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Thermal Modeling of Microbial Methane Generation Constrained by Laboratory Experiments

Sandu, Constantin *1; Bissada, Adry 2
(1) Lunar and Planetary Institute, Universities Space Research Association, Houston, TX.
(2) Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX.

Microbial methane gas can comprise a significant portion of many commercial natural gas accumulations worldwide. For this reason, understanding its mode and rate of biogenic gas generation is critical in forecasting the occurrence and magnitude of these types of gas reserves. The process of biogenic methane generation is very complex and involves a series of successive chemical reactions carried out by bacterial microorganisms whose metabolic rates are extremely difficult to be predicted from simple kinetic equations. Though temperature plays an important role, its effect on microbial productivity is neither linear nor monotonic. We derived a simple methanogenesis model based on theoretical biogeochemical concepts in order to predict the volumes of gas generated within a temperature interval relevant for the biogeochemical cycle. The model was calibrated against laboratory simulations performed on organic-rich samples that underwent microbial degradation in a controlled environment. Experiments were conducted at 25°C, 30°C and 45°C over a time period of 200 days. The simulations revealed significant systematic variation in CH4 and CO2 yield with time and temperature. The systematics helped constrain our theoretical model and allowed us to tune the empirical relations and to scale the generation rate with temperature. The results suggest that methanogenesis attains peak efficiency at about 30°C. The reaction rate for CH4 generation correlates strongly with the level of free H2 in solution, supporting the theory that CO2 reduction dominates microbial CH4 production. Carbon isotopic composition of CO2 and CH4 was also monitored over time. The observations indicate evolution towards the isotopically heavier carbon (more enriched in δ13C) over time. The trend suggests that certain types of organic substrates might be prone to generate isotopically heavy microbial methane to start with.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California