Numerical Simulation of the In-situ Upgrading of Oil Shale

Abstract

Abstract Oil shale is a highly abundant energy resource, though commercial production has yet to be realized. Thermal in-situ upgrading processes for producing hydrocarbons from oil shale have recently gained attention, however, in part because of promising results reported by Shell using their In-situ Conversion Process. This and similar processes entail heating the oil shale to about 325ºC, when the kerogens in the shale decompose through a series of chemical reactions into liquid and gas products. In this paper we present a detailed numerical formulation of the in-situ upgrading process. Our model, which can be characterized as a thermal-compositional, chemical reaction and flow formulation, is implemented into Stanford's General Purpose Research Simulator (GPRS). The formulation includes strongly temperature-dependent kinetic reactions, fully-compositional flow and transport, and a model for the introduction of heat into the formation through downhole heaters. We present detailed simulation results for representative systems. The model and heating patterns are based somewhat on information in Shell patents; chemical reaction and thermodynamic data are from previously reported pyrolysis experiments. After a relatively modest degree of parameter adjustment (with parameters restricted to physically realistic ranges), our results for oil and gas production are in semi-quantitative agreement with available field data. We also investigate various sensitivities and show how the produced hydrocarbon components are impacted by heater temperature and location. The ability to model these effects will be essential for the eventual design and optimization of in-situ upgrading operations. Introduction Oil shales are organic-rich sedimentary rocks that contain significant amounts of kerogens and generate oil upon pyrolysis or retorting (Peters et al., 2005). Oil shale deposits are found in many parts of the world, and the kerogens in oil shales are recognized as one of the most abundant energy resources. A conservative estimate suggests that the total worldwide oil shale resources are equivalent to around 3×1012 barrels of oil (Dyni, 2005). The Green River formation in the western US is the largest known oil shale deposit in the world, containing approximately 1.5×1012 barrels of shale oil (Dyni, 2005). Although technologies for oil shale processing have been actively investigated in past decades (Biglarbigi et al., 2007), there has been renewed interest in recent years, due to both high oil prices and the promising field test results of Shell's In-situ Conversion Process (Crawford et al., 2008). Assuming oil shale production technologies are able to successfully address issues related to water use and carbon dioxide emissions (Brandt, 2008), it is quite possible that oil shales may contribute to the future oil supply. Kerogens in oil shales are believed to originate from the same types of organic matter that generated existing oil and gas. In the case of oil shales, however, the natural maturation process, which would eventually lead to oil and gas, is at a very early stage and may still require millions of years. This process can be accelerated through either surface retorting or in-situ upgrading. Surface retorting requires that the oil shale be mined, followed by surface pyrolysis and further processing. This method is only suitable for shallow formations that can be mined economically.