|Fig. 1: Diagram of steam injection TEOR. Steam is injected through an injector well, transfers its heat to and displaces the oil, and hot oil is extracted at the producer well. (Source: Wikimedia Commons.)|
Burgeoning global demand for oil and the resulting rising prices have prompted companies to seek economical means for producing unconventional petroleum resources. In particular, resources such as heavy oil, tar sands, and oil shale have received renewed interest as these have become progressively more economical to extract. Furthermore, as possible human effects on climate change becomes of increasing concern to governments and the broader public, it is becoming necessary for petroleum producers to find more environmentally sustainable methods for extracting petroleum resources.
One approach which has received interest in recent years is using solar energy - specifically, solar thermal setups - to aid in oil extraction.  Here, we explore the applications of solar thermal energy to petroleum recovery, current implementations of this technology, and potential ways this technology may shape the energy landscape in the coming decades.
Since the early days of the oil industry, engineers have sought to improve the amount of original oil in place (OOIP) extracted. These methods, known as enhanced oil recovery (EOR) methods, typically increase the OOIP recovered through a combination of increasing the temperature to lower viscosity, injecting water or other fluid to displace the oil, and introducing a solvent. In practice, solvents are seldom used due to cost and environmental concerns. Nearly all widely-used EOR methods use a combination of raising the reservoir temperature and injecting fluid to displace the oil; collectively, these methods are known as thermal enhance oil recovery (TEOR).
Fig. 1 depicts the basic workflow for steam injection-based TEOR, the most common in the family of TEOR methods. In the figure, we see that steam is first injected into the reservoir through an injector well. The steam then offloads some of its thermal energy to the oil, condenses into water, and displaces the hot oil. The hot oil is then extracted from a producer well some distance away. While there are many engineering challenges to overcome in this setup, here we are most interested in producing the steam.
Typically, steam for TEOR is produced by burning natural gas. While intuitively it may make sense to use natural gas produced at the well to create the steam, in practice the cost of processing the gas at the production site is too high, so the gas produced at TEOR sites is usually flared off and off-site gas is brought in to create the steam. This off site gas increases the cost of production - not to mention the carbon emissions involved - significantly. For this reason, oil companies would like to find a less expensive source of steam for TEOR.
One solution to generating cheaper steam for TEOR that has emerged in recent years is generating steam with solar thermal energy. Instead of creating steam by burning natural gas, solar thermal setups use solar energy to heat a liquid medium and pass this through a thermal exchange to create steam.
Solar TEOR setups have significant environmental the economic advantages over gas-based setups. On the environmental side, several studies have demonstrated the advantages of solar-based EOR setups. Specifically, TEOR can potentially reduce carbon emissions of EOR from 23.8 g CO2/MJ with a gas setup to 0.1 g CO2/MJ with a solar setup. [2,3]
Economically, solar-based setups also offer several advantages. Solar-based setups, after initial investment, do not have the high marginal costs of production as do gas-based setups, and are not subject to market fluctuations in natural gas prices.  showed that while solar setups are subject to significantly higher initial investment costs, the lowered marginal costs make solar TEOR economically viable at a much lower price point in both the near- and long- term time horizons. While not as definitive as the environmental advantages, the data suggests that solar TEOR setups are at least as economical or even slightly more desirable than gas-based setups.
Solar TEOR presents clear environmental advantages and comparable economics compared to natural gas-based TEOR setups. This then begs the question: why has solar TEOR not be implemented on a wider scale? If solar is so great, why isn't every oil company using this?
|Fig. 2: GlassPoint Solar's greenhouse solar trough setup. (Source: Wikimedia Commons.)|
The answer comes down to both the practicalities of solar TEOR—indeed, solar energy in general—and the base realities of the petroleum industry. On the practical side, solar energys perennial shortcoming is its inability to produce energy except during specific hours in regions with high solar flux. Some work (e.g. Agarwal and Kovscek) has shown that with sufficiently high solar flux, the steam injection rate experiences minimal fluctuations throughout the day.  However, this is predicated on having sufficiently high solar incidence in the first place, and there are very few petroleum producing regions that also experience high enough solar irradiance.
Further discouraging oil companies from adopting this technology are the exceedingly high initial capital investments involved with a solar TEOR setup. In an industry as turbulent as petroleum, companies are weary of investing huge amounts of capital in projects that may only have a nominal economic advantage in the near-term time horizon. The real economic advantages of solar TEOR are in the long-term time horizon, but many recovery projects do not last into the long-term financial time horizon, meaning the near-term time horizon, where the economic advantages of solar TEOR are murkier, determines financial decisions.
Right now, there are two competing designs for solar-based EOR that have been implemented. BrightSource Energy has proposed a solar tower design, and in conjunction with Chevron has built a prototype model in Coalinga, CA that has provided a portion of the steam to the oil field there. GlassPoint Solar, a competing firm, has a concentrated solar design with solar troughs are enclosed in a greenhouse. This setup is shown in Fig. 2. GlassPoints design is less efficient than Brightsource's, but also far less expensive. GlassPoint has partnered with Petroleum Development Oman to build a field-scale version of their greenhouse design at the Amal oilfield in Oman. The facility was completed in late 2012, and today provides over 20% of the steam for the field.
Economic viability is, and always will be, the primary determinant of petroleum production. Environmental advantages are simply not enough, and until oil producers view solar-based EOR projects as equally economical with gas-based methods, solar TEOR will not become common. Much work remains for bringing this technology into widespread use, but with the success of GlassPoints facility in Oman, this technology is promising. With further research and investment in pilot projects, the future will be bright for solar TEOR technology.
© Timothy Anderson. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. 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.
 A. Kovscek, "Emerging Challenges and Potential Futures For Thermally Enhanced Oil Recovery," J. Petrol. Sci. Eng. 98-99, 130 (2012).
 A. R. Brandt and S. Unnasch, "Energy Intensity and Greenhouse Gas Emissions from Thermal Enhanced Oil Recovery," Energy Fuels 24, 4581 (2010).
 J. Sandler et al., "Solar-Generated Steam For Oil Recovery: Reservoir Simulation, Economic Analysis, and Life Cycle Assessment," Energy Convers. Manage. 77, 721 (2014).
 T. Anderson, "Economic Analysis of Solar-Based Thermal Enhanced Oil Recovery," One Petro SPE-173466-STU, 29 Oct 14.
 A. Agarwal and A. R. Kovscek, "Solar-Generated Steam for Heavy-Oil Recovery: A Coupled Geomechanical and Reservoir Modeling Analysis, One Petro SPE-165329-MS, 19 Apr 13.