An Efficient Modeling Approach to Simulate Heat Transfer Rate between Fracture and Matrix Regions for Oil Shale Retorting

Abstract

The conversion of hydrocarbons in oil shale into liquid fuels has gained interest due to decreasing conventional oil reserves. Thermal conversion involves heating fractured rock and recovering gas and liquid phase products. The efficiency of this process is markedly dependent on heat transfer limitations between fracture porosity and rock matrix. Computer models are useful tools for process optimization. Explicit modeling of heat transfer processes within rock fragments would require great computational effort, making inverse modeling and forward process optimization very difficult if not impracticable. In this article, we evaluate the feasibility of using first-order heat transfer formulations to approximate these processes by comparing first-order model results with a rigorous explicit formulation and by comparison with laboratory retorting experiments published in the literature. Comparison of the two modeling approaches indicates that the first-order heat transfer approximation can be used without significant loss of accuracy if the block size and/or heating rate are not too large, as quantified by a proposed dimensionless heating rate. However, computational effort can be decreased by an order of magnitude compared with explicitly simulating diffusive heat transfer. List of symbols bHeating rate d m Rock fragment diameter E m Rate of heat generated (+) or absorbed (−) by reactions H fm Heat transfer rate from the matrix to fractures (+) or visa versa (−) r Radial distance from the center of the rock fragment r H Rate of temperature increase in fractures r m Radius of rock fragments r H Rate of rock surface temperature increase R H Dimensionless heating rate S m Specific heat of the rock matrix, T m is the rock matrix temperature t Time T f Fracture-rock interface temperature at the corresponding location T 0 Initial rock temperature T m Average rock matrix temperature β Linearized heat transfer coefficient λ m Thermal conductivity of the rock matrix ρ m Rock matrix density defined as the solid phase mass per total fracture-matrix volume

Numerical Simulation of in Situ Conversion of Continental Oil Shale in Northeast China

“…A deposit of oil shale having economic potential is usually the one that is at or near enough to the surface to be developed by open-cast or conventional underground mining or by in situ methods [3]. Costs and environmental issues tend to favor in situ processes [6].…”

Heat transfer characteristic of oil shale particle during the retorting

“…Shih and Sohn [4] developed a rigorous model incorporating the intrinsic kinetics of the decomposition and heat transfer effects, which could describe the retorting of oil shale more accurately than the shrinking-core model or the uniform-temperature model. Zhang and Parker [14] developed a first-order heat transfer model that is capable of efficiently simulating oil shale retorting accurately if particle sizes are not too large or heating rates are low. Pan et al [15] evaluated a two-dimensional heat transfer model to predict mass loss and particle center temperatures for large particles.…”