Water influence on CH4 and CO2 generation from tar sandstones: Insights from incubation experiments in the Pirambóia Formation, Paraná Basin

Abstract

Processes controlling the biogenic production of CH4 and CO2 are relevant not only to environmental issues related to greenhouse gases emissions, but also in the energy scenario, since the biogenic generation of CH4 can be associated with the formation of natural gas reserves over time. This work aimed to evaluate the influence of water and calcium sulfate in the production of CH4 and CO2 from petroleum biodegradation in tar sandstones. Water saturation and sulfate concentration are taken to be important factors of influence in the oil biodegradation process. Long-term incubation experiments (up to 363 days) were carried out with samples of sandstone impregnated with high-viscosity hydrocarbons. The rocks used in this study were collected from outcrops of the Pirambóia Formation, a known heavy-oil reservoir of the Paraná Basin, Brazil. The net accumulation of both CH4 and CO2 was crescent during all the experiments performed, but with decreasing production rates through time. Average potential production rates obtained were 18 mL CH4/t.y (milliliter per ton of sandstone per year) and 54,000 mL CO2/t.y. The addition of water resulted in an average increase of 11% and 10% in the CH4 and CO2 cumulative production, respectively, in comparison with the dry incubation experiments. The water influence was stronger in the first 30 days of incubations when average production was around 70% higher in wet samples. Once the pH remained constant in the presence of calcium sulfate, the experiments indicate that calcium sulfate may function as buffer for pH, limiting acidification, which has a known negative influence on CH4 and CO2 production. Thus, the present work aims to alert for the potential of CO2 and CH4 production from tar sands in Brazil and for the importance of water as a positive influence on CH4 and CO2 generation from tar sandstones. This helps to evaluate the influence of environmental conditions on biogenic gas systems as well as on greenhouse gases emissions from geological sources, which are taken to be important sources of carbon to the atmosphere in a context of climate change.