Reservoir Engineering Analysis of Microbial Enhanced Oil Recovery
- Steven L. Bryant (U. of Texas at Austin) | Thomas P. Lockhart (EniTecnologie)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- October 2002
- Document Type
- Journal Paper
- 365 - 374
- 2002. Society of Petroleum Engineers
- 5.4.1 Waterflooding, 5.1 Reservoir Characterisation, 4.3.4 Scale, 4.1.2 Separation and Treating, 5.3.2 Multiphase Flow, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.3.4 Reduction of Residual Oil Saturation, 5.8.7 Carbonate Reservoir, 3 Production and Well Operations, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.2.1 Phase Behavior and PVT Measurements, 5.3.3 Particle Transportation, 5.4.2 Gas Injection Methods, 1.8 Formation Damage, 5.4.10 Microbial Methods
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We assess processes for enhancing oil recovery by means of microbes (MEOR) from the perspectives of reservoir and reaction engineering. In this work, MEOR refers to recovering incremental oil by increasing displacement and/or volumetric efficiency; it does not include well stimulation treatments. MEOR processes address the same physical parameters as chemical enhanced oil recovery (EOR) processes; hence, they are subject to the same technical difficulties: identifying and locating the target oil and retention, dissipation, and consumption of chemicals in the formation. The essential difference between MEOR and chemical EOR resides in the method of introducing the recovery-enhancing chemicals into the reservoir.
Although quantitative measures of microbial performance (reaction rates, stoichiometry, required product concentrations) are lacking, it is possible to demonstrate quantitative relationships between microbial performance, reservoir characteristics, and operating conditions (well spacing, injection rates, residual oil saturation). These relationships are the focus of this paper. From them, 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 risk is associated with the many and severe performance constraints that a microbial system would have to satisfy 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.
Every oil reservoir, whether mature, recent, or yet to be discovered, is a candidate for EOR because significant quantities of oil remain in place 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-1980s 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 shutoff). Nevertheless, interest in developing MEOR methods has persisted, largely on account of their perceived potential to provide incremental oil production at low cost. To assess this potential, we have undertaken a study to examine the fundamental premises of MEOR processes.
In doing so, we have adopted a reservoir engineering perspective, focusing on issues such as scaleup of laboratory results, process design, and field implementation and operation. Many discussions of MEOR in the literature, in contrast, reflect 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 the success or failure of field applications of microbial technology is often hindered by the absence of specific, quantitative understanding of microbial activity.
This study focuses on enhanced recovery processes based on the induction or promotion of microbial activity that, in turn, generates appropriate chemicals within the reservoir. Enhanced recovery in the classical sense, and as used in this work, means reducing residual oil saturation and/or increasing volumetric (sweep) efficiency. 1 Many reported applications of MEOR address well productivity, rather than these factors, and should be classified as stimulation treatments. The distinction matters because the results of a process must be judged against its objectives. However, some of our analysis is relevant to both applications.
A wide range of microbial reaction products is commonly cited2 as being relevant to enhanced oil recovery (see Table 1). With the exception of biomass, all the microbial products correspond to families of chemicals already used or proposed for use in enhanced recovery processes. The effects of these chemicals correspond exactly to those cited in the literature of those processes. Moreover, the claimed effects of biomass also correspond to effects achievable by means of other chemicals. In short, current MEOR processes propose no fundamentally novel mechanism of oil recovery. Hence, MEOR differs from other EOR processes only in the manner in which the chemicals are introduced into the reservoir (i.e., in-situ generation) and, therefore, should be evaluated on the same basis as other EOR processes. It follows that any advantage of MEOR will arise only from being more efficient than another EOR process. In-situ generation presents a potential advantage in logistics, especially if residual oil can be used as an in-situ carbon source. This is likely to be the most important, and possibly the only, advantage over other processes. In-situ generation introduces a new set of technical difficulties beyond those facing other EOR processes. In subsequent sections of this paper, we define a base case for MEOR implementation and examine its scaleup and engineering aspects. The analysis treats both the case of reservoir inoculation with function-specific microbes and the activation of indigenous microbes through nutrient injection. In either case, the use of microbes introduces reaction engineering considerations into the analysis. Consideration of reservoir and reaction engineering parameters, which largely have been neglected in the past, leads to the identification of several important technical challenges for this process. It also provides a basis for reviewing the MEOR laboratory and field studies reported in the literature. The admittedly ambitious goal of the latter review is neither to promote nor to criticize any particular MEOR process, treatment, or experiment, but to foster a constructive, science-based debate on the past and future of this technology.
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