Abstract

In-situ combustion (ISC) has long been viewed as a low cost means to input thermal and driving energies into petroleum formations, and could be used to enhance oil recovery from both light and heavy oil reservoirs. However, field trials of ISC processes, in the past half a century, showed mixed results: a few successful and the majority disappointing. This indicates that ISC has not been properly understood and optimized for field applications.

This paper provides a general discussion of the limitations of using ISC in flooding/displacement processes for heavy oil reservoirs, and the means to circumvent the limitations in alternative gravitystabilized processes. Firstly, it outlines two generic limiting factors in ISC flooding processes; i.e., (1) gas override/front instability caused early gas (or oxygen) breakthrough and low sweeping efficiency, and (2) oil blocking caused high fuel deposition and combustion front stalling. Secondly, it discusses the advantages of ISC and gravity drainage combined processes (such as, COSH, THAI, and top-down ISC). And, thirdly, it describes the potential of utilizing low temperature oxidation in ISC/gravity drainage processes.

Introduction

The idea of utilizing in-situ combustion (ISC) in heavy oil reservoirs as an enhanced oil recovery process has been around for more than half a century. The potential benefits of in-situ combustion are the low cost of delivering thermal energy in the reservoir to increase reservoir temperature, improve oil mobility, and achieve high recovery.(1) Because of these potentials, it has attracted broad attention of the oil industry. In-situ combustion processes can be viewed as alternative processes to steam processes. Therefore, for every major steam processes, there exist similar or corresponding combustion processes. Some of them are listed below:

Table of Steam Processes vs. ISC Processes (Available in full paper)

In steam processes, the focus is on the maximization of heat delivery efficiency of steam into the reservoir and the minimization of heat loss to under and/or over burdens and non-producing areas, while the focus of ISC is to deliver oxygen into reservoir and generate sufficient heat in the formation. Generally speaking, the ISC processes are more complicated than the steam processes, due to the involvement of heat generation chemical reactions in the formation.

Extensive laboratory studies and field pilots have been pursued.(2,3) From these focused studies and trials, it has been revealed that the ISC processes are much more complicated than initially envisioned, and a successful field project requires in-depth understandings of the process physics and proper design of every aspects of the project.(4)

The present paper will discuss the in-situ combustion processes from the reservoir engineering point of view. After a brief summary of the oil oxidation characteristics, it will firstly focus on the limitation factors for combustion flooding processes; secondly on the potentials of gravity stable combustion processes; and thirdly on the potentials of low temperature combustion reactions for gravity stable combustion processes.

OIL OXIDATION CHARACTERISTICS

Various laboratory studies(4,14) have revealed that there are generally two predominant exothermic chemical reaction modes in ISC: low temperature oxidation (LTO) at relative low temperature, and high temperature oxidation (HTO) at a higher temperature, as shown in Figure 1 for Athabasca bitumen (8 ° API).(4)

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