Using Solar to Produce Oil
We’re all familiar with solar as a way to generate electricity and hot water, but now it is being tested as a way to produce oil in one of America’s oldest oil fields. I recently took a Chevron-sponsored trip to their demonstration plant near Coalinga, in California’s Central Valley. The plant is the first in the world to try using steam generated by a solar thermal plant for enhanced oil recovery (EOR).
Constructed by solar thermal leader BrightSource Energy and completed in August 2011, the 29 megawatt plant uses an array of 7644, 7-foot by 10-foot mirrors mounted on 3822 heliostats to track the sun throughout the day and focus it on a 327-foot-tall central tower.
There, a closed-loop system heats to 700° F, which is then run through a heat exchanger to turn water pumped up from the oil field into 500° F steam, which is in turn injected into the field to release the oil.
The heliostats are independently controlled by an automated system, and can tolerate up to 25 mph winds in full operation. In the event of a power outage, an on-site uninterruptible power supply turns the heliostats away from the tower, and then a generator fires up and circulates the fluids in the tower until the system cools down.
In order to minimize the amount of water needed to keep the mirrors clean in this dry desert environment, Chevron uses an Israeli-made machine that needs less than 1 liter of water to clean both mirrors on a single heliostat. Using de-mineralized water avoids the need to squeegee the mirrors and leaves no spots behind.
Steam injection, or “steamflooding,” is one of several EOR methods used to extract the last dregs of old oil fields. This particular oil field was first discovered with a well dug by hand in 1899, and entered production in the 1890s. Production quickly ramped up to a peak around 40 thousand barrels per day (kbpd) in the early 1900s, then went into a typical long decline as the natural pressure of the field began to fall. Only about 5 to 10 percent of the oil in the field can be produced with “primary” recovery under natural pressure. Then hot water flooding was initiated, which is able to extract 15 to 25 percent of the oil. Steamflooding, which is thought to be able to extract 50 to 80 percent of the oil, began in 1964 and pushed production up to a new peak around 140 kbpd by 1990.
The steam spreads out over the top of the oil-bearing sands below, raising their temperature to about 250° F and lowering the viscosity of the thick, gooey “heavy oil” they contain. (Oil from this field weighs in at 12 to 13 API specific gravity, meaning it can just barely float in water.) The oil then drains down into production wells at 2000 to 4000 feet of depth, where it is pumped back to the surface. The steam essentially “cleans” the sands from the top-down.
Today, the roughly 10-square-mile Coalinga oil field produces around 7 kbpd from over 800 active wells. The oil is separated from the produced water, and sold to Shell. The water is cleaned up and recycled back through the system to be turned into steam again. Total recovery is thought to be only around 50 percent so far, so there is a lot of oil left to produce from this over 100-year-old oil field.
However, making the 52,000 barrels of steam needed each day is expensive. Optimizing oil production with the minimum amount of steam is a key operational objective, and Chevron has focused a great deal of effort on getting it right. They now claim to have a steam-to-oil ratio of around three—about half that of its nearest competitor, AERA.
Today the steam is generated from a combination of older traditional natural-gas fired boilers, plus three newer co-generation units. The co-generation units run 24×7 and use natural-gas fired turbines to produce 3 megawatts of power plus 4,000 barrels of steam per day each, and are 98 percent efficient.
Together they generate about 48 megawatt-hours a day of excess power, which is sold to the local utility, PG&E. In total, the field consumes about 33,000 thousand cubic feet (mcf) of gas per day, most of which is obtained on-site (natural gas is formed by the same process as oil and is commonly found associated with it in an oil field).
Despite occupying about 100 acres of land, the new solar thermal plant contributes a small portion of the field’s steam needs. It produces about 350 barrels of steam per hour when in operation, or about 5 percent of the total steam needs of the field, according to Chevron. Construction supervisor Ray Guidry says the system offsets about 225,000 million BTU worth of gas each year, which I calculate to be just under 2% of the field’s total gas consumption. At full daytime output, the solar plant takes the place of just 2.6 of the 11 steam generators used in the field.
Although Chevron would not disclose the cost of the solar plant, it is highly unlikely that it pencils out in that particular location, particularly when natural gas is currently trading at a historical low around $2 per mcf in the U.S. Even so, the solar plant probably offsets around half a million dollars worth of gas per year, and more importantly, about 12,000 metric tons of carbon dioxide.
The real value of the Coalinga project is to show that solar can provide the process heat needed to perform EOR in a heavy oil field, and yield valuable information about how to optimize that process. With that knowledge in hand, Chevron will be able to apply its technological know-how to unlock the substantial heavy oil resources that exist around the world, particularly in places where gas is either unavailable or too expensive to be a practical heat source. The same technology may also be useful at Chevron’s Kern River heavy oil field about 100 miles away in Bakersfield, which currently uses gas piped in from Northern California and is currently evaluating PV as a way to power its oil pumps.
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