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| The solar collectors were contained in a greenhouse enclosure to protect against dust. An automatic washer is seen on the roof, with a storage water tank on the right. |
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The Sultanate of Oman has large heavy-oil reserves, which are best produced with thermal EOR methods. Natural gas is traditionally used as the fuel for these projects. However, concerns about future gas supply, CO2 emissions and possible cost escalation led Petroleum Development Oman (PDO) to investigate solar technology to power EOR projects. The result was the solar steam generation pilot (SSGP) built at Amal West field in southern Oman.
This pilot project deployed a new, enclosed-trough solar thermal design, in which solar radiation is concentrated, using parabolic trough mirrors that are enclosed to protect them from dust and wind loading. The sunlight is reflected onto receiver tubes carrying water, which is heated to produce 80% quality steam at 100 bar, matching typical EOR specifications. The system’s “once-through” design mimics conventional oilfield once-through steam generators (OTSGs), using the same feedwater and delivering steam to the field’s main injection header.
The key objective for the pilot was to prove that the system is able to be deployed, practically and economically, at scale in the region. To do this, several key elements had to be field-proven. First, it was important to prove that the steam output could be modeled and predicted with certainty. This article will discuss performance vs. model, and the enhancements implemented during the year to improve steam production and quality. Secondly, the oil fields of Oman lie in a region with about 15 times higher dust or “soiling rate” than locations where concentrating solar power (CSP) is typically deployed. The performance of the system in extreme weather was tested. The data point toward the feasibility of a full-field deployment of solar EOR in the region, despite some challenges that lie ahead in its adoption.
SOLAR STEAM GENERATION
Thermal EOR projects require a massive, long-term thermal energy supply to heat the reservoir. Concentrating solar power could provide this energy, at a low cost, after the initial capital investment. Hence, thermal EOR and solar energy are two processes that are well matched, especially in locations with high levels of solar radiation.
For over 100 years, commercial systems, capable of generating steam from sunshine, have been deployed around the world. Collecting thermal energy at steam temperatures requires concentrating sunlight on small targets using mirrors that track the sun, keeping the intensely focused light on a target, known as a “receiver.” The most widely deployed approach uses linear receivers—pipes—with parabolic mirrors that resemble troughs tracking the sun in one axis. The receiver, and the fluid it contains, is heated by the intense, incident light. The larger the mirror, and the smaller the target, the more thermally efficient the system can be; however, as this ratio (known as the “concentration ratio”) increases, so do the requirements for optical precision. Higher concentration ratios allow higher temperatures to be reached, generally with a trade-off of higher costs for the system; however, they are much more sensitive to deflections or misaligned mirrors causing light to miss the target, and thus, require higher total precision in the optical apparatus.
The application of solar energy to steam generation for EOR is not new. In 1983, ARCO Solar constructed a 1-MW, thermal, solar steam generation pilot in Taft, Calif. In February 2011, GlassPoint Solar delivered the world’s first commercial, solar EOR installation for Berry Petroleum in California. Later, in 2011, Chevron and Bright Source Energy opened a 29-MW solar-to-steam facility at Coalinga field in California. The challenges that have remained include project economics and the practical aspects of applying concentrated solar power to thermal EOR in an oilfield environment. In Oman, interest in solar EOR was triggered by limited fuel gas and its potential cost escalation.
EOR APPLICATIONS
Most technology development in CSP has been in support of thermal electric power generation. In such configurations, the efficiency of steam turbines strongly depends on the inlet steam temperature. Accordingly, engineering efforts have focused on the delivery of superheated steam at the highest practical temperatures. Such systems universally employ very pure water, simplifying the selection of materials of construction. Since electricity can be moved economically for long distances, solar arrays can be sited at the cleanest, highest-radiation, remote locations where land is inexpensive.
