Effect and mechanisms of chelating agents on pore structure variation and permeability enhancement of low permeability coal seams

China’s coal seams are characterized of low porosity and low permeability. To improve the production of coalbed methane (CBM) wells, chemical stimulation was proposed to enhance the connectivity of coal fractures and cleats by dissolving the cementing inorganic minerals, hance increase coal permeability. Considering the drawbacks of conventional acid such as severe corrosion and second precipitation, the possibility of using chelating agents for reservoir reconstruction was proposed. Taking coal samples from No.3 coal seam, Chengzhuang coal mine, Qinshui basin as research object, chemical titration, static dissolution experiment, micromorphology observation, core-flooding experiment, evaluation of corrosion rate of N-80 coupons and mercury intrusion porosimetry experiment were conducted. The corrosion effects of four chelating agents, EDTA, HEDTA, GLDA and MGDA, on minerals in coal under different pH conditions and the dynamic change process of permeability in coal cores during the process of expulsion were investigated, and finally the corrosion rates of chelating agents and conventional hydrochloric acid on the N-80 specimens were compared, and the influence law of the preferred chelating agents on the pore structure of coal and their permeability-enhancing action mechanism were clarified. Results showed that among the four chelating agents, HEDTA has the strongest chelating ability to Ca2+ and Fe3+; the coal dissolution rate decreases with the increase of the concentration of chelating agent under acidic conditions; the coal matrix soaked with 5% GLDA produces micro-cracks and pores, which is beneficial to the desorption and migration of coalbed methane; after injecting 80PV of 5% GLDA into the coal, its permeability increased 68.75%; the total porosity of the coal sample increased from 5.91% to 28.8% due to the increase of pore volume with diameter of 10-100 nm and >1000 nm; the corrosion rate of 5% GLDA to N-80 coupon is only 7.53% of that of 5% HCl, no additional corrosion inhibitor is needed, which is more cost-effective. The environmentally friendly chelating agent can effectively dissolve the inorganic minerals (calcite, iron dolomite, etc.) in the coalbed through acidification and chelation to generate stable and soluble chelating compounds, which can improve the permeability of the coal bed and reduce the secondary damage to the reservoir. These findings can provide valuable ideas for efficient exploration and development of low-permeability coalbed methane.