RESULTS OF SOIL GAS AND GROUNDWATER MONITORING AT A CARBON SEQUESTRATION TEST SITE, BLACK WARRIOR BASIN, ALABAMA

The Pottsville Formation in the Black Warrior basin of Alabama contains important coalbed methane reservoirs that are potential sinks for carbon dioxide. Injection of CO₂ can further be used to enhance coalbed methane recovery, which could increase reserves by more than 20 percent. An injection test, sponsored by the Southeastern Regional Carbon Sequestration Partnership (SECARB) and El Paso Exploration and Production, Incorporated, was completed in Blue Creek Field during 2010. In preparation for this test, a study of soil gas composition and soil CO₂ flux was conducted at a control site in Deerlick Creek Field. Soil CO₂ flux around the injection well in Blue Creek Field was monitored before, during, and after injection. A groundwater well was set in the shallow aquifer at the injection site to monitor water quality.

At the Deerlick Creek control site, soil gas samples were collected at each monitoring location at four depths from 21 stations. The carbon isotopic ratios of the soil CO₂ suggest a bacterial source and that bacterial activity intensifies downward in the soil profile. Wetter gas in the soil profile than in the adjacent well indicates these gases are locally derived.

Soil flux was monitored for 10 months at the control site; at the injection site, soil CO₂ flux was monitored for 17 months. Soil flux was highly variable among stations and over time. A seasonal pattern was identified with highest flux during summer and lowest flux during winter. No change in soil CO₂ flux was observed during or after injection.

The shallow groundwater aquifer, used by some local residents for drinking water, was sampled monthly from February through January 2011. Samples were analyzed for organic and common inorganic nonmetal and metal constituents. Results indicate no change in groundwater quality.

It is important to document the baseline groundwater quality, soil gas composition and CO₂ flux ranges for comparison to any sampling during or post-injection. Continued monitoring of groundwater, soil gas and CO₂ flux can augment other monitoring activities, helping confirm containment of the gas. Changes in flux rate, soil gas, or groundwater quality could indicate the need for further investigations. Additionally, soil gas composition can also help determine the origin of the gases and the potential for leakage.