Thermal EOR, however, universally employs saturated steam, at pressures determined by formation characteristics, but broadly within a range of 60 bar to 130 bar. Given the “once-through” nature of EOR operations, water treatment is a critical cost factor, requiring the boiler to accept thousand-fold higher mineral content. Since steam can be moved economically only a few kilometers, the solar array must operate adjacent to the EOR injection wells and be tolerant of the dirt and solar radiation conditions, which prevail in oilfield environments. Fundamentally, these are different sets of design requirements, and they ultimately result in different design solutions.
While previous solar EOR trials and pilots have utilized state-of-the-art CSP technologies developed or optimized for electrical power generation and applied them to EOR, the resultant systems have been found to be too costly to build and operate. The solar technology for EOR applications must be designed to cope with the harsh desert environment in a cost-effective manner. The operating temperature can be designed to match the steam conditions of the oilfield steam distribution network, which is generally in the 280°C–330°C range. Although land around desert oil fields may be plentiful, many factors limit the land freely available for solar deployment. These factors include terrain, pipeline and utility corridors, drill sites, and future developments of oil and gas.
The simulation studies on solar EOR show that, “the daily cycles in solar-generated steam do not have a negative impact on the oil recovery (compared to constant-rate steam injection), as long as the cumulative amounts of steam injected into the reservoir (during the same time-span) are the same”.1 The thermal storage technologies (molten salt systems), which have been developed for power generation, are of no use in mitigating the primary issue identified in solar EOR research: that of seasonal variations in sunshine. The enclosed trough technology described in this article is the result of a system specifically designed and optimized for solar EOR in a harsh desert environment.
SOLAR RADIATION AVAILABILITY
The measure of solar radiation used to quantify resources for concentrating solar is direct normal irradiance (DNI). The deserts of Oman receive DNI of over 2,000 KWh/yr in most locations, with higher altitudes reaching over 2,500 KWh/yr. The Amal location receives 2,057 KWh/yr. Due to Oman’s low latitude, solar irradiation does not show large seasonal variations.
ENCLOSED TROUGH TECHNOLOGY
The SSGP plant has a solar field footprint of 17,280 m2, with a peak output of over 7 MW thermal. Enclosed trough represents an entirely new approach to the design and construction of concentrating solar collectors, Fig. 1. The system is protected by a glass structure, which is similar to an agricultural greenhouse. Lightweight parabolic mirrors (4) are hung within the glasshouse (2). The glasshouse provides structural support and isolates the solar collectors from wind and moisture, substantially reducing the total cost of the solar energy system. These greenhouses are similar to the design deployed at scale globally in a wide variety of environmental conditions.
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| Fig. 1. Enclosed trough solar system. Source: B. Bierman, Energy Procedia, 2013. |
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Ambient windborne sand, dust and humidity are substantial in many desert oilfield environments. The mirrors are of a low-cost, lightweight, aluminum honeycomb construction, and the structural design and low aspect ratio of the greenhouse allows it to withstand design wind loads at much lower cost and material usage than conventional CSP designs.
Roof washer. In many parts of the Middle East, overnight condensation occurs on dust-laden surfaces, resulting in “mud” that requires wet washing. The enclosed trough glasshouse structure is fitted with an automated roof washing system (1) capable of cleaning the entire roof surface each night, while the collectors are offline. The majority of wash water is returned in the gutter system and can be recovered for re-use. Dust infiltration is minimized by positive pressure from an air-handling unit (6), which provides filtered, dried air at slight overpressure within the structure in all conditions. This is designed to cope with intense dust storms of long duration. These measures have proven effective in delivering consistent energy output in oilfield conditions. The small losses due to roof glass transmission and structural shading are more than compensated by the soiling control and wind protection afforded by this architecture.
Reflector. Due to the absence of wind forces acting on the collectors, a lightweight parabolic reflector can deliver consistently high optical accuracy. Total weight of the mirror and frame is only 4.2 kg/m.2 The lightweight reflector enables a simple cable drive aiming system. Tracking angle is measured by inclinometers with 0.01° accuracy. Dedicated controls positioned every few meters along each trough reduce collector torsion. The fully automatic control system delivers less than 0.5 mrad pointing error at hundreds of points within the glasshouse. Pointing accuracy is unaffected by wind velocity, which is not the case for traditional CSP designs.
