Abstract

In the steam assisted gravity drainage (SAGD) process, the addition of small amounts of non-condensable gases to steam may improve oil recovery. The gas accumulates at the top of the reservoir where it provides an insulation effect and forces the steam chamber to spread laterally. The result is a more efficient use of steam and the potential for greater recovery of oil.

Six experiments were conducted in two different geometries to study the effect of non-condensable gas on the performance of SAGD. These experiments consisted of steam-only, steamcarbon dioxide and steam-n-butane injection. Three SAGD experiments were carried out in a scaled 3-D model packed with crushed limestone premixed with a 12.4 °API heavy crude. In these experiments, the steam-only case had the highest recovery, as expected. However, using carbon dioxide or nbutane with steam reduced the steam consumption. In both those cases, recovery was lower than the steam-only case.

The other SAGD experiments were carried out using limestone core plugs saturated with the same heavy oil. Similar trends were observed for core plug experiments; however, the recovery was better when n-butane was added to steam. The presence of n-butane had a positive effect on the oil recovery and required less steam consumption than the other two cases.

ifferences between the experiments were identified by means of analytical modeling. All the experiments were modeled with respect to Butler's SAGD theory and Reis' linear model. The results of the 3-D experiments were matched by all models, whereas the results of the core scaled experiments were better represented with Reis' linear model because of the heterogeneity present in the core plugs.

The addition of non-condensable gas to steam in a SAGD operation was evaluated using physical models of different geometries. The experimental results indicated that for both geometries, steam consumption was reduced by using either carbon dioxide or n-butane. More experimental studies are needed to asses the effect of non-condensable gas addition on increasing oil recovery.

Introduction

The basic mechanism of the SAGD process for heavy oils was initially proposed and demonstrated by Butler. It is based on simple physical concepts: rising steam heats the formation and hot liquids flow downward. Generally a large portion of the original oil in place can be produced by gravity drainage, resulting in low residual oil saturation. In this process, parallel horizontal wells are used for both injection and production due to the large contact area that they provide for the process. The steam is injected through the upper well, where it condenses on the cold sections and heats the oil. The viscosity of heavy oil decreases the oil becomes mobile and drains by gravity with the condensed steam to the lower production well1.

A major limitation of SAGD is the requirement for a large amount of steam, particularly in thin, low quality reservoirs. This means that high energy is required for production of continuous steam. Das and Butler proposed that this limitation could be avoided or decreased in two ways:

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