Oilfield Technology correspondent, Gordon Cope explains how although combining enhanced oil recovery with carbon sequestration sounds like a win-win situation, significant hurdles stand in the way.

The oil and gas industry possesses billions of bbls. of hydrocarbons whose location is proven but still remain unrecoverable. Residual oil zones, or ROZs, are pockets of immobile oil within a reservoir that refuse to budge, even when water flooding is conducted to sweep oil toward production wells. Fortunately, under certain conditions, much of this oil can be extracted through the use of enhanced oil recovery (EOR) techniques, in which carbon dioxide (CO₂) is injected into the field.

CO₂ itself is a colourless, odourless gas. In addition to promoting plant growth, CO₂ acts as a greenhouse gas (GHG), trapping heat in the atmosphere. Mankind primarily emits CO₂ through the burning of fossil fuels. Since the industrial revolution, the amount of CO₂ in the atmosphere has climbed from 280 ppm to 400 ppm. Some climate change scientists say that unless this trend is reversed, higher temperatures will cause sea levels to rise, species extinction and significant environmental degradation.

Many countries throughout the world have taken steps to mitigate CO₂ emissions through the promotion of renewable energy, fuel-efficient vehicles and the switch from coal to natural gas for electricity production. But these measures are insufficient, and other large-scale schemes are deemed necessary. Much attention has focused on carbon capture and storage (CCS), in which carbon emissions at large utility and industrial plants can be pulled out of the air and disposed underground.

Clearly, the captured CO₂ has tremendous value as a way to increase oil production. But to establish that value, a number of serious challenges need to be addressed.

How EOR works

When first production occurs, petroleum reservoirs often contain sufficient formation pressure to push oil into the wellbore, where it can be lifted to the surface by pumps. This is called primary recovery, and can amount to approximately 33% of oil-in-place. The life of an oilfield can be extended through secondary recovery, which often takes the form of a water flood, in which water is injected laterally to push oil toward a production well. Typically, secondary techniques can double recovery rates, and offshore fields in the North Sea achieve recovery rates of around 60% with water floods.

Some fields can be further produced using EOR techniques, including surfactants, polymers and miscible gas injection. In a miscible gas injection EOR, natural gas or CO₂ is injected under high pressure into a reservoir. The CO₂ acts as a solvent in the oil, reducing its viscosity, so it can mobilise more easily. It also reduces the interfacial tension bond between the remaining oil and the reservoir rock. Finally, it swells the oil, so the reservoir pressure increases. Miscible gas projects typically last over several decades and increase oil recovery by 7 - 15% of original oil in place.

Most of the CO₂ EOR projects are found in Permian Basin fields located in Texas and adjacent states. This is due to the presence of several large, natural CO₂ deposits (such as the McElmo Dome and Sheep Mountain) that contain abundant supplies of pure gas that can be inexpensively shipped by pipe to reservoirs. In 2012, the region produced approximately 350 000 bpd of EOR oil.

But the Department of Energy estimates that ROZs could contain 100 billion bbls. of the 1.124 trillion bbls. of technically recoverable oil in place in US reservoirs, and the consistently high price of oil in recent years has spurred companies outside of the Permian Basin region to coax that oil out. Rex Energy announced that it was targeting the Lawrence field in southern Illinois, which held 1 billion bbls. of oil in place. Since the 1950s, 400 million bbls. have been produced, leaving plenty of ROZs. Rex is currently operating an alkali-surfactant polymer flooding on a pilot site, and expects to launch a full-scale injection in latter 2013.

Other companies are relying on CO₂ that is stripped from natural gas. Occidental Petroleum used to release the CO₂ separated at its Sandridge plant in Texas into the atmosphere, but since 2010, it has been capturing 5 million tpy and shipping it to nearby oilfields for EOR. In China, CNCP began a commercial CO₂ EOR flood in the Jilin Oil Field in 2009. Approximately 200 000 tpy is extracted from a natural gas processing plant and transported 200 km to the field. CNCP expects to increase volumes to 1 million tpy by 2015.

Part 2 of this article can be reached here.

Adapted by David Bizley from an article in the September 2013 issue of Oilfield Technology. Sign up for a free trial here.

Read the article online at: https://www.oilfieldtechnology.com/drilling-and-production/18092013/what_can_you_do_with_co2_part_1/