservoir heterogeneities, wellbore hydraulics, and geomechanical impact were not considered in this study.

The results of this study showed a relationship between ultimate recovery, heat accumulated in the reservoir, and the thickness of the water sand (bottom or top water). For the base case run, an average oil rate of 80 m3/d was maintained for 1,400 days before it started to decline. Ultimate recovery was approximately 70% of the OOIP after nine years of steam injection, and the cumulative OSR was 0.3 m3/m3 (CWE). The presence of a bottom water sand had a lesser impact on recovery than when an overlying water sand was present. Recovery efficiency decreased with increasing water sand thickness. In the case of overlying water sand, larger areal coverage of the overlying water sand severely reduced the recovery efficiency of the process, as heat was diverted (or channeled) into the "thief' zone. When a bottom water layer was present, the BHFP (bottom hole flowing pressure) of the horizontal producer could be operated at or above the pressure of the aquifer to prevent water coning and hence only affected the heat source slightly.

Introduction

Successful application of the SAGD Process at the Underground Test Facility (UTF) demonstrated it to be a commercially viable recovery method to exploit heavy oil and oil sand reservoirs(1–4). Some of these studies examined the mechanics of the SAGD process, while others discussed the implementation of this process (including operational constraints) at UTF. In this paper, an examination of SAGD application to an oil reservoir in communication with water sand, as well as the effects of confined and unconfined water-bearing zones, was evaluated. Heat balance calculations were also performed to establish relationships between heat accumulated in the reservoir to the thickness of the water sand and the recovery efficiency.