Introductory Chapter: Lithological Dynamics – CO2-EOR at the Forefront of Enhanced Oil Recovery Evolution

1. Introduction

The evolution of Enhanced Oil Recovery (EOR) methods reveals a shift from thermal to CO2-EOR, with reservoir lithology influencing method selection. In sandstone, thermal and chemical methods like High-Pressure Air Injection (HPAI) and polymer flooding thrive. Conversely, carbonate reservoirs favor polymer flooding, with modest chemical contributions. Gas flooding methods, including CO2, find application in both lithologies. CO2-EOR emerges as a dual-benefit solution, capturing significant CO2 while boosting oil production. Despite challenges, CO2-EOR is globally recognized as an attractive carbon storage method.

Over the decades, the evolution of EOR methods is evident in the data presented in Figure 1. From the early years with a focus on thermal techniques, such as steam injection, to the growing significance of CO2-EOR and diverse chemical and thermal methods, the industry has witnessed dynamic shifts in project distribution and technology adoption.

Figure 1.

Global distribution of enhanced oil recovery (EOR) projects, 1971–2017. Adapted from IEA [1], the data illustrates the evolution of EOR methods, showcasing trends and technology adoption over the decades.

The lithological composition of a reservoir is a screening factor that restricts the suitability of certain EOR methods [2]. The information presented in Figure 2 adopted from Manrique et al. [3], consisting of 1450 projects, demonstrates the implementation of thermal, gas, and chemical EOR techniques in both sandstone and carbonate reservoirs. It clearly emphasizes the primary emphasis on EOR applications in sandstone reservoirs. According to Figure 2, sandstone reservoirs are more amenable to EOR projects due to extensive testing and successful evaluations of various technologies. In contrast, carbonate reservoirs, with low porosity and natural fractures, pose challenges like fluid bypassing. Gas injection is the primary method in carbonates, while polymer flooding stands as the sole proven chemical technique. Thermal methods have minimal contribution, but High-Pressure Air Injection (HPAI) is gaining traction, particularly in light oil carbonate reservoirs. In this chapter, a concise exploration of the three primary EOR methods—thermal, gas, and chemical—applied in both sandstone and carbonate reservoirs is provided.

Figure 2.

EOR field projects categorized by reservoir lithology.

2. Thermal method: sandstone versus carbonate

In sandstone reservoirs, thermal EOR methods, such as cyclic steam injection, steam flooding, and Steam-Assisted Gravity Drainage (SAGD), are widely employed [4]. Optimization efforts involving solvents, gases, and additives for steam injection are ongoing, and SAGD stands out as a significant thermal EOR method [5].

In contrast, thermal methods, notably steam injection, exhibit limited application in carbonate formations. Nevertheless, according to Tang et al. [6], steam injection proves highly efficient for heavy oil reservoirs containing carbonate, and the authors outlined its potential recovery mechanism. Furthermore, as indicated by Xu et al. [7], thermal recovery through steam injection appears to be the preferred option for heavy oil reservoirs containing carbonate formations. Instances include steam drive projects in specific United States fields and small-scale testing in various international locations. HPAI has gained prominence in carbonate reservoirs, with successful projects in the United States, such as in Montana, North Dakota, and South Dakota, showcasing economic viability and controllable risks. Ongoing exploration of air injection processes, particularly HPAI, in carbonate reservoirs, suggests potential future developments influenced by the outcomes of the global recent projects [8].

3. Chemical methods: sandstone versus carbonate

In sandstone reservoirs, chemical EOR methods, notably polymer flooding, are well-established and globally implemented. Ongoing projects explore advanced technologies like Colloidal Dispersion Gels (CDGs) for improved sweep efficiency; however, as indicated by Wang and Seright [9], there is no credible evidence that CDGs can propagate deep into the porous rock of a reservoir. The development of Alkali-Surfactant-Polymer (ASP) technology, particularly in China’s Daqing field [10], has reignited interest in chemical floods, evident in active ASP and surfactant-polymer (SP) projects globally. Despite oil price fluctuations, the industry maintains a strong focus on chemical flooding, with growing interest and projects in various countries like Argentina, Canada, India, and the United States [11].

