Heavy oil mobilization from underground coal gasification in a contiguous coal seam

Abstract

Underground coal gasification (UCG) is a process to convert coal in-situ to combustible gases which are of versatile end use from power generation to chemical synthesis. It extracts the energy of coals which are normally too deep/thin to recover with conventional technologies, thus increasing enormously the recoverable coal reserves worldwide. It is generally advised to operate UCG in a coal seam with a thickness no less than 2 m as excessive heat loss to the surrounding environment significantly undermines the gross thermal efficiency. However, UCG can still be attractive in a thin seam when a heavy oil reservoir adjoins. This paper aims to shed light upon the feasibility of mobilizing heavy oil with UCG in a contiguous thin coal seam. A 3-D UCG model was integrated into a heavy oil reservoir model. Modelling results showed that, with a creative layout of process wells, it is possible to grow a UCG fire in favour of mobilizing neighbouring heavy oil. UCG can mobilize significant oil due to a remarkable reduction in oil viscosity in a short time. The characteristics of production wells as well as the expansion of a thermal cavity were also studied. Furthermore, the effects of coal thickness, oil well deployment and solution gas in live oil were investigated with respect to thermal cavity propagation and oil production, which can serve as a guide in designing future reservoir development with UCG.