Abstract

To assess the essential features of the hot water and steamdrive processes in heavy oil formations, which are dominated by bypass zones, laboratory and numerical simulations were performed and compared. It is shown that for viscous extra-heavy oil Reservoirs, both final recovery and maximum obtainable oil/steam ratios become strong function of pressure drop. A simplified model summarizing numerous laboratory and computer simulated recovery processes is presented. The model can be used to predict the effect of interwell pressure drop on the recovery process for simplified reservoir situations and to assess the effect of depth, interwell distance, and various additives on recovery processes.

Final recovery in a hot-water-assisted additive process compares satisfactorily with the steamdrive process. However, lower oil/steam ratios observed for hot water processes than those observed for steamdrive result in the hot water processes being less economical than steam.

Introduction

Four large Canadian deposits of oil sands are located in the northern part of Alberta: Athabasca. Cold Lake, Peace River and Wabasca they cover an area of more than 7 million ha. Without considering the carbonate reservoirs, the Alberta Energy Resources and Conservation Board estimates that approximately 215 billion m3 of bitumen are contained within Alberta in these reservoirs; more than 200 billion m3 are too deep to be economically mined. New in situ recovery techniques aimed at producing bitumen from deeper formation at marketable prices are now under development.

Background

Cyclic steam stimulation (huff and puff) or the "one-well process" is employed by a large number of pilots and commercial projects in heavy and extra-heavy oil sands. The process consists of several competing mechanisms such as sand deformation, gravity drainage and gas/steam flashing, and does not require interwell communications. Temperature measurement in Athabasca Pattern II-(Texaco) and physical simulators (Figure 1) indicate that in cyclic stimulation. a large amount of heat is withdrawn from the reservoir during the drawdown stage. This is largely the result of sudden vaporization of formation water which was previously heated at above saturation pressure1,2. Development of the active recovery zone around the injection/production well depends on sand deformation and induced fractures3 and on the effectiveness of gravity drainage during the soaking and drawdown stages of each cycle. Unless fracture and sand microchannelling are effective4, the active recovery zone is limited to distances of 10 to 20 m from the well. Therefore, cyclic steam would require a well density In excess of 0.5/acre, and have a low overall thermal efficiency. This is why, for most of the Alberta's oil sands reservoirs, the one-well cyclical steam stimulation could only be considered as the first stage in a strategy aiming to develop interwell communications5.

Bypass Reservoir Flow

For more than five decades, the stability of the sequence steam-condensate-oil fronts was given major emphasis In research efforts. Recently, in recognition that frontal advancement is not a relevant basis for flow models for thermal recovery of heavy and extra-heavy oil reservoirs, the bypass flow model began to receive special attention.

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