When he says "it's good to the last drop," Wayne Ahr isn't talking about his morning coffee. Research by this Texas A&M University professor aims to squeeze some of the last remaining drops of oil out of mature fields, potentially gaining needed energy reserves from sources drillers thought were used up.
"An enormous amount of oil remains in the ground at developed oil fields," Ahr said. "Only about 30 percent of the available oil is recovered from any field during 'primary recovery'. Nearly 70 percent of the oil stays there in the rock where it was trapped."
"Recent improvements in geological, geophysical, and engineering methods have enabled us to discover and develop a tremendous number of new fields worldwide," he observed. "Now we need to devise methods to allow us to go back and recover between one-third and one-half of the oil that primary recovery left in the ground."
Ahr, a petroleum geologist, is part of a project funded by the U.S. Department of Energy to develop those methods. Working with Texas A&M petroleum engineer Tom Blassingame, computer modeling experts at the University of Alabama in Tuscaloosa, geophysicists at McGill University in Canada, and industry experts from smaller, independent oil companies, Ahr shares a $1.3 million grant to figure out how to get to that residual oil.
"Most people think oil lurks beneath the surface of the ground in underground lakes, just waiting to be sucked out like soda through a straw," Ahr said. "Actually, the oil is embedded in porous rocks, much as water saturates a sponge. Residual oil - forming the 70 percent that primary recovery leaves behind -- sticks to the rock grains and has to be forced out with another fluid.
"The channels connecting oil-containing pores between the rock grains can be connected in any number of ways," he observed. "If the pores dead-end in the rock, no amount of pressure can force the oil out. However, if the rocks' underground plumbing is connected just right, we can use its natural structures to channel oil to the wellbore."
Another problem involves finding the right fluid to use in flushing the rocks.
"We're exploring the use of detergents, somewhat like dish-washing liquid," Ahr said. "Just as dish detergent cuts grease on your plates, it can help pry oil from rock grains. The oil will then mix with the detergent and be pumped to the surface, where the two liquids can be separated and the detergent recycled to be used again."
So far improved recovery methods have been more expensive than new exploration techniques.
"But as we discover novel techniques to reduce costs, and as it gets harder and more dangerous to explore for oil in far-flung, potentially politically hostile environments, I believe oil recovery from mature fields will become more attractive to large corporations," Ahr said.
Ahr's current project focuses on two mature oil fields in Alabama.
"Tom Blassingame and I are looking for new ways to dramatically improve the amount of oil recovered," Ahr said. "We know exactly what we want to do. Mapping the size, shape, and configuration of the rock pores is my part of the project."
Ahr slices cores - plugs of rock from bore holes - into micro-thin sections and examines them inch by inch under the microscope at medium, then high magnification, tracking the size and shape of the rock pores.
"I want to construct 3-D pictures of an oilfield's underground plumbing," Ahr said. "Then petroleum engineers will take over, honing in on specific 'flow' zones and figuring out how to introduce outside fluids to move the oil through the reservoir to the surface."
Next, University of Alabama experts will use computer programs to simulate the recovery process. The McGill group will take the lead in using seismic records from the fields to help determine which rock zones are 'flow' units.
"Computer technology means we don't need a rock sample - a borehole core -- for every location," Ahr said. "Once we have an idea of what the pores look like in particular sections of rock, we can concentrate on forcing the oil out of the larger, more easily accessible pores first, then use fluids such as detergents or gases like carbon dioxide dissolved in water to bubble the oil loose from the smaller pores, where it sticks more tightly to the rock grains."
Ahr, a certified petroleum geologist, is Williford Professor in the College of Geosciences, Department of Geology and Geophysics, at Texas A&M.
"If we can find a way to reduce the cost of forcing this residual oil from these rock sponges, we'll improve recovery of a potentially enormous asset," Ahr said. "If we can get the 'leftover' two-thirds of all the oil in old fields at a price that is competitive with exploration, we could change the whole ballgame of petroleum economics."