Abstract

An economic model for thermal recovery has been developed. This paper evaluates the optimum recovery rates and production schedule under both steam drive and in-situ combustion processes, possible future production, and the economics of each method based on production, and the economics of each method based on today's prices.

The recovery model for steam injection is based on the work of Ramey and Raghavan which presented a computer program to determine surface steam line and wellbore heat losses, vertical heat losses to the adjacent formation, an oil recovery schedule, and associated steam injection calculations. The recovery model of in-situ combustion is based on the Gates and Ramey method of making engineering calculations of air-oil ratios and oil rates as a function of oil recovery.

Once recovery rates for steam drive and in-situ combustion are obtained, a cash flow analysis may be made. Capitalization and operating costs are considered and income is discounted to net present worth. Thus, an engineer may be able to use this method to help evaluate which processes would be more profitable for a particular reservoir.

Introduction

The commercial use of thermal oil recovery methods has been in existence for over 25 years. Steam injection and in-situ combustion are the two processes that dominate thermal recovery. Both of processes that dominate thermal recovery. Both of these methods introduce heat into the reservoir. However, the difference between producing the heat insitu, such as is done in in-situ combustion, and generating the heat at the surface and injecting it in the formation can greatly influence the recovery and economics of a thermal recovery project. This study compares the economics and productivity characteristics of continuous steam injection, which is strongly influenced by associated heat losses from the system, and in-situ combustion, which is primarily influenced by air requirements and the burned volume.

Heat transmission is the controlling mechanism in continuous steam injection. Hot water and steam are employed as a transport medium because of their high heat capacity and availability. Steam generators usually produce high quality steam, but require constant surveillance and maintenance to ensure efficient operation. The important considerations associated with steam generators are the type and availability of fuel, availability and treatment of feed-water, maintenance, and operating costs. Heat transmission to the reservoir and associated heat losses become important once the steam or hot water is generated. Heat is lost in the generating unit, in the surface lines, within the wellbore, and finally from the heated portion of the reservoir to the adjacent strata.

Recently, Gates concluded that the surface energy requirements per barrel of oil produced by continuous steam injection is approximately 80% greater than that for in-situ combustion. His calculations considered the typical heavy oil operations in California. The main reason for this conclusion is the fact that fuel must be used to generate a sufficient amount of heat to compensate for the heat losses from the surface lines and the wellbore, while the process heat of in-situ combustion originates from the residual oil-in-place. This implies that in-situ combustion is more energy conservative than continuous steam injection. Gates's observation also reopens the debate on the economic attractiveness of continuous steam injection vis-a-vis in-situ combustion in heavy oil recovery.

A re-examination of the economic evaluation techniques involving the thermal drive processes is pertinent. Previous technical and economic studies of pertinent. Previous technical and economic studies of steam injection and in-situ combustion were either valid for specific cases employing assumptions and restrictions that were particular to a given application or were erroneous. This study attempts to alleviate the confusion and illustrate that both processes should be considered. Only after a sufficient amount of laboratory and field test data has been accumulated and a detailed geological analysis made, should one of the thermal drive methods be chosen.

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