The amount of crude oil remaining in depleted oil fields has been estimated to be about 50 – 80% of the total estimated oil reserves. In the past, CO2 has been injected into such wells for enhanced-oil recovery — a technique that displaces the oil and stores the greenhouse gas. Now, a completely different approach to monetizing the carbon is being considered by Inpex Holdings Inc. (Tokyo; edlinks.chemengonline.com/6902-536). The idea is to use the depleted wells as bioreactors to convert the oil and CO2 into natural gas (mainly CH4).
In the concept, pressurized CO2 is injected into the depleted well. There, two types of naturally occurring microbes metabolize the carbon sources at about 50°C; one type producing hydrogen from the oil, the second type converting the H2 and CO2 into CH4.
In water drawn from the depleted oil fields at Yabase, Akita, Japan, the researchers have discovered ten specific H2 -producing microbes and four specific microbes that produce CH4. Inpex, in cooperation with Chugai Technos Co. (Hiroshima) and support from JOGMEC (see p. 14), has demonstrated in the laboratory that cultivating the specific microbes at 55°C and 2 to 50 bar of nitrogen and CO2 (9-to-1 mole ratio), crude oil is converted to CH4 after about 200 days. In bench-scale tests, a CH4 -production rate of 0.13 NmL/h is achieved from 1 L of oil-well brine taken from a depleted well.
Inpex is looking for ways to increase the methane production rate, and plans to apply the technology to its existing depleted oil fields in Japan in the early part of the next decade.