Analysis of recovery rate for underground coal gasification based on strength and permeability

The medium and deep underground coal gasification (UCG) is a new technology of coal utilization, and there is still lack of criterions to evaluate recovery rate. The strength of remaining wall between the gasification chambers and the gas permeability are two important factors indicating the safety of the formation. To ensure the safety of the formation after gasification, the recovery rate was defined according to the wall strength and seam permeability theory. Firstly, the stress distribution of target coal seam under the influence of the multiple factors was analyzed through the finite element method, and the critical conditions of crack under different loads were analyzed. The minimum wall thickness between the gasification chambers was determined under different conditions, such as different shape of the gasification chamber, different thickness of the coal seam, and different formation pressure and working pressure. Secondly, the Darcy Law was used to analyze the coal seam permeability quantitatively. Thirdly, the amount of coal could be used by the underground coal gasification process was calculated, and the recovery rate of underground coal gasification was defined based on the mining rate of coal and the recovery rate of crude oil. Finally, the recovery rates of different formation thicknesses and different gasification chambers were calculated and analyzed. The research results reveal that the recovery rate of UCG cannot be measured by the mining rate. Due to the cylindrical/conical shape of the gasification chamber, the recovery rate is lower than that of coal mining. Because a wall between the gasification chambers is kept to support the roof rock, the recovery rate of UCG is reduced further. With the increase in the wall thickness, the recovery rate decreases. However, the influence of wall thickness on the thick coal seam is minor, while the influence on the thin coal seam is greater. The thicker the coal seam, the thicker the wall between the chambers. On the other hand, the recovery rate of thick coal seam is greater than that of thin coal seam, especially for the pear-shaped gasification chamber. Different from coal mining, the recovery factor of UCG is affected significantly by the thickness of the coal seam. If a pear-shaped gasification chamber is used, the mining value of thin coal seam is low. Underground coal gasification provides a solution for deep coal mining and utilization. Since no UCG recovery rate evaluation method has been established universally, the results of this work provide a reference for the site selection, resource evaluation and gasification process of underground coal gasification, as well as the formulation of UCG recovery rate criterions.