DOE: Regional Partnerships Find New Use for Unmined Coal

July 21, 2008 // Published as a news service by IHS

The project represents a promising strategy for safely storing this greenhouse gas while simultaneously increasing natural gas production in the region, said DOE.

The project is the first of the Regional Carbon Sequestration Partnerships to inject CO2 into a coal seam in the U.S.

The phase II pilot project, headed by the DOE Office of Fossil Energy National Energy Technology Laboratory (NETL) and the Illinois and Indiana State Geological Surveys, is testing the viability of turning unmined coal deposits into a source of useable energy by extracting natural gas, specifically coalbed methane, trapped in the coal.

According to DOE, the bituminous coals of the Illinois Basin have a high content of methane. The Illinois State Geological Survey estimates that there is up to 3.6 billion tons of storage capacity and over 10 trillion cubic feet of recoverable coalbed methane from unmined coal seams in basin.

Traditionally, operators drill production wells and, in a process called dewatering, pump water out of the coals to extract the gas. MCSG scientists are turning this research pilot project into a two-for-one opportunity by coupling the gas extraction with CO2 sequestration.

Dewatering does not work well with carbon sequestration; it limits the amount of CO2 that can be sequestered because the dewatering process creates a low-pressure environment for the injected CO2 that reduces storage capacity, said DOE.

The Illinois Basin project eliminates the need for dewatering. The project is designed to use one injection well and three production wells. Trucked-in CO2 is pumped through a heater and injected into the coal seam as a gas, increasing the pressure underground and desorbing the coalbed methane. CO2 is preferentially adsorbed, meaning that as the CO2 flows through the coal seam methane is displaced from the surface of the coal in favor of the CO2.

According to DOE, this method presents a major challenge: coal swelling. As CO2 is injected, coal swells to absorb the gas. CO2 molecules are larger than methane, and the coal can absorb three molecules of CO2 for every molecule of methane that is released. Injecting too much CO2 at once can lower the permeability of the coal, and it has the potential to reduce the injection rate and limit the overall amount of CO2 that can be injected and sequestered into the seam.

To alleviate coal swelling, MGSC scientists are field testing injecting the CO2 in pulses or cycles. CO2 is injected for eight- to 24-hour periods followed by a similar duration of shut-in to let the CO2 soak. This gives the CO2 time to adsorb, the pressure to dissipate and avoids any pathway congestion or roadblocks. At the end of the test, researchers anticipate up to 250 tons of CO2 will be injected over three months.

This project will also help demonstrate that geologic sequestration is a safe and permanent method to mitigate greenhouse gas emissions into the atmosphere. The Illinois Basin project will monitor the injected CO2 both in the lab and in the field, measuring changes in CO2 injectivity, the amount of CO2 that is retained by the coal and the amount of methane gas that is displaced by CO2.

The project is one of six small-scale pilot field tests being conducted by the MGSC during the current validation phase of the Regional Carbon Sequestration Partnerships program, which is managed by NETL. During this phase, field tests are being conducted to validate the most promising sites to deploy sequestration technologies. Other field tests are determining the ability of some oil fields to sequester CO2 while enhancing oil production and they are examining the injection of CO2 into saline formations a mile or more below the surface.

Source: DOE Office of Fossil Energy National Energy Technology Laboratory (NETL).