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Gaining from a low-salt diet
When an oilfield grows old and too costly for production to continue, much of its oil may be left behind. But new techniques are increasing our ability to make the most of the world’s precious petroleum resources. One approach is to flush out more of the remaining oil with saline water. But we have learned to go easy on the salt.
Oil droplets that stick to clay in salty water let go when the water’s salinity level falls.
An oilfield’s production rate begins to slow after as little as 20% or so of its oil has come to the surface. Companies then often pump water into the field to sweep out more oil. Typically, they tap an available water source, such as seawater if the field is offshore; or groundwater produced along with the oil if onshore. In both cases the water is salty. “Saline water flooding enables us to get perhaps another 20% of the oil out of a field,” says Dave Brooks, Team Lead Improved Oil Recovery at Shell. “But we can do much better.”
Laboratory and field tests have shown that using less-salty water can further boost oil recovery by as much as 10 percentage points. The improvement varies from field to field, since it depends on the original composition of the oil, the rock and the water in the field.
Engineers take samples of rock containing oil from a field and flush water with different salt compositions through them to determine which works best. They then use that information to adjust the settings of new water-treatment equipment that Shell is now developing. The equipment combines a desalination unit with a water-softening system to ensure that the injected water contains exactly the right amount of salt.
“If this approach is adopted across Shell’s operations, we may be able to draw out millions of extra barrels of oil from our developed fields,” says Dave. “That’s like getting an additional decent-sized field.”