Water-free oil extraction - More efficient and economic drilling
A carbon dioxide thickener could make crude oil extraction more efficient and economical by precluding the need for water.
Current extraction methods usually involve pushing oil from underground sandstone or limestone reservoirs using a combination of water and CO2, in what is known as the water-alternating-gas method. Without the water, CO2 is not viscous enough and tends to ‘finger’ through the oil rather than sweep it up to the production well, resulting in an inefficient rate of recovery from the field.
In an effort to engineer a way around this, Professor Robert Enick from the Department of Chemical and Petroleum Engineering at the University of Pittsburgh, USA, has developed a small molecule thickener that could increase the viscosity of the gas by up to 100 times.
Made from inexpensive oxygenated hydrocarbons or silicones, the molecules dissolve in and thicken the CO2. Enick explains that there are two basic ways to thicken any fluid. One involves using ultra-high molecular weight polymers, however, ‘because CO2 is a feeble solvent compared to water and oils, this is not a promising path for even the most carbon dioxidesoluble known polymers,’ he says. ‘The second path, which our project focuses on, involves the design of small molecules with a solvent-philic segment and solvent-phobic segment that not only dissolve in the solvent, but also self-assemble into long, narrow cylinders, lines, rods or helices. These aggregates of dissolved molecules give the illusion of an ultrahigh molecular weight polymer and are an effective thickener.’
At present, there are no other commercial carbon dioxide thickeners, although Enick and his colleague Eric Beckman previously made one that was a high molecular weight polymer composed of fluoroacrylate (CO2–philic) and styrene (CO2–phobic associating group). ‘It was a technical success, but it was extremely expensive and therefore could not be used in the oilfield,’ he says.
To put into practice, the new method will require the use of a high-pressure mixer to introduce the thickener to the pipeline. ‘It may be a powder, a liquid or a sticky compound,’ says Enick. ‘In the best case, the thickener would start out as a low viscosity liquid that could be easily pumped and readily dissolved in CO2, with the thickening properties of the molecule becoming activated via reaction with the gas.’
According to Enick, the main benefits of using a thickener are that water injection can be greatly reduced if not eliminated altogether, the amount of CO2 required to produce a barrel of oil can be decreased by up to 50%, and more oil can be economically recovered from the formation.
As our easy-to-access resources diminish, the use of flooding and the need for thickeners is only going to increase, says Enick. ‘CO2 flooding accounts for roughly 5% of domestic oil production and that percentage continues to increase each year. It is currently limited by the supply of natural CO2, therefore if we can produce more oil with less CO2, more of these type of projects can be initiated. CO2 flooding is the most rapidly growing improved oil recovery technique.’
He adds that if CO2 sequestration is ever mandated by legislation, flooding will be the first destination for the CO2 from coal-fired power plants. ‘When the CO2 flood ends after several decades, a vast amount will be trapped within the oil-depleted sandstone or limestone formations.’