ENHANCED oil recovery helps boost output in old U.S. wells

Sunday, May 20th 2001, 12:00 am
By: News On 6

U.S. oil production is plummeting while billions of barrels remain locked underground away from conventional pumps.

An estimated 67 billion barrels have not been discovered in the United States. But the country's known fields have pumped 168 billion barrels in the past century and a half. Technology exists to pump 23 billion more barrels from those places.

Left among known reserves are 377 billion barrels that conventional pumps cannot get out.

The U.S. Department of Energy, industry and university researchers nationwide are digging for ways to pump what remains, said Jerry Casteel with the DOE's National Petroleum Technology Office in Tulsa.

Enhanced oil recovery, or EOR, accounts for about 600,000 barrels of daily U.S. oil output, now at 5.85 million barrels. The country now imports more oil than it produces.

Companies consider enhanced oil recovery projects when prices are consistently higher and they think the investment will yield enough additional oil to pay the sizable expense and generate profits.

New wells depend on underground pressure to push oil out. But pressure falls as production dwindles. Oil companies then look at ways to get at oil locked away in the rocks.

Researchers are looking at vibration technology to shake oil loose from rocks. Other methods are more common, but still limited.

EOR processes do not work in all fields because of the nature of the underground rock formations and other factors. Common recovery methods include:

-- Water-flood. This technique has become widespread in older U.S. fields in the past 50 years and is not considered EOR, but a secondary way of boosting production.

Water is pumped down injection wells to the oil-bearing rock layer. The water adds pressure in the formation and pushes the oil from the rock to a nearby producing well. Once there, the oil and water are pumped out and separated by equipment on the surface.

Water-flood will not work in some fields where pores in the rocks are especially tight, Casteel said. The process is thought to leave two-thirds of the oil behind.

-- Carbon dioxide. Used in western Texas fields, carbon dioxide is more costly and not as available for EOR projects. But it can be more effective than water. Carbon dioxide has a tenth of the thickness of water and can penetrate tighter spaces.

If pressures are high enough, carbon dioxide pumped down an injection well will dissolve oil from the rock pores and cause it to be driven to a nearby production well.

But pressure must be 1,000 psi and wells have to be at least 2,000 feet deep to maintain adequate pressure without fracturing, or breaking, the rock, Casteel said.

Once the rocks break, larger cracks or crevices form for the fluid to flow through. Then it becomes harder for the thin carbon dioxide to sweep through portions of the rock with tighter pores and greater resistance to the flow.

-- Polymers. Polymers are chemicals that can be mixed with water to do the things water and carbon dioxide do, wash oil from the rocks to a production well.

Polymers thicken the water. Regular water is often injected behind it so the water will go through parts of the rock where it hasn't gone before.

-- Surfactants. Surfactants, or soaps, mixed with water can help free up oil through similar processes of water-flood or carbon dioxide. They basically do what soap does _ wash oil from the rocks.

-- Microbial EOR. This means putting microbes into the oil formation. When microbes eat on the oil, they generate surfactants _ soaps _ that make the oil flow. Microbes also generate polymers that help with production.

-- Thermally enhanced recovery. The use of steam injections to free up deposits of thick oil. Oil becomes thinner when it's heated. The method is used widely in the heavy oil fields around Bakersfield, Calif.