Coupled reservoir and geomechanical simulation for a deep underground coal gasification project

Abstract

Underground Coal Gasification (UCG) is an in-situ technology for extraction of energy from otherwise un-mineable coal seams. As coal is gasified, high temperature is generated and cavities are formed. Hence, the UCG imposes significant geomechanical changes to strata. The province of Alberta, Canada recently operated a deep UCG demonstration project, at a depth of 1400 m. The demonstration project successfully produced methane, hydrogen, and other gases. The current study aimed at conducting a sequentially coupled coal gasification and geomechanical simulation to study effects of the Alberta UCG on the coal seam and bounding seal system. A mechanical earth model was built for the test site utilizing geological layers reported for the site and under anisotropic in-situ stress magnitudes and orientations, particular to the Western Canadian Sedimentary basin. Ten chemical reactions along with their kinetics were implemented in a reservoir simulator. The Controlled Retraction Injection Point (CRIP) method was studied, in which four gasification chambers were simulated. The product gas compositions, over a period of 60 days, were in good agreement with the syngas composition measured at the demonstration project. By utilizing the coupling workflow, complex three-dimensional (3D) geometry of the UCG cavities as well as temperature and pore pressure, were passed along from the gasification module to a geomechanical simulator. This allowed simultaneous observation of geomechanical response of the strata as the gasification process advanced, syngas produced, and cavities developed.