Receiver. The low system weight allows the entire mirror system to be suspended from the fixed receiver system. This fixed receiver eliminates all moving parts from the high-pressure direct steam receiver system. This enables the pressures necessary for oilfield steam generation at low cost, and eliminates safety risks and maintenance requirements associated with articulated high-pressure receivers. The direct-steam system eliminates other costs and risks of traditional CSP systems, such as heat exchangers and flammable heat transfer oils.
A proprietary solar receiver technology is employed, based on a standard, 2-in., carbon steel boiler tube, similar to those used in oilfield boilers. The receiver tube is polished and coated, with a selective absorber coating that maximizes the absorption of solar radiation, while minimizing the losses via the emission of infra-red radiation. Tubular glass shields minimize heat losses from convection (3). The glasshouse structure carries the receivers and troughs. The receivers are suspended from the structure by steel rods (5). The troughs are supported from the receiver tubes using similar rods. These rods are held in tension and can accommodate the significant daily thermal expansion and contraction of the receivers and troughs, while maintaining precision alignment of the optical system.
Boiler. This system is specifically designed to mimic a conventional oilfield OTSG boiler process, and accommodates feed water, with total dissolved solids as high as 30,000 ppm, and produces 80% quality steam at 100 bar. The control system tightly calibrates the steam quality to avoid precipitating scale deposits within the evaporator tubes. The total heat flux, at peak output is less than 75% of the flux in typical oilfield steam generators, to minimize the risk of surface boiling. Even so, some scaling might occur, due to excursions in water quality or chemistry. The system design incorporates features to enable receiver cleaning by pigging in the same manner as a standard
oilfield boiler.
Steam quality is controlled via a separator and re-mixing system. The steam is separated in a vessel, and the flow of steam vapor and liquid is measured. The two streams are then re-mixed at the target steam quality. Excess liquid can be recycled to the insulated water supply tank, so energy is not wasted. A steam quality of 75% was the target during operations.
Water tank. An 80-m3 upright, insulated water tank was added to the system during the year to improve overall performance. The tank removed direct dependence on water supply, and allowed the recovery of waste heat into the feed water during transient periods.
The system is connected to the Amal West main steam header and is designed to deliver 50 tpd of steam on average for the year. This output represents a small percentage of the total steam capacity of the field and, as such, does not lead to significant pressure or rate variations in the steam distribution network. Future systems can incorporate turndown of fuel-fired OTSGs and steam distribution networks that will accommodate large fractions of solar energy.
PILOT PROJECT RESULTS
HSE performance. The project, to date, has been executed without a lost time injury in over 270,000 man-hr of construction and operations. This is a result of eliminating risks through engineering design and a high level of safety focus at the site. For example, the mirrors can be installed without the use of cranes or working at height. The main maintenance task, the cleaning of the roof, is completed nightly, using automated equipment, eliminating any requirement for work at heights.
System performance. A model was built to predict the ideal performance of the system based on incident solar radiation (DNI) and the sun’s position. This proprietary model is a combination of an optical model that calculates ray paths, reflection and focusing of light onto the receiver, and a thermal model that calculates heat transfer and losses in the steam system.
Three performance tests were designed to measure the system performance. The first was to measure instantaneous performance of the system in full sun (Test A). The second was to measure cumulative output vs. model for the first year of operations (Test B), based on the following ramp up:
- Months 1–3: post-commissioning: 80% of model output.
- Months 4–5: post-commissioning: 90% of model output.
- Months 6–12: post-commissioning: 95% of model output.
The final test (Test C) was to show that the system could be run autonomously for three days, with no human interaction.
TYPICAL DAILY OPERATION
Figure 2 shows a typical clear day’s performance. At the start of the day, water is circulated at minimum flowrate (A) until high-enough temperatures and pressures are reached to start exporting steam (B). Steam quality is tightly controlled at 75% ±5% during the operating day (C). At the end of the day, a small quantity of steam is delivered at slightly lower quality to maximize enthalpy delivered to the customer. The system operates completely automatically with the mirror aiming, water and steam flow, and steam quality regulation controlled. The goal of the control system is to maximize the energy delivered in the form of steam at up to 80% steam quality.
