Occurrence Mechanism and Risk Assessment of Dynamic of Coal and Rock Disasters in the Low-Temperature Oxidation Process of a Coal-Bed Methane Reservoir

The aim of this research is to explore the occurrence mechanism of dynamic disasters of coal and rocks in low-temperature oxidation of coal-bed methane (CBM) reservoirs. For this purpose, the occurrence probability of dynamic disasters at different oxidizing temperatures was assessed by using the comprehensive predictive index K for dynamic disasters involving coal and rocks. By employing nuclear magnetic resonance (NMR) technology, the evolution law of diameter and quantity of pores inside coal was detected during low-temperature oxidation of coal masses. In addition, fracture development of coal masses was monitored and analyzed by applying various instruments, including an instrument for measuring Δ P, hardness tester, gas chromatograph, and measuring system for rocks using the P-wave. The results showed that both the diameter and quantity of pores inside coal increased with the rise of the oxidizing temperature of the coal masses: the porosity increased by 72.2% as the temperature rose by 200 °C. The gas chromatograph and industrial analytical experiment proved that the whole fracture development process of coal masses was divided into two stages during low-temperature oxidation. In the initial stage of low-temperature oxidation (30–130 °C), water inside coal masses was lost and evaporated, resulting in the expansion and connection of micropores to mesopores. In the later period of low-temperature oxidation (130–230 °C), mesopores expanded and connected to macropores and microfractures because of oxygenolysis of the macromolecules and volatiles in coal. On the basis of the comprehensive predictive index K for dynamic coal disasters, the occurrence probability of dynamic disasters in low-temperature oxidation of CBM reservoirs was verified to increase with the increasing oxidizing temperature according to the measured data. In addition, the maximum allowable oxidizing temperature of the CBM reservoir where the specimen was collected was 130 °C.