Enhanced Oil Recovery: can the Future be Solar?

By Dr. DF Duvenhage

Introduction

Engineers and industry stakeholders have long sought innovative solutions to enhance the efficiency and sustainability of Enhanced Oil Recovery (EOR) techniques to maximize oil recovery from mature reservoirs. Traditionally, EOR methods have relied on thermal, chemical, or gas injection processes, each presenting challenges and limitations. For example, traditional EOR approaches, particularly thermal methods, often rely on fossil fuels for steam generation, raising concerns about greenhouse gas emissions and environmental impact. Furthermore, amidst the growing imperative for sustainable energy practices, there is a burgeoning interest in exploring alternative approaches to EOR that leverage renewable energy sources.

One promising avenue is the integration of solar energy into EOR processes. Utilizing solar technologies in oil recovery operations offers the potential to improve energy efficiency, reduce environmental impact, and enhance project economics. By harnessing the power of sunlight, engineers can explore new avenues for enhancing oil recovery while aligning with the broader goals of sustainability and energy transition.

Solar-enhanced oil recovery (SEOR) is a technique that utilizes solar energy to enhance the recovery of oil from reservoirs, particularly those containing heavy or viscous crude oil. Unlike traditional thermal-enhanced oil recovery methods, which rely on external heat sources such as natural gas for steam generation, ultimately used in steam injection to reduce the viscosity of the oil and improve its mobility, SEOR harnesses the natural energy of the sun to generate steam [1].

The basic principle of SEOR involves placing solar collectors or panels on the surface or near the wellhead to capture sunlight. These collectors absorb solar radiation and convert it into heat energy, which is then used to generate steam or heat water. The steam or hot water is injected into the reservoir through injection wells, which helps reduce the oil’s viscosity, increase its mobility, and facilitate its flow towards production wells [2].

The most notable difference between SEOR and traditional thermal EOR methods is the heat source used for steam generation. In conventional thermal EOR, steam is typically generated using external energy sources such as natural gas, electricity, or the combustion of hydrocarbons. This process requires significant energy input, contributing to operational costs and environmental impact [3]. Table 1 highlights the major differences between SEOR and traditional thermal EOR methods.

Table 1: Key differences between SEOR and conventional thermal EOR.

In contrast, SEOR offers several advantages over traditional thermal EOR techniques. Firstly, it harnesses a renewable and abundant energy source – sunlight – which reduces dependence on fossil fuels and lowers greenhouse gas emissions. SEOR systems can be modular and scalable, making them suitable for deployment in remote or off-grid locations where access to traditional energy sources may be limited. Additionally, SEOR can potentially reduce operational costs and improve project economics over the long term by reducing energy expenses [4].

However, SEOR also presents its own set of challenges and considerations. Factors such as weather conditions, time of day, and geographic location can affect the efficiency of solar collectors, which may impact steam generation rates and overall performance. Additionally, the upfront capital costs associated with installing solar infrastructure may be higher compared to traditional thermal EOR systems. However, the co-location of major oil fields and high solar energy potential offers a promising alternative to the reliance on a valuable product like natural gas, which is also subjected to price fluctuations and could add more yield to the facility’s productions.

Current use of fossil fuels for thermal EOR

Thermal Enhanced Oil Recovery encompasses two distinct approaches: one involves generating heat within the reservoir itself, while the other entails injecting heat into the reservoir. Of the various thermal EOR techniques, steam injection stands out as the most widely employed method worldwide. It is widely recognized as the most successful thermal recovery technique and is often regarded as the most effective among all EOR methods [5].

The primary mechanism employed across all thermal recovery methods involves the injection of heat. This elevation in temperature induces several effects, including the reduction of viscosity, enhanced mobility of the oil, modification of reservoir rock wettability, and the generation of force to augment oil flow rates into the production well. Additionally, phenomena such as fluid and rock expansion, compaction, and distillation may also occur. Thermal recovery methods are further categorized into hot water flooding, steam drive, cyclic steam injection, in-situ combustion, and SAGD.

Cyclic steam injection (commonly known as Huff & Puff), steamflooding, and more recently, Steam-Assisted Gravity Drainage (SAGD) have emerged as the primary methods for recovering heavy and extra-heavy oil from sandstone reservoirs over the past several decades. These thermal Enhanced Oil Recovery (EOR) techniques have been predominantly utilized in regions such as Canada, the Former Soviet Union, the United States, Venezuela, Brazil, and China, albeit to varying extents.