In contrast, carbonate reservoirs primarily rely on polymer flooding as the proven chemical EOR technology, applied mostly during early waterflooding stages. However, carbonate reservoirs have modestly contributed to the success of polymer flooding [12]. While Alkali-Polymer (AP), SP, and ASP floods are exclusive to sandstone reservoirs, ongoing lab testing of ASP in carbonate formations suggests potential future applicability. Strategies based on chemicals, such as employing gels and foams for gas and water shutoff, are anticipated to enhance the efficiency of water, gas, or Water-Alternating-Gas (WAG) projects in carbonate reservoirs in the imminent future.

4. Gas methods: sandstone versus carbonate

In sandstone reservoirs, gas flooding methods are commonly used for light, condensate, and volatile oil, with nitrogen (N2) injection proposed to vaporize light fractions. Ongoing projects, notably in Hawkins Field (Texas) and Elk Hills (California), utilize N2 injection. Hydrocarbon gas injection, excluding pressure maintenance or double displacement, has marginal contributions, particularly on Alaska’s North Slope, with potential differences in offshore sandstone reservoir dynamics. Non-hydrocarbon gases like N2 and CO2 provide alternatives, preserving reservoir pressure and enhancing oil recoveries. CO2 flooding, widely used for medium and light oil in United States sandstone reservoirs, has successful global applications [13].

In carbonate reservoirs, gas flooding methods, including nitrogen and hydrocarbon gas injection, are widely employed for EOR. The majority of injection projects in carbonate reservoirs, accounting for 61%, utilize CO2, while 36% involve hydrocarbon gas injection, and a mere 3% are dedicated to N2 injection [7]. N2 flooding, once effective, has waned in interest due to high costs, with HPAI emerging as a cost-effective alternative, especially in regions like Montana, North Dakota, and South Dakota. Hydrocarbon gas injection in onshore carbonate reservoirs has a marginal contribution to total oil recovery. Utilizing natural gas for pressure maintenance or in WAG and foam-assisted water-alternating gas (FAWAG) processes is considered practical, preserving reservoir energy and maximizing oil recovery [14]. CO2-EOR in carbonate formations relies on natural CO2 sources, especially in the United States Permian Basin, with expected growth unless more viable EOR strategies are developed [15]. Globally, CO2-EOR is seen as an attractive CO2 storage method, particularly from anthropogenic sources, despite challenges like limited storage capability and high costs. Additionally, acid gas injection, involving a mixture of H2S and CO2 [16], serves as an EOR strategy in carbonate formations, with ongoing or planned projects in various locations, including Zama field (Canada), Tengiz field (Kazakhstan), and Harweel (Oman).

CO2-EOR is a method capable of capturing significant amounts of CO2 while boosting crude oil production from established oil fields. This process yields oil with a potentially lower carbon footprint compared to conventional methods. CO2-EOR, employed for over four decades, utilizes a closed-loop injection and recycle system to trap CO2. Injected into reservoirs, CO2 serves as a solvent, expanding oil volume, reducing viscosity, and facilitating oil movement from injection to production wells. This established technique maximizes hydrocarbon recovery in new fields and prolongs the lifespan of mature oil fields. United States, Middle East, North Africa, Eastern Europe, and Russia contain most of the potential for CO2-EOR and its associated storage volume (see Figure 3).

Figure 3.

Global potential for EOR using CO2 in conventional fields [17].

In summary, the lithology of reservoirs significantly influences the selection of Enhanced Oil Recovery (EOR) methods. Sandstone reservoirs favor thermal and chemical approaches, with ongoing advancements such as HPAI. Chemical EOR, particularly polymer flooding, is widespread globally in sandstone. Carbonate reservoirs primarily rely on polymer flooding, with limited chemical contributions. Gas flooding methods, including N2 and hydrocarbon gas injection, are common in both sandstone and carbonate reservoirs. HPAI emerges as a cost-effective alternative in carbonate formations. CO2 flooding, successful in United States sandstone, is applied globally, including carbonate formations. Despite challenges, CO2-EOR is considered an attractive CO2 storage method. The evolving EOR landscape reflects ongoing projects and technological advancements in different lithologies.