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| Fig. 2. Typical operating day overview. |
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Operational days are all days when external factors don’t preclude plant operations, Fig. 3. The goal of the test laid out at the start of the operations was exceeded by 2%. Close to 14,000 t of steam were generated during the first 12 months of operational days.
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| Fig. 3. Cumulative measured output from the solar steam pilot exceeded target performance. |
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The solar steam pilot operated with extremely high uptime, averaging 98.6% for the full year and over 99.5% for the final quarter, Fig. 4.2,3 The solar field uptime significantly exceeded expectations, although a number of remedial actions were addressed. Most of these could be addressed at night, when the plant was non-operational and cooled off.
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| Fig. 4. Operating efficiency as a fraction of available days. |
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During the pilot, the SSGP was operating during the commissioning and start-up of the Amal steam project. As such, the pilot was impacted by several planned oilfield commissioning shutdown events. The availability of feed water was also limited by shutdowns, and this was the most common cause of downtime. Both of these issues are particular to the commissioning of the field and should not be representative of normal operations. However, when installing a solar system in an active oil field, such unplanned events are inevitable, and the system must not require uninterrupted power and water. This is important, in consideration of energy storage, such as the use of molten salt storage common in CSP systems used for power generation.
DESERT OPERATING CONDITIONS
Soiling rate. During the course of the year, various soiling rate measurements were made. The rate of soiling varies significantly, depending on atmospheric conditions. However, mean values could be calculated. By changing the frequency between wash cycles to leave the roof unwashed for one, two or three nights, a correlation could be made between the fall-off in performance with time.
The system shows approximately a 2.5%-per-day reduction in output from soiling for the enclosed trough, but 100% recovery of performance with a single wash, even after the most severe dust storms. These rates are high, but if they were extrapolated to infer the loss in performance of a parabolic trough system or tower, the losses would be much higher. The dust would scatter the light at three passes, striking the glass face of the mirror, exiting the glass face of the mirror, and at the vacuum receiver tube. In addition, the lower height above grade for conventional systems would further increase soiling rates.
Others have observed a relationship between airborne dust and elevation. A Desert Research Institute study of windblown dust from vehicular traffic, a common oil field soiling source, found a 55% reduction in dust at 4.6 m of elevation, compared with 1.8 m of elevation, and an 87% reduction at 9.1 m. In conclusion, the higher the optical surface is from the ground, the less vulnerable it is from soiling and abrasion. The primary optical surface of the enclosed trough is the glasshouse roof.
Extreme weather events. A severe dust storm was experienced in early April 2013. During this event, winds exceeded 40 kmph for over 24 hr, with peak gusts over 60 kmph. Surface visibility was reduced to less than 100 m. The plant continued to operate during this storm, producing 48 tons of steam. Although the windblown dust forced a shutdown of outdoor activities, at the 6-m level of the glasshouse roof, there was sufficient direct sunlight to continue normal plant operation. The optical model has calculated that over 99.5% of the energy will enter via the roof glass and less than 0.5% through the end walls.
When operation resumed after the storm, performance had degraded nearly 12%. After measuring the performance with the soiled roof for a full day, the roof was cleaned overnight using the automatic roof washing system, and performance was measured again, Fig. 5. Performance had returned to the pre-storm baseline. Several conclusions can be drawn from these data. First, the automatic roof washing system effectively removes soiling, due to airborne dust. Second, that there was no significant soiling of the mirrors inside the glasshouse during the storm. Third, the enclosure allows normal operation in high winds, with negligible attenuation from airborne particles or loss of optical alignment.
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| Fig. 5. Performance before, during and after extreme weather events. |
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LESSONS LEARNED
During the course of the pilot, several lessons were learned regarding practical operation of the pilot. Many of these items have now been corrected, and the lessons learned incorporated into future designs.
