An experimental study on oxidizer treatment used to improve the seepage capacity of coal reservoirs

It is difficult to stimulate coal reservoirs in physical approaches, and the recovery factor of coalbed methane (CBM) can be enhanced by applying strong oxidizers through oxidation to stimulate coal reservoirs. At present, however, there have been very few studies on the oxidation of CBM and fewer experimental studies for systematically evaluating the effect of oxidation on the seepage capacity of coal reservoirs. In this paper, the coal samples taken from coal seams of the Jurassic Xishanyao Formation in the Heishan Coal Mine, Toksun, Xinjiang, were selected as the study objects. Hydrogen peroxide solution immersion experiments were carried out on columnar and powdered coal samples, respectively to measure the permeability of columnar coal samples, the dissolution rate of powdered coal samples and the property parameters of hydrogen peroxide solution. Then, the reaction mechanisms between coal samples and hydrogen peroxide and the mechanisms to improve the seepage capacity of coal reservoirs were analyzed by means of X-ray diffraction (XRD), scanning electron microscope (SEM), infrared spectrum and wetting angle measurement. Finally, the stimulation effect of oxidation was compared with that of acidification. The following research results were obtained. (1) The permeability of coal samples is increased significantly after oxidation to 1.4–3.2 times the original permeability. (2) A large number of micro-fractures and dissolved pores are formed in the coal samples after oxidation, and consequently pore connectivity is improved greatly. Thus, the amount of associative hydroxyl and carboxyl functional groups on the surface of the coal samples increases, and the water wettability on the surface reduces. (3) Organic matters and pyrites are oxidized and consumed easily, and the generated H+ and micro-molecular aliphatic acids further dissolve inorganic mineral components. (4) Oxidation also has the advantage of acidification for dissolving inorganic mineral components, so there is a low and controllable probability of generating coal powder. In conclusion, applying strong oxidizers has the potential to become a new technology for coal reservoir stimulation. Keywords: Coal rock, Oxidation, Permeability, Pore connectivity, Acidification, Micro-fracture and dissolved pore, Dissolving inorganic minerals, Coal reservoir stimulation