As the Alaska Oil and Gas Conservation Commission approaches a decision on how much production to allow for a natural gas pipeline from the Prudhoe Bay oil field, the commission's Cathy Foerster explained reservoir issues at stake in a presentation to the Alaska Natural Gas Development Authority's board.
In a Jan. 23 presentation, Foerster said there are three things that can happen in the reservoir when gas is produced that could impede the long-term recovery of oil.
One is that as gas is withdrawn from the large gas "cap" overlying the oil-saturated rocks in the field, the reservoir pressure in the cap declines. That causes oil fluids to move up from the oil strata below the gas into rock that previously had not held oil.
"The previously 'dry' reservoir rock becomes coated with oil. Although some of this oil can be produced, a substantial portion - in some fields 20 percent to 30 percent - sticks to the rock and will never come out," Foerster said. "In short, producing gas without replacing the gas cap fluids will cause some oil to stick to the rock and result in a decrease of ultimate recovery of oil."
A second factor is that as overall pressure in the reservoir declines, it loses its energy and its ability to produce.
"Typically operators of oil reservoirs maintain reservoir pressure and energy by re-injecting produced gas and injecting water to replace produced oil," she said. "They continue to do this until they have recovered all the oil. Then, when no commercially recoverable oil is at risk, they 'blow down' the gas cap. They do this because producing gas from an oil reservoir and not replacing it will result in a decrease of reservoir energy and, therefore, a decrease in oil recovery."
Finally, as the reservoir energy decreases, some of the oil changes from liquid to gas. The remaining oil becomes thicker. Think about soup cooking; as water evaporates the remaining liquid becomes thicker.
"In an oil field this thickening makes it harder for the oil to flow and thus decreases oil recovery," Foerster said. "We all know it is easier to suck water up a straw than molasses." It's similar with producing thick oil.
There will inevitably be tradeoffs with the Prudhoe Bay field, she said.
"The commission must balance oil recovery with gas recovery to ensure production of hydrocarbons, and this will be no trivial task," Foerster said.
Tim Bradner can be reached at tim.bradner@alaskajournal.com.