Glass panes. By deploying conventional agricultural greenhouses, most of the risks of a new design were removed. However, the conditions in Amal have more extreme temperatures, both interior and exterior, than would be seen to grow any crops. Overall, the glasshouse design performed very well, and the design to allow expansion during diurnal temperature cycles was efficient. Only eight glass panes broke out of the many thousands that comprise the glasshouse during the initial months, which is normal for a new glasshouse. Since September, no additional panels have failed. When a glass pane fails, it is quickly detected from the pressure and flowrate through the air-handling unit, as positive pressure remains effective. The glass is tempered and forms small harmless “pebbles” of glass, and is replaced during the night.
Roof sealing. The sealing system for the roof using plastic weather strips did fail. This was due to high-temperature, differential thermal expansion and intense UV radiation. An all-aluminum sealing system was installed and has resolved the leak issues. A new design of this element will be used for all future installations.
Waste heat capture. It was initially envisaged that a hot water tank would be used to capture waste heat from excess condensate and shutdowns. However this was replaced with a heat exchanger to reduce system costs. In the end, the heat exchanger introduced transients in water temperature in the solar receivers that became unstable. To resolve this and increase overall efficiency, an 80-m3 hot water tank was added to the system. It was commissioned late in the year, but contributed to increased performance at year-end, and decoupled the system from periodic interruptions to the water supply.
Steam leaks were more of a problem than in a traditional fuel-fired steam system, which can largely be related to the daily cycles of operation from hot to cold. In particular, screw fittings led to steam leaks in cyclic operations, even where they fully complied with standard codes for design. As a solution, welded connections were used. In the future, where welds cannot replace flanged connections, engineered compression washers will be used. Wedge gate valves also caused problems, and sliding gate valves or metal-seated ball valves are recommended. Most of these leaks were discovered and resolved in the first six months of operations. These shortcomings have now all been addressed in the pilot, and specific design and component selection guidelines have been developed for future projects.
CONCLUSIONS
Through one year of operations, the pilot project has demonstrated the technical feasibility of solar steam generation for EOR in the Middle East desert environment. The solar radiation in most parts of Oman and GCC is at a high-enough level to produce solar steam suitable for EOR, and other oilfield and industrial applications.
Actual performance has matched the modeled performance to within a few percent, and steam output continues to exceed the contract target performance in all tests. The uptime of the solar field was 98.6% and continues to improve, to be over 99%. Due to the priorities of the commissioning and operations of a new thermal development, there were frequent field outages, and the system was tolerant of periods without power and water. Several enhancements have been made to the system to improve performance and reduce operating costs for future large-scale installations. These relate primarily to the sealing of the glasshouse, the mirror-aiming mechanisms, and the use of a tank to recover waste heat and water. In particular the insulated tank will be an essential part of future designs, as it increases uptime, efficiency and reduces water consumption. The system has proven its performance in a typical desert location in Oman. The passing dust storms and sand storms do not adversely affect the enclosed trough design. The roof washing system proved particularly effective in all weather events. 
REFERENCES
- van Heel, A.P.G., J.N.M. van Wunnik, S. Bentouahi and R. Terres, “The impact of daily and seasonal cycles in solar-generated steam on oil recovery,” SPE paper 129225, presented at the SPE EOR Conference at Oil & Gas West Asia in Muscat, Oman, April 11-13, 2010.
- Bierman, B., J. O’Donnell, R. Burke, M. McCormick and W. Lindsay, “Construction of an enclosed trough EOR system in South Oman,” paper presented at SolarPACES 2013, Las Vegas, Sept. 17-20, 2013.
- Bierman, B., C. Treynod, J. O’Donnell, M. Lawrence, M. Chandra, A. Farver and P. von Behrens, “Performance of an enclosed trough EOR system in South Oman,” paper presented at SolarPACES 2013, Las Vegas, Sept. 17-20, 2013.
ACKNOWLEDGMENT
This article is adapted from “Construction, operations and performance of the first enclosed trough solar steam generation pilot for EOR applications,” SPE paper 169745, presented at the SPE EOR conference held in Muscat, Sultanate of Oman, March 31-April 2, 2014.
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