The inception of steam injection dates back approximately five decades ago, with notable projects such as those in the Mene Grande and Tia Juana fields in Venezuela, and the Yorba Linda and Kern River fields in California, exemplifying successful steam injection endeavors spanning over four decades [6] [7] [8]

During the initial phase, shown in Figure 1, known as the huff stage, vapor is introduced into a well for approximately one month. Subsequently, in the soaking stage, which follows, the well is temporarily shut down for approximately five days to allow the steam to heat the oil surrounding the wellbore. This process serves to reduce the oil’s viscosity and enhance its mobility [9]. In the final stage, referred to as the puff stage, the well is returned to production until it achieves optimal production rates. This steam-injection cycle is reiterated multiple times until the response to stimulation is no longer economically viable. At that juncture, the Continuous Steam-Solvent (CSS) process transitions into a continuous steam flood project [10]. In addition to viscosity reduction, CSS enhances oil productivity by augmenting the thermal expansion of crude oil and improving permeability through the removal of accumulated paraffinic and/or asphaltic substances around the wellbore [11] [12] [13].

Figure 1: Cyclic steam stimulation process [14]

Steam Assisted Gravity Drainage (SAGD) is a significant thermal Enhanced Oil Recovery (EOR) method employed to augment oil production from oil sands reservoirs. With its suitability for unconsolidated reservoirs exhibiting high vertical permeability [15], SAGD has garnered attention in regions abundant in heavy and extra-heavy oil resources, notably Canada and Venezuela, which possess extensive oil sands deposits. Although pilot tests of SAGD have been reported in countries such as China, the United States, and Venezuela, commercial applications of this EOR technique have been predominantly observed in Canada [16] [17] [18].

The SAGD method involves the utilization of two horizontally displaced wells drilled parallel to each other in a perpendicular plane, as depicted in Figure 2 [19]. The upper well is positioned approximately 10-30 feet above the lower well. Initially, vapor is introduced into both wells for a period of 2-4 months to preheat the crude oil surrounding them [20]. Subsequently, steam is injected into both the upper and lower wells to initiate oil production. During this process, the steam expands upward and laterally, creating a steam chamber. The injected steam elevates the temperature of the crude oil to above 400°F, reducing its viscosity to approximately 10 centipoise (cp) and enhancing oil mobility. The condensed steam and oil then flow towards the production well under gravitational forces. The anticipated recovery factor typically ranges from 50% to 70% [21] [22].

Figure 2 SAGD concept through steam injection [23]

Concentrated Solar Power; the Heart of SEOR

CSP systems use mirrors to concentrate sunlight onto a receiver, which creates heat that is used to generate electricity. The benefit of CSP’s exploitation of the thermal component of solar irradiation is that the energy can be stored as heat, or thermal energy storage (TES), which provides a workable solution to the challenge of reduced or curtailed energy production when the sun sets or during periods of weather-related intermittency [24]. CSP systems can store thermal energy for many hours, which enables them to be flexible and dispatchable options for providing clean, renewable energy [25]. CSP can provide power grid stability and flexibility, making it a valuable asset as grid demands evolve [26].

Additionally, CSP can provide inertia, which is the kinetic energy stored in spinning generators and is useful in handling transient conditions such as temporary short circuits and momentary disruptions. This inertia is especially useful for low-frequency control [27]. CSP has the potential to reduce the cost of renewable energy from various sources and provide a reliable source of clean energy [28].

CSP utilizes three alternative technological approaches: Parabolic Trough (PT), power or solar tower (ST), and dish/engine [29]. CSP systems use mirrors or lenses to concentrate sunlight onto a receiver, which creates heat that is used to generate electricity.

The ideal conditions required for the economic feasibility of CSP include areas of high solar radiation, low humidity, and minimal dust and clouds. These factors affect the amount and quality of sunlight that reaches the receiver, and thus the thermal energy output. The availability of land, water, grid connection, and environmental impact should also be considered [30].

Countries with CSP in operation and under development include Algeria, Australia, Canada, Chile, China, Denmark, Egypt, France, India, Italy, Mexico, Morocco, South Africa, Spain, the United Arab Emirates, the United Kingdom, and the United States [31].

