Investigation of the Evolution of Stratum Fracture during the Cavity Expansion of Underground Coal Gasification

Abstract

The evolution of fracture zone controls the safety of underground coal gasification (UCG) in terms of gas emission and water leakage. In order to understand the fracture propagation in the confining rock of a UCG cavity with various influence factors, this paper implemented a set of numerical models based on different geological and operating conditions. Analysis was implemented on the mechanism of fracture propagation and its evolution characteristics, suggesting that (a) continuum expansion of the cavity leads a near-field fracture circle in confining rock initially, followed by the roof caving and successive propagation of shear band. (b) The key observed influence factors of fracture propagation are the grade of confining rock, overburden pressure, dimension of the cavity and gasifying pressure, the linear relationships between them, and the fracture height. Additionally, the fracture depth in the base board was mainly caused by tensile fracture. (c) A model was proposed based on the evolution of fracture height and depth in roof and base board, respectively. Validation of this model associated with orthogonal tests suggests a good capacity for predicting fracture distribution. This paper has significance in guiding the design of the gasifying operation and safety assessment of UCG cavities.

Critical review of methods for intensifying the gas generation process in the reaction channel during underground coal gasification (UCG)

“…As a result of the chaotic caving of the upper rocks, additional pathways are formed for gases that evade contact with the coal seam surface. Expansion of gasification channels due to these flows results in reduced efficiency of the underground gasification process [144]- [147].…”

Pure oxygen condition forced oxidation ignition process of underground coal gasification

“…[1][2][3] The technology can be divided into drilling type (underground with shaft) and mine type (underground without shaft). 4,5 The drilling type is suitable for coal resources that are difficult or impossible to mine. [6][7][8] Figure 1 shows a schematic diagram of drilling type UCG process.…”

“…Underground coal gasification is a new chemical technology for mining the underground solid coal in situ, and industrial experiments have been carried out in Ukraine, Russia, and China 1–3 . The technology can be divided into drilling type (underground with shaft) and mine type (underground without shaft) 4,5 . The drilling type is suitable for coal resources that are difficult or impossible to mine 6–8 .…”