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ABSTRACT

We assess processes for enhancing oil recovery by means of microbes (MEOR) from the perspective of reservoir and reaction engineering. In this work, MEOR implies recovering incremental oil by means of increased displacement and/or volumetric efficiency; it does not include well stimulation treatments. MEOR processes address the same physical parameters as chemical EOR processes such as surfactant flooding, polymer flooding, alkaline flooding, etc. They are hence subject to the same technical difficulties: identifying and locating the target oil; retention, dissipation and consumption of chemicals in the formation, etc. The essential difference between MEOR and chemical EOR resides in the method of introducing the recovery-enhancing chemicals into the reservoir.

An examination of the literature shows that MEOR generally does not recover as much remaining oil in place as other chemical EOR processes, either in the laboratory or the field. Efforts to explain this difference are severely limited by the lack of quantitative measures of microbial performance (reaction rates, stoichiometry, required product concentrations, etc.) It is possible, however, to demonstrate quantitative relationships between microbial performance, reservoir characteristics, and operating conditions (well spacing, injection rates, residual oil saturation, etc.).

We conclude that MEOR is potentially a “high risk, high reward” process, depending on whether it can use residual oil as an in situ carbon source for the production of recovery-enhancing chemicals. The reward in this case is that the difficulty and the logistical costs of implementing the process would approach those of implementing a waterflood. The high risk is associated with the many and severe performance constraints that a microbial system would have to satisfy in order to take advantage of an in situ carbon source. The current state of knowledge fails to provide satisfactory evidence that existing systems can meet the constraints identified in this study; thus an ambitious program of research and development would be required to determine MEOR feasibility.

INTRODUCTION

Every oil reservoir, whether mature, recent or yet to be discovered, is a candidate for enhanced oil recovery (EOR). This is because reservoirs still contain significant quantities of oil after conventional primary and secondary recovery operations. Conversion of candidates into projects is a function of economic climate, available technology, and operator priorities.

The prevailing low oil price since the mid-1980's has led to a marked shift in focus from EOR to “improved oil recovery” processes based, for example, on well technology (completions, well stimulation, water shut-off, etc.). Nevertheless, interest in developing microbial enhanced oil recovery (MEOR) methods has persisted, largely on account of their perceived potential to provide incremental oil production at low cost. To better assess this potential, we have undertaken a study to examine the fundamental premises of MEOR processes and to assess the current state-of-the-art.

In doing so we have adopted a reservoir engineering perspective, focusing on issues such as scale-up of laboratory results, process design, and field implementation and operation. This approach provides a consistent framework for comparing MEOR with other EOR processes. Many of the discussions of MEOR in the literature have adopted, consciously or unconsciously, a microbial science perspective. An unfortunate consequence is that the applicability or relevance of microbial activity to reservoir engineering design is often obscure. Conversely, explaining success or failure of field applications of microbial technology is often hindered by the absence of specific, quantitative understanding of microbial activity.