Abstract:

A geomechanical analysis of underground coal gasification (UCG) requires integrated modeling in complex multidisciplinary areas of coupled fluid flow, rock and cavity mechanics. In this study, the results from an existing thermal and multi-phase fluid flow simulator are imported into a geomechanical module to solve for the vertical displacement and the stress variation around a propagating cavity induced by a coal gasification process. A Controlled Retracting Injection Point (CRIP) well configuration is applied in the thermal reservoir simulator to model chemical reactions and geochemistry. The high temperature nature of the UCG process as well as creation of void space within the rock mass continuum require the use of efficient rock constitutive models. Three different constitutive models are investigated in this study: linear elastic, hyperbolic, and elasto-plastic. We have then applied these constitutive models on a case study to compare the results. The displacement of a coal layer and the surrounding blocks as well as stress arching in regions away from the cavity can efficiently be captured by the use of all the above constitutive models; however, a stress analysis around the induced cavity necessitates implementation of a constitutive model which can efficiently capture the shear softening after the rock failure.

Introduction

Coal accounts for almost 30% of global primary energy consumption, which makes it the second largest primary energy source in the world after oil. Coal is also the largest provider of electricity. Over 40% of global power production derives from coal [1]. Coal's significant position in the global energy mix is mainly because it is abundant, low-cost, and the most wide-spread fossil fuel in the world [2]. Despite a decline in the coal demand growth in 2014, International Energy Agency's forecast shows the growth of almost 1% per year through 2020 [1]. Moreover, based on the 2013 survey of World Energy Council, coal consumption is forecast to increase over 50% to 2030. This rise will be mainly due to the escalating electricity rates in the developing countries [2]. Therefore, more coal extraction is necessary to meet the global future energy demand.

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