|Fig. 1: Developing miscibility by CO2 injection. |
Most of the current world oil production comes from mature fields. Increasing oil recovery from the aging resources is a major concern for oil companies and authorities. In addition, the rate of replacement of the produced reserves by new discoveries has been declining steadily in the last decade. Therefore, the increase of the recovery factors from mature fields under primary and secondary production will be critical to meet the growing energy demand in the coming years.  Nearly 2.0 × 1012 barrels of conventional oil and 5.0 × 1012 barrels of heavy oil remain in reservoirs worldwide after conventional recovery methods have been exhausted. Much of this oil would be recovered by Enhanced Oil Recovery (EOR) methods, which are part of the general scheme of Improved Oil Recovery (IOR).  The major EOR processes include gas injection, thermal recovery and chemical methods. The choice of the method and the expected recovery depends on many considerations, economic as well as technological. In the U.S., chemical and thermal EOR projects have been in constant decline since mid-1980s, with gas injection methods as preferred recovery methods in the last decade. Gas injection projects, mainly CO2 floods, are becoming more widespread and outnumbered thermal projects since 2002. 
|Fig. 2: Conditions for different types of oil displacements by solvents. |
Technology advances, higher oil prices, reduced costs, and environmental needs have aligned to create a strong growth opportunity for a well-established method for enhancing oil recovery in the U.S.: CO2 flooding. CO2 flooding is the fastest-growing EOR technique in the United States. While production volumes and the number of projects for thermal, chemical, and other EOR processes have fallen off sharply since 1980, the number of CO2 projects has more than tripled since 1980, to more than 70 projects as of yearend 2004. Meanwhile, CO2 production volumes have jumped twentyfold since the early 1980's. 
Large volumes of oil are left unrecovered after completion of primary and secondary oil recovery methods. The reasons for these large volumes of unrecovered oil include: oil that is bypassed due to poor waterflood sweep efficiency; oil that is physically unconnected to a wellbore; and, most importantly, oil that is trapped by viscous, capillary and interfacial tension forces as residual oil in the pore space. The main mechanism by which CO2-EOR can recover this trapped oil is by creating miscibility between the residual oil and the injected CO2. Additional mechanisms such as viscosity reduction, oil swelling and improved reservoir contact further contribute to efficient oil recovery.  The displacement of crude oil by CO2 injection is normally divided into miscible and immiscible, depending on the reservoir conditions (pressure and temperature) and the oil compositions.
Miscible CO2-EOR is a multiple contact process involving interactions between the injected CO2 and the reservoir's oil. During this multiple contact process, CO2 vaporizes the lighter oil fractions into the injected CO2 phase and CO2 condenses into the reservoir's oil phase. This leads to two reservoir fluids that become miscible (mixing in all parts), with favorable properties of low viscosity, enhanced mobility and low interfacial tension, thus remobilizing and dramatically reducing the post-waterflooding residual oil in the reservoir's pore space. Fig. 1 provides a one-dimensional schematic showing the fluid dynamics of the CO2 miscible process.
Immiscible CO2-EOR occurs when insufficient reservoir pressure is available or the reservoir's oil composition is less favorable (heavier). The main mechanisms involved in immiscible CO2 flooding are: (1) oil phase swelling, as the oil becomes saturated with CO2; (2) viscosity reduction of the swollen oil and CO2 mixture; (3) extraction of lighter hydrocarbon into the CO2 phase; and, (4) fluid drive plus pressure. This combination of mechanisms enables a portion of the reservoir's remaining oil to be mobilized and produced (details of these mechanisms are discussed later in this paper). In general, immiscible CO2-EOR is much less efficient than miscible CO2-EOR in recovering the oil remaining in the reservoir. 
