CHAPTER 1 INTRODUCTION TO ENHANCED OIL RECOVERY: THE PROCESSES AND THE RESULTS
| Jurisdiction | United States |
(May 2015)
INTRODUCTION TO ENHANCED OIL RECOVERY: THE PROCESSES AND THE RESULTS
Professor
Department of Petroleum and Geosystems Engineering
University of Texas at Austin
Austin, Texas
[Page 1-1]
LARRY W. LAKE is a professor in the Department of Petroleum and Geosystems Engineering at The University of Texas at Austin. He holds B.S.E and Ph.D. degrees in Chemical Engineering from Arizona State University and Rice University, respectively. Dr. Lake is the author or coauthor of more than 100 technical papers and four textbooks, and the editor of three bound volumes. He has served on the Board of Directors for the Society of Petroleum Engineers (SPE), won the 1996 Anthony F. Lucas Gold Medal of the AIME and the 2002 Degoyer Distinguished Service Award, and has been a member of the National Academy of Engineers since 1997.
Enhanced oil recovery (EOR) is oil recovery by the injection of materials not originally present in petroleum reservoirs. This definition covers all modes of oil-recovery processes (drive, push-pull, and well treatments) and most oil-recovery agents. Materials can mean chemical of various sorts, heat (usually through steam injection) and through various electromagnetic and seismic sources. Aspects of these technologies also apply to carbon dioxide (CO2) storage. Implied in the definition--the word reservoir--is that the processes operate on entire reservoirs (or large portions thereof) not just around wells. Finally, the definition applies to oil recovery; although enhanced gas recovery is entirely possible in the future, such does not occur today.
The definition does not restrict EOR to a particular phase (primary, secondary, or tertiary) in the producing life of a reservoir. Primary recovery is oil recovery by natural drive mechanisms: solution gas, water influx, gas-cap drive, or gravity drainage. As illustrated in Fig. 1, it is the first production phase in the life of a reservoir. Secondary recovery refers to techniques, such as gas or more commonly water injection, that have the main purpose of boosting or at least maintaining reservoir pressure. Tertiary recovery is any technique (usually based on chemical alterations of reservoir fluid properties) applied after secondary recovery. Nearly all EOR processes have been at least field tested as secondary displacements. Many thermal methods are commercial in both primary and secondary modes. Most practice has been as tertiary EOR, but the definition imposes no such restriction.
A definition related to EOR is improved oil recovery, which is EOR plus several technologies that are also intended to increase ultimate oil recovery. Examples of these are hydraulic fracturing, horizontal and multilateral wells, infill drilling, well stimulation, and optimizing the production or injection rates of individual injection wells.
Definitions of EOR can be important in areas where regulatory agencies give tax or price credits to promote use of EOR. The definition given above is the one used throughout this paper. The definition does exclude waterflooding and is intended to exclude all processes based solely on pressure maintence.
[Page 1-2]
Solely is because the distinction between pressure maintenance and displacement is fuzzy because displacement occurs in most pressure-maintenance processes. Moreover, agents such as methane in a high-pressure gas drive, or carbon dioxide in a reservoir with substantial native CO2, do not satisfy the definition, yet both are clearly EOR processes. The same can be said of CO2 storage. Usually, the EOR cases that fall outside the definition can be clearly classified by the intent of the process.
Fig. 1--Oil-recovery classifications (adapted from the Oil and Gas Journal).
Figure 1 also shows at the bottom the major categories of EOR. This paper is largely organized around the solvent, chemical, and thermal categories. Although EOR does not include waterflooding, this technology is the mother of all displacement techniques. So-called unconventional recovery, or oil and gas recovery from very low-permeability media through fracturing, is part of primary recovery.
[Page 1-3]
Fig. 2--Schematic of production phases.
Another perspective on the recovery phases is provided in Fig. 2. This figure shows oil rate (in volumes produced/time) on the upper plot, pressure (well pressure Pwf, average reservoir pressure P, and injection well pressure PInj) on the middle plot, and average oil saturation on the bottom plot, all on a common time axis. The oil saturation is the oil content in the reservoir; the remainder of the pore space is usually water. The figure is a schematic that treats injection and production as occurring through a single well (most fields have many wells). The time axis is divided into primary, secondary, and tertiary recovery phases as indicated.
Much depends on the economically limiting rate (labeled EL) and the limiting pressure (PLim) on the upper and middle plots respectively. EL is the rate at which the revenue from oil production equals the operating costs of the field. PLim is the well pressure below which fluids cannot flow to the surface without external support. Of the two, EL is the most important because it is determines project live and the ultimate recovery. Additionally, it is the portal between the engineering and economic worlds.
[Page 1-4]
Primary production is production by fluid expansion and pore-volume contraction caused by pressure decline. In this period, there is no injection and the average oil saturation remains constant or nearly so. The rate is characterized by a rapid increase, limited mainly by the rate at which wells are drilled, followed by a constant rate or plateau period and then a decline. The plateau period ends when the well pressure falls to PLim. The average pressure falls throughout, and primary recovery ends when the drawdown pressure (P-PLim) is insufficient to maintain the oil rate above the EL. Often there is little or no water production during primary production. The absence of water production and the prevalence of natural (unassisted) flow usually make this the most profitable phase of the field life.
Secondary production is production by injection of a second fluid, occasionally natural gas, but most commonly water. In this period and throughout the remainder of the field life, the producing well pressure is at PLim (wells are said to be pumped off), and the average pressure rises. There is a new pressure PInj for the injection well. Of course we must have PInj 〉 P 〉 Pwf = PLim to maintain flow. When P becomes constant, the pressure is said to be "maintained." Production rises because of the increased drawdown.
The increased drawdown, however, causes the injected fluid to be produced, as indicated by the dotted line in the upper plot. An increase in P would cause fluids to contract; therefore, production would proceed from this point mainly by displacement. The displacement causes the average oil saturation to decline. Secondary production ends when the rate again approaches the EL, at which point water production rate can be many times the oil production rate.
Tertiary production commences when waterflooding becomes uneconomical or when the rate to below the EL. At this time, chemical agents (surfactants or solvents) or heat are injected that cause physical or chemical changes in the oil. The entire nature of the recovery is based on displacement now, and the pressures change little with time. All the production occurs because of declining average oil saturation.
Several points deserve to be noted in Fig. 2.
1. Production can be increased by lowering PLim throughout the life of the field. Much oilfield technology is directed to exactly this goal.
2. In the same fashion, production can be increased by increasing the proportionality constant between drawdown and rate, the so-called productivity index (PI) of a producer. Much oilfield technology, including some forms of EOR, is devoted to this.
[Page 1-5]
3. EL need not be constant with time. Indeed, it is likely that EL will increase during tertiary recovery when agent costs become a principal factor in production. EL is exceptionally sensitive to oil price during all phases.
4. The lengths of the periods in the figure are highly variable; usually, primary production occurs over a shorter time than secondary production. The lengths for primary and...
To continue reading
Request your trial