SOIL GAS MONITORING FOR CARBON SEQUESTRATION THE BLACK WARRIOR BASIN, TUSCALOOSA COUNTY, ALABAMA

Coal beds of the Pottsville Formation in the Black Warrior basin of Alabama are important coalbed methane reservoirs and are potential sinks for carbon dioxide. In addition to CO₂ sequestration, there is potential for enhanced coalbed methane recovery that could increase reserves by more than 20 percent. In preparation for injection testing sponsored by the Southeastern Regional Carbon Sequestration Partnership (SECARB), a study of soil gas composition and soil CO₂ flux around an active coalbed methane well was conducted in Deerlick Creek Coal Degasification Field. A sampling pattern, consisting of 21 monitoring locations was established within a 150-meter radius of the Jobson 24-14 #11 well (Alabama State Oil and Gas Board Permit 4001-C).

Soil gas samples were collected at each monitoring location at four depths (surface, 30cm, 60cm, and 60cm). The samples were analyzed for nitrogen, argon and oxygen, and carbon dioxide concentrations, as well as the concentration of several light hydrocarbons and the isotopic composition of the CO₂. The soil gas contains on average 78% nitrogen, 21% oxygen and argon, and less than 1% carbon dioxide; the coalbed methane produced by the Jobson well contains only 0.01% CO₂. The carbon isotopic ratios of the soil CO₂ suggest a bacterial source and that bacterial activity intensifies downward in the soil profile. The soil contains between 0.46 ppm and 1,103 ppm hydrocarbons, primarily methane and ethane. The soil hydrocarbons have a dryness index of 0.98, which is less than the dryness index of gas produced from the Jobson well (dryness index = 1.00). Wetter gas in the soil than at reservoir depth indicates these gases are locally derived.

Soil flux was monitored for 10 months. Measurements from two stations were taken weekly, and all stations were measured monthly. Soil moisture and temperature correlate negatively. Soil flux was highly variable among stations and over time. Differences of up to 7.77 mmols/m²/s² were measured at two stations on the same day and of 5.13 mmols/m²/s² were measured at a single station from one week to the next. A seasonal pattern was identified with highest flux during summer and lowest flux during winter. This pattern correlates with soil moisture during warm, dry weather.