Parabolic trough linear concentrating systems are used in one of the longest-operating solar thermal power facilities in the world, the Solar Energy Generating System (SEGS) located in the Mojave Desert in California. The facility has had nine separate plants over time, with the first plant in the system, SEGS I, operating from 1984 to 2015, and the second, SEGS II, operating from 1985 to 2015 [32]. Parabolic trough systems are commercially mature and have been in operation for over 30 years. They have a lower cost per unit of capacity than power tower systems [33].

Solar irradiance is concentrated by long, curved reflectors onto collector tubes spanning the length of the parabolic troughs. These reflectors consist of a reflective material (such as mirrors or polished metals) that has been bent and orientated to reflect the sunlight onto a focal line [34]. The collectors consist of a glass outer tube, a high-conductance, a coated metal tube filled with a heat transfer fluid (HTF), and a vacuum between the two tubes. The selective coating on the metal tube allows high radiation absorption from the mirrors and limits heat loss due to the emission of infrared radiation from its surface [35].

The HTF within the metal tubes at current PT power plants that are operational or under development are either synthetic oils, molten salts, or steam (water). In the case of using an HTF other than water, heat is exchanged within a boiler, between the HTF and water, to produce high-temperature, high-pressure steam. In the case of direct steam generation, water is cycled through the metal tubes, producing the critical steam [36]. Finally, the steam drives a conventional Rankine-cycle turbine and generator and is condensed either by wet-, dry- or hybrid cooling.

The PTs within the solar field are modular and are arranged in long parallel rows. The structures on which the reflectors and collectors are mounted, in the solar field, track the sun on one axis only. They are aligned in a North-South orientation and track the sun from East to West throughout the day [37]. The HTF must be circulated through the entire solar field, hence they can reach great lengths. This results in the HTF having to be pumped kilometres before reaching the power block where it can exchange heat with water, generate steam, and drive the turbine and generator [38]. As a result of this fact, PT power plants generally need to be located in areas that are not steep and are near flat. This is another key difference between PT and ST technologies.

The temperature of the heat transfer fluid flowing through the pipe, usually thermal oil, is increased from 293ºC to 393ºC, and the heat energy is then used in the thermal power block to generate electricity in a conventional steam generator.

The key difference between ST and PT plants is that the solar irradiance is concentrated by many mirrors in a solar field onto a single point. These mirror structures are called heliostats and track the sun on both the vertical (tilting)- and horizontal (rotating) axis [39]. They reflect and concentrate the sunlight onto a central receiver mounted on a tall tower. Here, the sunlight can be concentrated 600-1000 times, or more, resulting in temperatures in excess of 1000 degC [40].

The fact that the source of the heat is at a central location results in much less need for piping to transport the working fluid. The power block, storage, and condensers can all be located at the base of the tower, meaning that the working fluid does not have to be circulated for such long distances, as is the case with PT technology. The dual-axes tracking system required for the vast solar field is however more complicated than the single-axis system for PT power plants and requires more sensitive calibration.

The HTF used in ST power plants can also range between synthetic oils, molten salt or direct steam. In addition to these mediums, air can also be used as the working fluid because of the high temperatures reached [41]. If any medium other than water is used as the HTF, then heat must be exchanged between it and water to produce the steam required for the Rankine cycle. If direct steam is used, then the same principles as for PT power plants apply. The higher temperature, super-critical steam from ST power plants does however result in higher Rankine efficiencies. Beyond this fact lies the improvement in thermal storage capacity due to the higher temperatures since hotter molten salts can retain heat for longer periods [42].

Spain has several power tower systems. Planta Solar 10 and Planta Solar 20 are water/steam systems with capacities of 11 and 20 megawatts, respectively. Gemasolar, previously known as Solar Tres, produces nearly 20 megawatts of electricity and utilizes molten-salt thermal storage [43]. South Africa currently has two power towers; Khi Solar One, near Upington, and Redstone near Postmasburg. Khi, a 50MW direct-steam power tower, has been operating since 2017, while Redstone, a 100MW molten salt tower, is currently under construction. Power tower systems are less mature than parabolic trough systems, but they have the potential to achieve higher operating temperatures and efficiencies [44].

