Analysis of Steamflood Oil Recovery in a Light-Oil Naturally-Fractured Reservoir
Authors: Chenboonthai N. , Hazlett W.G. 
Published: December 2007

The purpose of this paper is to present an analysis of a steamflood in the Teapot Dome Field, Wyoming. This field has two features that are uncommon in steamflooding. First, the oil is significantly lighter than the oil usually steamflooded. Second, the reservoir is naturally fractured. In order to evaluate these features, black oil (set up as a "dead oil") and compositional thermal models were built. The two models were compared to field data to determine which thermal simulation model best represents the reservoir behavior. Natural fractures were then added to both models. From the results of our study, the black oil model cannot adequately characterize the hydrocarbon phase behavior. We conclude that a fully compositional thermal model that includes non-interconnected fractures is required to model the phase behavior, heat transfer and effect of fractures in this light-oil naturally-fractured reservoir.

Introduction

Typically, steamfloods are performed in heavy oil reservoirs with high viscosities. Computer simulation models that have been used in the past assumed these oils were "dead" and ignored the low hydrocarbon content in the vapor phase because the amount of hydrocarbon vapor is so small. The models consist of liquid hydrocarbon, water, and steam vapor. The petroleum industry, however, is currently injecting steam or hot-water into light-oil reservoirs where there is a significant hydrocarbon component that moves from liquid to vapor phase as the reservoir is heated. The light-oil reservoir can generate a large amount of hydrocarbon vapor. As a result, the reservoir behavior will be influenced significantly. If these light-oil reservoirs are modeled as "dead" oil, production forecasts and reservoir management decisions will be inaccurate.

The Teapot Dome field, the Department of Energy's Naval petroleum Reserve NO. 3 (NPR 3) is a light-oil reservoir where steamflooding is occuring in the Shannon Sandstone. Chappelle et al. screened and evaluated this field and found the Shannon Sandstone formation was a good candidate for enhanced oil recovery (EOR). They described pilot testing and a geologic characterization study which showed that EOR could be successful. The EOR techniques that could be applied in the Teapot Dome field, such as waterflooding polymer-improved waterflooding or fire flooding, do not satisfy economic constraints. A steamflood project, however, that was first proposed for heating the reservoir prior to the in-situ combustion pilot, improved oil recovery unexpectedly. In 1989, Lawrence-Allison & Associates West, Inc. estimated that steamflooding portions of the Shannon could add 12.6 million barrels of otherwise unrecoverable oil.

As a result, a steamflood was predicted to be the most suitable EOR technique for the Teapot Dome field. Nevertheless, it is difficult to predict performance and manage the reservoir at the present time. When injecting steam to the formation, the properties of the hydrocarbons change and some of the hydrocarbons which are in the liquid phase move to the gasous phase. Consequently, the reservoir oil exhibits a more pronounced decrease in API gravity with time.

In order to optimize the thermal operation, black oil and compositional thermal simulation models were first built. The fully compositional steamflood model was developed to account for the effect of changing phase when some components of the hydrocarbons are heated. The two models were compared to determine which thermal simulation model best represents the reservoir behavior. The simulation models do not include the natural fractures as part of the reservoir. These fractures are believed to be a major producing mechanism. The second phase of the study included the natural fractures in both models. The results are compared to field data to determine which kind of model gives the more realistic results.

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