Key pretreatment technologies for coalbed gas bioengineering and review on their mechanism of action

Coalbed gas bioengineering (CGB) is a new technology for the purpose of realizing the clean utilization of coal resources and the enhanced recovery of coalbed gas. To explore the key characteristics of the pretreatment technologies of CGB, hydrocarbon generation potential and its mechanism of action, conclusion and summary was made for various pretreatment technologies herein. Meanwhile, the pretreatment technologies were divided into three types according to the action mode of coal (organic matter) and bacteria (microorganism): microflora optimization and improvement, coal dissolution and oxidation, and biological stimulation and synergy. By comparing the capability of coal to generate methane through microbial degradation with different pretreatment technologies, it is considered that different pretreatment technologies can stimulate the yield of biomethane. Specifically, the microflora optimization and improvement technology has the best effect on stimulation, reaching 428.6%-2 335.2% (1 025.4% on average), which realizes methane stimulation by increasing the abundance of methanogens or the number of bacteria, suitable for the bituminous coal reservoir and the conditions with sufficient trace elements. Then, the coal dissolution and oxidation technology ranks in the second place with methane increased by 17.6%-733.3% (223.4% on average), of which the mechanism of action is to increase the hydrolysis sites of biological enzymes, reduce the intermolecular force of coal, shed the oxygen-containing functional groups, lower the coal crystallinity and destroy the aromatic structure of coal, so that the coal is easier to be used by microorganisms. Therefore, it is suitable for high-rank coal reservoir and the conditions with low porosity and low permeability. Last one is the biostimulation and synergistic technology with methane increased by 28.3%-620.5% (148.6% on average), which is to increase the biomethane by stimulating the bioenzyme activities in microorganisms and increasing the contact area between bacteria and coal in different ways, thus suitable for the lignite reservoir and the guanidine gum fracturing conditions. In general, the research results are of great scientific significance for enriching the basic theory of CGB and guiding the field engineering application.