Case Studies of SEOR

The concept of solar EOR has been explored through various experimental and commercial applications. One of the earliest commercial projects was deploying the world’s first commercial solar EOR project at an oilfield operated by Berry Petroleum in Kern County, California, USA, in 2011. This system used the sun’s radiant heat to produce approximately 1 million British Thermal Units (BTUs) per hour of solar heat, preheating water to 190°F used as feed water for Berry Petroleum’s gas-fired steam generators [45].

In October 2011, Chevron Corp. and BrightSource Energy revealed a 29-megawatt solar-to-steam facility at the Coalinga Oil Field in Fresno County, California. The Coalinga solar EOR project spans more than 400 square meters and consists of 3,822 mirror systems, or heliostats, each with two 3-meter by 2.3-meter mirrors mounted on a 2-meter steel pole focusing light onto a 100-meter tower [46].

BrightSource was contracted to provide the technology and EPC services, and Chevron Technology Ventures managed the project operations. The facility began construction in 2009. It was reported that Chevron spent more than its initial budget of $28 million on the contract, and BrightSource has lost at least $40 million on the project and disclosed it will lose much more [47].

Another notable and commercially successful project is the Middle East’s first solar EOR project, a 7MW system developed in partnership with Petroleum Development Oman (PDO), the largest oil company in the Sultanate of Oman, in May 2013. This system produced an average of 50 tons of emissions-free steam daily, 27 times larger than the original operations at the berry field. The steam was fed directly into existing thermal EOR operations at the oil field [45]. The fully automated system successfully achieved all performance tests and production targets in the first year of operations. The system recorded an uptime of more than 98%, significantly exceeding PDO’s expectations. Even during severe dust and sandstorms, the system has maintained regular operations [48].

The largest commercial project to date is the Miraah project, a 1,021-megawatt solar plant in Oman, which was announced in 2015. This project is a turnkey system that will be owned and operated by PDO, generating 6,000 tons of steam per day to extract viscous oil from Oman’s Amal oil field [49]. The project uses GlassPoint’s enclosed trough technology, which encloses lightweight, trough-shaped mirrors with piping in a standard agricultural greenhouse, concentrating the sun’s energy to create steam, as shown in Figure 3.

Figure 3: The greenhouse enclosure used to protect the troughs from adverse conditions in the desert at the Miaah project [50].

In the Miraah project, solar thermal energy generates high-pressure steam, which is then injected into the oil reservoir to heat heavy oil. This process reduces the viscosity of the oil, making it easier to extract. The project’s design allows for efficient steam production, generating 6,000 tons of steam per day, making it the largest solar EOR project in the world.

The use of greenhouse enclosures in large-scale Solar Enhanced Oil Recovery (SEOR) projects in the desert is a crucial component of the technology. These enclosures, also known as glasshouses, are designed to protect the solar mirrors and other delicate components of the system from the harsh desert environment, including sandstorms, wind, and humidity [51].

The glasshouse structure is strategically placed approximately 6 meters above ground level, which significantly reduces the soiling rate by 50% compared to the use of typical exposed solar designs where mirrors are placed around 1 meter above ground [52]. This design choice is particularly important in the desert environment where sand and dust storms are common, as it prevents the mirrors from being damaged and reduces the need for frequent cleaning [53].

The enclosed trough technology used in these projects, such as the Miraah solar plant in Oman, employs large, curved mirrors to focus sunlight onto a boiler tube containing water. The concentrated sunlight boils the water to create high-pressure steam, which is then fed directly to the oilfield’s existing steam distribution network [54].

The use of greenhouse enclosures offers several advantages in these desert based SEOR projects. Firstly, it reduces the cost of the system by minimizing the need for frequent cleaning and maintenance due to reduced soiling rates. Secondly, the enclosed design enables the use of thin and lightweight components, significantly reducing material usage compared to exposed solar thermal systems. Finally, the greenhouse structure provides better performance by protecting the mirrors and other components from the harsh desert environment, ensuring optimal energy conversion and steam production [55].

