Reservoir-Geomechanics of Underground Coal Gasification (UCG)

• Author / Creator

  Akbarzadeh Kasani, Hossein

• The Underground Coal Gasification (UCG) is still not in a worldwide operation despite inheriting an old idea which is nearly one and a half century old. Aside from effects by other fuel sources on development of the UCG and from a technical perspective, this delay is due to the complex nature of the process. The chemical process of gasification may develop temperature in the order of 1000 oC, turn the initial solid coal to char and ash, generate pore pressure, create cavities, and develop cracks in the coal seam and rock layers. The later will, in turn, influence the gasification process due to the enhanced porosity and permeability of the strata. Hence, the UCG represents a coupled thermal-hydro-chemical-mechanical process. Understanding how coal, a fractured organic sedimentary rock, responds to the temperature and pore pressure changes around a gasification chamber is crucial for a successful UCG operation. How pore pressure is influenced by volumetric deformation and the ultra-high temperature around a gasification cavity is of significant importance which has not been well studied. Previous high-temperature experimental studies of coal are scarce yet distributed in multiple disciplines; including chemical, petroleum, and geomechanics. Moreover, previous modeling of the UCG mostly included separate gasification simulations or geomechanical simulations of idealized cavities. This research encompassed experimental and simulation studies. An inter-disciplinary research of coal was conducted to investigate impact of elevated temperature on weight loss, thermal deformation, microcrack generation, transport properties, as well as strength and stiffness of coals from different ranks. Furthermore, a High-Pressure High-Temperature (HPHT) triaxial set up was upgraded and utilized in this study to accommodate measuring permeability of Alberta coal to nitrogen gas along with geomechanical test. Several coal specimens were cored from large coal blocks acquired from the Genesee coal mine in Central Alberta. The coal specimens were tested under different confining stresses and temperatures. This experimental program measured thermal deformation, stress-strain, elastic properties, permeability as well as permeability evolution during progressive shearing. The simulation studies included parametric geomechanical analyses of an idealized UCG cavity to understand fundamentals of the formation response to evolution of a cavity which included high-temperature syngas. Impacts of different operational pressures and coal material properties on thermally-induced pore pressure as well as deformation and stresses around the UCG cavity were investigated. Moreover, a numerical modeling workflow was devised in order to sequentially couple coal gasification capability of a reservoir simulator to a geomechanical software. This coupling workflow facilitated simultaneous observation of gasification process as well as geomechanical effects to the strata. This coupling workflow was applied to a reservoir scale modeling of the Swan Hills, Alberta UCG project implementing the in-situ stress magnitudes and orientations. The gasification modeling was run for a 60-day period utilizing the Controlled Retraction Injection Point (CRIP) operational method and produced syngas compositions which closely matched the field measurements. Simultaneously, deformations, stresses, and mechanical failure in the strata were observed as the gasification front advanced and the tear-drop shape cavities grew in dimensions.

• Subjects / Keywords

  • Cavity
  • High-Pressure High-Temperature Triaxial
  • Coal
  • Sequentially Coupled Modeling
  • Permeability
  • Geomechanics
  • Elevated Temperature
  • Underground Coal Gasification (UCG)

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3SB3X40J

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.