Enhancing biogenic methane generation in coalbed methane reservoirs – Core flooding experiments on coals at in-situ conditions

Abstract

Stimulation of microbial methanogenesis on intact coal under in-situ conditions is demonstrated in a series of 18 anaerobic core flooding experiments. The experiments used core samples from producing coalbed methane fields in the Surat Basin and Bowen Basin, with formation water containing methanogenic microbial consortia sourced from within the matching basin. The formation water was amended with nutrients containing phosphorus and nitrogen before being used in core flooding experiments of between 6 and 20 weeks duration. Since the core floods used degassed coal core at reservoir pressure, gas generated during the core flood could be adsorbed and retained by the coal. Therefore the quantity of methane generated was determined at the end of the experiment by decreasing the pore pressure and measurement of the volume of gas desorbed from the core sample. The amount of methane generated, averaged on a per week basis, varied considerably across the different core floods but was up to 7.75 μmol methane per gram of coal per week, comparable to previous studies where biostimulation experiments were conducted on crushed coal particles. During the core floods, the nutrient concentration was measured periodically in inflow and outflow water samples and the rates of nutrient consumption determined. The concentration of methane dissolved in the water samples was also determined and used as an indicator during a core flood of the relative rate of methanogenesis. Plateaus in gas generation were observed in the majority of core floods despite constant nutrient supply, indicating nutrient availability is not the single limiting factor to sustained methanogenesis. Higher gas generating core floods displayed higher rates of utilisation of phosphorus, underlining its importance in stimulation of microbial activity, while nutrient utilisation and gas generation measured in the absence of continuous nutrient delivery suggests the possibility of injection and production from a single CBM well. Results demonstrate the ability to stimulate gas generation on intact coal under reservoir conditions, an important step in development of technology for increasing gas content in depleted or undersaturated CBM fields.