Challenges and limitations of SEOR

Solar-enhanced oil recovery (Solar-EOR) presents several challenges and limitations that need to be addressed for widespread adoption and successful implementation. These challenges primarily revolve around the technical, economic, and operational aspects of integrating solar energy into oil recovery processes. Here are some key challenges and limitations of Solar-EOR:

1. Energy Intensity and Solar Resource Availability: One of the primary challenges of Solar-EOR is the high energy intensity required for steam generation, especially in regions with lower solar irradiance or during periods of inclement weather. Solar collectors must be highly efficient in generating sufficient heat for steam injection, which can be challenging in certain geographic locations.
2. Intermittency of Solar Energy: Solar energy is inherently intermittent, dependent on daylight hours and weather conditions. This intermittency can pose challenges for continuous steam generation, impacting the consistency and reliability of the steam supply needed for oil recovery operations. This intermittency, however, can be overcome by thermal energy storage coupled with CSP technology. This is generally regarded as one of the major benefits of CSP since it can store large amounts of energy in a thermal form for steam generation during night hours or during times of intermittent irradiation due to weather conditions.
3. Initial Capital Costs: Implementing Solar-EOR requires significant upfront capital investment for acquiring and installing solar thermal systems, including solar collectors and associated infrastructure. These initial costs can be a barrier to adoption, especially for smaller operators or in regions with limited financial resources. While the cost of electricity generated by CSP has declined in the recent past, the cost for direct steam generation and integration into oil recovery systems at inland oil fields have not due to lower adoption rates.
4. Complexity of Integration: Integrating solar thermal systems with existing oil recovery infrastructure can be complex and requires careful planning and engineering. Designing efficient heat transfer mechanisms, steam distribution networks, and control systems to optimize solar energy utilization adds complexity to project development. Furthermore, Solar-EOR may not be suitable for all reservoir conditions and oil types. The effectiveness of solar-driven steam generation depends on reservoir depth, geology, and the characteristics of the crude oil. Certain reservoirs may require additional heating or treatment methods beyond what solar energy can provide.
5. Scale and Adaptation: Scaling Solar-EOR projects from pilot to commercial scale can pose challenges related to system reliability, maintenance, and operational scalability. Adapting solar technology to meet the specific needs of oil recovery operations requires ongoing research and development.

Conclusion

While SEO is a promising technology for improving the sustainability of oil recovery operations, it is still uncertain how widely it will be adopted. CSP technology itself is mature and has been demonstrated to be technically feasible and effective at many large-scale sites globally for electricity generation. The worldwide focus has been on reducing the environmental impact of the power sector, particularly in electricity generation. However, oil extraction itself is a large contributor to emissions and could potentially be used to reduce the use of hydrocarbon-based fuel for the extraction thereof, thereby reducing the lifecycle emissions of oil and its petrochemically derived products.

The co-location of inland oil fields and high solar irradiance in desert regions offers a promising coincidence of factors, making using CSP for steam generation and its use in enhanced oil recovery an almost natural fit. Combine this with the large expanses of uninhabited and unproductive desert land, and CSP seems even more well-suited to provide the energy for steam generation.

The harsh environment of the deserts, however, does complicate the implementation of the large fields of mirrors to concentrate sunlight required as the primary energy input. The winds, dust, and occasional severe dust storms can reduce the solar fields’ effectiveness and damage them. The use of greenhouse enclosures in large-scale SEOR projects in the desert is a critical component of the technology, offering advantages in terms of reduced costs, improved performance, and increased efficiency. These enclosures protect the solar mirrors and other components from the harsh desert environment, ensuring optimal energy conversion and steam production for the oil recovery process.

Solar EOR has emerged as a promising technology for extracting oil from maturing oil fields. The technique has shown significant potential for reducing the environmental impact and costs associated with traditional oil recovery methods through various experimental and commercial applications. The Miraah project, in particular, highlights the scalability and efficiency of solar EOR, making it a potentially attractive option for oil producers seeking to adopt more sustainable practices. While it has the potential to play a significant role in the global oil industry, it faces several challenges and limitations that need to be addressed to ensure its widespread adoption and success.

The cost of SEOR is expected to be lower than traditional methods of oil extraction, as it uses renewable energy sources and reduces the need for fossil fuels. According to a study by Total Energies, adopting SEOR could reduce the cost of oil production by up to 15% [56]. This is because SEOR can reduce the energy required for oil extraction, which can lead to lower production costs. Additionally, SEOR can reduce the environmental impact of oil production, leading to lower costs associated with environmental regulations and cleanup. According to a study by the International Energy Agency (IEA), adopting SEOR could reduce greenhouse gas emissions from oil production by up to 20% [57].

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