Numerical and experimental study of strata behavior and land subsidence in an underground coal gasification project

Abstract

Abstract. Underground Coal Gasification, with enhanced knowledge of hydrogeological, geomechanical and environmental aspects, can be an alternative technique to exploit the existing unmineable reserves of coal. During the gasification process, petro-physical and geomechanical properties undergo a drastic change due to heating to elevated temperatures. These changes, caused due to the thermal anisotropy of various minerals, result in the generation of thermal stresses; thereby developing new fracture pattern. These fractures cause the overhead rock strata to cave and fill the gasification chamber thereby causing subsidence. The degree of subsidence, change in fluid transport and geomechanical properties of the rock strata, in and around the subsidence zone, can affect the groundwater flow. This study aims to predict the thermo-geomechanical response of the strata during UCG. Petro-physical and geomechanical properties are incorporated in the numerical modelling software COMSOL Multiphysics and an analytical strength model is developed to validate and further study the mechanical response and heat conduction of the host rock around the gasification chamber. Once the problems are investigated and solved, the enhanced efficiency and the economic exploitation of gasification process would help meet country's energy demand.

Fault Reactivation Can Generate Hydraulic Short Circuits in Underground Coal Gasification—New Insights from Regional-Scale Thermo-Mechanical 3D Modeling

“…Luo et al [57] developed a UCG subsidence prediction methodology and showed that overlying strata and ground surface subsidence behavior in UCG operation is relatively similar to that in conventional longwall coal mining. Depending on UCG panel design, channel dimensions and coal seam depth, the maximum surface subsidence calculated by Li et al [58] (0.039 m) and Sirdesai et al [10] (0.038 m) is in the range of our simulation results (0.055 m). Even if the strength of overlying strata above the gasified coal seams can increase under the influence of temperature as suggested by Li et al [58] and observed in experiments by Liu et al [48] at temperatures between 100 and 450 • C due to cementation reactions effecting weakening, increasing temperature tends to a general decrease in rock strength [13,54,59].…”

“…However, the complexity of simultaneously occurring physical and chemical processes, such as chemical reactions and their kinetics, transport phenomena (i.e., heat and mass), turbulent flow patterns in the reactor, water influx from the surrounding rock mass and thermo-mechanical processes related to geological boundary conditions and stress regimes is remarkably high [27][28][29]. Almost all previous UCG studies applied one-dimensional (1D) or two-dimensional (2D) models to account for selected coupled processes [10], which only have a limited validity in predicting the performance of UCG field trial operation, especially when considering the generally high heterogeneity and complexity of geological systems. For instance, it is known that 2D models may overestimate the magnitude of ground surface subsidence and stresses in pillars [30].…”

“…Furthermore, potentially induced changes in overburden hydraulic conductivity may result in pollution of shallow aquifers [9,10]. Especially thermo-mechanical effects and/or fault reactivation may introduce potential migration pathways for hazardous environmental contaminants, mainly composed of organic and inorganic pollutants [4,9,11].…”

Geological and Rock Mechanics Perspectives for Underground Coal Gasification in India

“…Similar studies should be carried out for Indian coalfields for assessing feasibility of UCG in India. In fact, Sirdesai et al [17] have carried out a computational study using COMSOL Multiphysics software to understand subsidence behaviour during UCG. They have concluded that cavities of width less than 100 m cause lower subsidence.…”

Elastic modulus of thermally treated fine grained sandstone using non-contact laser extensometer

“…The magnitude and the duration of heating in the above-mentioned processes varies significantly. Temperatures of over 1200°C can be encountered in processes like UCG [11,16,17]. Whereas, processes like nuclear waste disposal and geo-thermal energy extraction encounter relatively low temperatures but for a longer period of time.…”