Abstract

This study addresses the important role of initial gasto-oil ratio (GOR) in steam assisted gravity drainage (SAGD) operations. A numerical model using CMG's STARS was validated through history matching of laboratory experiments conducted at the Alberta Research Council. The impacts of initial GOR on process performance were then studied using field scale numerical simulations. The results indicate that high initial GOR may have beneficial effects, namely, reduction of oil viscosity, and improvement of the oil-tosteam ratio (OSR). A detrimental impact, however, is also shown as the gas impedes the rate of steam chamber growth, and hence reduces oil production rates. Further analysis indicated that this is because of a "dynamic vacuum" effect due to steam condensation at the front of the steam chamber. This dynamic vacuum effect dominates the diffusion process and creates a gas-rich zone at the front of the steam chamber, thereby resisting further growth of the steam chamber and slowing oil production. The same effect occurs when noncondensable gas was co-injected with steam in either live oil or dead oil reservoirs.

Introduction

It is generally assumed that when the steam assisted gravity drainage (SAGD) process is applied to a heavy oil reservoir with a high initial gas-to-oil ratio (GOR), there is better production than for a reservoir with low initial GOR. This is a fairly common assumption for good reasons; the mobility of live oil in porous media is usually higher [1] than that of dead oil. The present numerical study was initiated to investigate this assumption.

The approach involves history match of 1-D laboratory tests and field scale simulations. The history match was used to verify the numerical techniques and to provide reliable parameters for the use of field scale simulations. The results support the commonly believed assumption of viscosity reduction in 1-D live/dead oil experiments. In field scale simulations, production predictions were compared for reservoirs with different initial GOR. There were additional effects beyond viscosity reduction. These include the long-term reduction of production and improvement in the oil-to-steam ratio (OSR).

In this paper, we summarize these activities, analyze the results and discuss possible mechanisms.

1-D EXPERIMENTS

A cylindrical model (inside diameter of 9.05 cm and height of 169.1 cm) was used for the experiments. A dynamic boundary control system allowed the surface temperature of the cylinder to be matched with that of the centre, to minimize radial heat loss. The model was packed with sand of 1.5 Darcy at porosity of 36.8%. After packing, the sand was saturated with water, which was then displaced with either "dead" oil or "live" oil. The live oil contained an initial GOR of 7.2 (m3/m3). Steam was controlled by an injection pressure set-point, which delivered slightly superheated steam to the model at about 2100 kPa. Measurements of oil drainage rates and temperature profiles ahead of the steam chamber were made for the two experiments performed.

HISTORY MATCH

A radial grid in a 2-D configuration was used to simulate the experiments, as shown in Figure 1.

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