To explain the different processes in miscible flooding, ternary diagrams are widely used. Fig. 2 summarizes the different processes in terms of miscibility development. Since the dilution path (I2-J3) in Fig. 2 does not pass through the two-phase region or cross the critical tie line, it forms first contact miscible displacement. The path (I1-J1), which entirely lies on the two-phase region, forms immiscible displacement. When the initial and injected compositions are on the opposite side of the critical tie line, the displacement is either a vaporizing gas drive (I2-J1) or a condensing gas drive (I1-J2). 
Regardless of how CO2 flooding is applied and whether or not miscibility is reached, the recovery of oil is assisted, to a varying degree, by a number of mechanisms all stemming from mixing and mass exchange between oil and CO2. These include:
Swelling of Oil: The swelling of oil due to CO2 dissolution is important for two main reasons. Firstly, the residual oil left in the reservoir after flooding is inversely proportional to the swelling factor, i.e. the greater the swelling, the less stock tank oil abandoned in the reservoir. Secondly, disconnected oil blobs may become reconnected as the oil swells and forces water out of the pore space. This creates higher oil recovery and more favorable relative permeability curves at any saturation condition.
Viscosity Reduction: This reduction, like heating of oil in thermal recovery, can yield viscosities one-tenth to one-hundredth of the original oil viscosity. The magnitude of viscosity reduction is greater in heavier oil samples.
Oil Extraction: At high pressure conditions, in addition to CO2 dissolution into the oil phase, light and intermediate hydrocarbon components may be vaporized into the CO2 and recovered. This extraction may also result in very low interfacial tensions (IFT) and consequently reduction of residual oil saturation.
Solution Gas Drive: Just as CO2 goes into the solution with an increase in reservoir pressure, after termination of the injection phase of flood, gas will come out of the solution and continue to drive oil into the wellbore. 
Currently available CO2-EOR technologies, including both miscible and immiscible CO2 injection, are in commercial use today. However, today's CO2-EOR technologies still underperform compared to their theoretical potential as established by laboratory testing, reservoir simulation and a handful of forward-looking, highly instrumented projects. As evidence for underperformance, field data shows that currently practiced CO2-EOR technology recovers only 5% to 15% of a reservoir's OOIP as opposed to theoretically possible oil recoveries using "next generation" CO2-EOR technology of over 20% of OOIP.  Because CO2 typically has much lower viscosity compared to crude oil, the displacement process severely suffers from viscous fingering and sensitivity to heterogeneity. Therefore, CO2 flooding process generally involves alternating injection of CO2 and water (WAG) to control the mobility of the CO2 where the displacement is horizontal or nearly so.  The "next generation" CO2-EOR technology options include: (1) increasing the volume of CO2 injected into the oil reservoir to increase sweep efficiency; (2) optimizing well design and placement, including adding infill wells, to achieve increased contact between the injected CO2 and the oil reservoir; (3) improving the mobility ratio between the injected CO2/water and the residual oil; and, (4) extending the miscibility range, thus helping more reservoirs achieve higher oil recovery efficiency. 
According to the 2008 EOR Survey published by the Oil and Gas Journal, approximately 250,000 barrels per day of incremental domestic oil is being produced by 105 CO2-EOR projects, distributed broadly across the U.S. Since 1986, when CO2-EOR was first used in commercial production, over 1.3 billion barrels of incremental oil have been recovered using this technology. CO2-EOR offers the potential for storing significant volumes of carbon dioxide emissions while increasing domestic oil production. There are several benefits which accrue from integrating CO2 storage and enhanced oil recovery. First, CO2-EOR provides a large, value added market for sale of CO2 emissions captured from new coal-fueled power plants. Second, storing CO2 with EOR helps bypass two of today's most serious barriers to using geological storage of CO2 (establishing mineral rights and assigning long- term liability for the injected CO2). Third, the oil produced with injection of captured CO2 emissions is 70% carbon-free, after accounting for the difference between the carbon content in the incremental oil produced by EOR and the volume of CO2 stored in the reservoir. [4,7]
© Amir Salehi. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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