Influence of Thermal Cracking Permeability Enhancement in Spontaneously Combustible Coalbed Methane Reservoirs on the Characteristics of Coal Spontaneous Combustion

ABSTRACT

Thermal cracking permeability enhancement (TCPE) is a method to improve the coalbed methane (CBM) extraction efficiency and yield in low-permeability CBM reservoirs, but this method can change the physical and chemical properties of coal and thus affects the characteristics of coal spontaneous combustion. In order to explore the influence mechanism of TCPE on the characteristics of coal spontaneous combustion, the evolutions of oxygen adsorption capacity and chemical groups of coal under different thermal cracking parameters were measured by experiments. The following research results were obtained: On the whole, the pore and free radical structures of coal are changed after TCPE at low temperature and for a short time. The oxygen adsorption capacity and chemical activity of coal are increased, which leads to the increase of the risk of coal spontaneous combustion. However, when the coal is subjected to long-term high-temperature TCPE, the oxygen absorption capacity, and chemical reactivity are weakened. Ultimately, the risk of coal spontaneous combustion and the intensity of oxygen composite reaction become lower compared with the original state of coal. Based on these findings, it is suggested to consider long-term high-temperature thermal cracking during TCPE in CBM reservoirs.

KEYWORDS:

• Thermal cracking permeability enhancement
• coal spontaneous combustion
• CBM extraction
• free radicals
• pore structure

Highlights

1. TCPE increases the adsorption capacity of oxygen while increasing the porosity of coal.

2. TCPE can increase the number of free radicals in coal, thus increasing the potential of spontaneous combustion.

3. Different parameters of TCPE have different effects on the risk of coal spontaneous combustion.

4. Higher temperature and longer time TCPE can reduce the hidden danger of coal spontaneous combustion.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [Grant Nos. 52074118, 51774135 and 51974120], Natural Science Foundation of Hunan Province [Grant No. 2019JJ50152], Scientific Research Fund of Hunan Provincial Education Department [Grant Nos. 18B210, 19A167 and 20A192], Scientific & Technological Innovation of Hunan Transport Department [Grant No. 201943].