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Abstract

Numerical simulation that provides history match of a SAGD pilot in a Cold Lake type reservoir may be expanded to include steaming of an offset well. In this theoretical configuration, the offset well is equidepth and parallel to, but 50 metres away, from the SAGD producer. After three years of SAGD operation, steam stimulation of the offset well with concomitant increased steaming of the SAGD injector provides early steam communication between the SAGD steam chest and the offset well. The result is markedly increased rate of production of bitumen and reduced steam-oil ratio. Mindful of such results, laboratory work is being undertaken to verify, and understand these results. We also encourage field-pilot testing of this process.

Introduction

Butler¹ initially proposed and demonstrated the basic mechanism of the steam-assisted gravity drainage (SAGD) process for heavy oils. The process was subsequently reduced to practice in the field² at the Underground Test Facility (UTF) in the Athabasca Oil Sands in northern Alberta in 1987. Since then, there have been many other demonstration projects of the SAGD process, in all of the three Alberta oil sands deposits and in the heavy oil deposits of Saskatchewan. Recently, one company has announced that they were taking the SAGD process to a commercial stage.

The steam-assisted gravity drainage process is effective as it is based on simple physical concepts: heat rises and liquids flow downward. The process is made possible through the use of horizontal wells, drilled either from underground tunnels as in the initial UTF wells or more commonly now from the surface. A pair of horizontal wells, as shown in Fig. 1, is placed near the bottom of the oil sands pay zone, about 5 m apart. The producer is at the bottom, as close as possible to the base of the pay zone, with the injector on top. Steam is injected through the upper well, where it condenses on the cold oil sands to heat the bitumen. The mobilized bitumen and condensed steam (hot water) drain by gravity to the lower production well.

The method is very effective because the injected steam continuously rises to cold sections of the oil sands to heat the bitumen. The heated bitumen drains by gravity to the lower production well with a minimum distance to flow in the constantly heated reservoir. Production is controlled at a temperature near the steam temperature (steam trap control mechanism). This improves the utilization of steam in the reservoir, as none is blown off into the production well. Polikar and Redford³ indicated that SAGD was a very gentle process with low pressure drops, no cycling of pressure and low flow rates through the sand surrounding any section of the horizontal production well.

The new process presented in this paper is called Fast-SAGD. It incorporates the main features of the SAGD process, and adds some features of cyclic steam stimulation to improve the overall process. Numerical simulations were conducted, for a Cold Lake type reservoir, to test this new concept. The results of these simulations are presented in this paper.

Background

Cyclic steam stimulation (CSS) has been successfully used in Venezuela and California heavy oil deposits for some forty years. The process relies on stimulating drive mechanisms existing in the reservoir rather than imposing an artificial drive mechanism. The thermal energy imparted to the reservoir heats the reservoir rock around the wellbore, creating a high mobility zone through which reservoir fluids can flow back to the well. CSS gives a rapid payback on invested capital since production begins soon. However, ultimate recoveries are low, and this process is preferably followed by another process such as steam drive or SAGD.