Multiphysics simulation of phase field interface development and geomechanical deformation in radio frequency heating of oil shale

Abstract

A novel multiphysics finite element framework, developed to numerically model in-situ pyrolysis of oil shale by radio frequency heating in a two-dimensional (2D) domain, is extended to include deformation analysis of the target formation and adjacent regions as a result of dielectric heating. This framework is constructed by explicitly coupling equations describing thermal, phase field front tracking, mechanical deformation and electromagnetics (TPME) to track the conversion of high yield oil shale to liquid oil. The finite element method is used to model heat generation by the conversion of electromagnetic energy in the formation then the solid-liquid conversion interface is tracked by the Allen-Cahn phase field method. The objective of this work is to analyze the evolution of the conversion interface and the subsequent mechanical deformation of the target formation by evaluating disparate geologic model descriptions to capture the anticipated subsurface behavior. These geologic models possess variations in complexity so that the appropriate level of detail in the geological model description can be assessed for further TPME simulation studies of radio frequency heating. The evaluated geologic models include those which are comprised of: heterogeneous or homogeneous high yield kerogen-rich properties in addition to the inclusion or omission of under- and overburden surrounding the target formation at varying electrode separation distances and applied electromagnetic frequencies. Parameterization of the numerical model was performed following characteristics of the Green River oil shale. Results show the variation in conversion interface of solid kerogen to liquid oil by way of interface arrival times at domain boundaries as well as the extent of formation deformation under the disparate geological model descriptions. Based on these results oil shale conversion timelines are compared and highlight the need to include mechanical deformation analysis of the target formation, including a kerogen-poor under- and overburden, in the numerical modeling of radio frequency heating as a process of oil shale in-situ pyrolysis.