Changes in Gas Storage and Transport Properties With Continued Bioconversion of Coal

Abstract

This study was aimed at identifying the changes in coal properties affecting gas deliverability in coal-gas reservoirs, when treated with microbial consortia to generate/enhance production of methane. The work expanded on the technology of bio-conversion, first proposed by Andrew Scott in order to imitate the natural/microbial process of biogenic gasification leading to recharging coalbed methane reservoirs or setting up natural gas reservoirs in non-producing coalbeds, to coal waste, typically in the form of fines/ultra-fines. The pressure parameter was considered critical since, with continued production of methane, the produced gas would diffuse into the coal matrix and get adsorbed with increasing pressure. During production, the pressure would decrease and the process is reversed, gas diffusing out of the coal matrix and arriving at the cleat system. The experimental work tested the sorption and diffusion properties for the coal treated and, more importantly, the variation in the relevant parameters with continued bio-conversion since these are the first two physical phenomena in CBM production. During the first phase, single component sorption-diffusion experiments were carried out using methane and CO₂ on virgin coals retrieved from the Illinois basin. Coals were then treated with nutrient amended microbial consortia for different periods. Gas production was monitored over thirty and sixty days of treatment after which, sorption-diffusion experiments were repeated on treated coals, thus establishing a trend over the sixty-day period. The sorption data was characterized using Langmuir pressure and volume constants, obtained by using the Langmuir model. The diffusion coefficient, D, work also established the variation trend as a function of pore pressure. The results indicated an increase in the sorption capacity of coal as a result of continued bioconversion. This was attributed to increased pore surface areas as a result of microbial actions due to change in the pore size or creation of new pores. It was further shown that the rate of diffusion increased, especially for methane, which exhibited rates higher than that for CO₂. These findings clearly support improved gas storage capacity with continued bio-version as well as significantly enhanced diffusion rates. As a continuation of this, change in permeability, the second gas transport phenomenon in coal-gas production, is being evaluated.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016