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A thermoplasticity model for oilshale

The United States and other regions of the world have abundant oilshale resources, but accessing this energy supply poses a number of unique challenges. Of particular note is the complex thermomechanical behavior of the material. When heated to sufficient temperatures, thermal conversion of kerogen to oil, gas, and other products takes place. This alteration of microstructure has a dramatic impact on the strength of the material. In this work, we develop a thermoplasticity model for oilshale. The model is based on a Critical State Plasticity framework, a general class of material models often used for clays and soft rocks. The model described here allows for both hardening due to mechanical deformation and softening due to thermal processes. In particular, the preconsolidation pressure—defining the onset of plastic volumetric compaction—is controlled by a state variable representing the kerogen content of the material. As kerogen is converted to other phases, the material weakens and plastic compaction begins. We calibrate the proposed model to a suite of high-temperature uniaxial and triaxial experiments on core samples. We also describe avenues for future experimental and modeling work to improve understanding and prediction of the geomechanical behavior of oilshale formations.

Author(s):

Joshua White    
Lawrence Livermore National Laboratory
United States

Alan Burnham    
American Shale Oil
United States

David Camp    
Lawrence Livermore National Laboratory
United States

Shabnam Semnani    
Stanford University
United States