Revisiting the Assumption of Local Thermal Equilibrium in Thermal Floods

Abstract

AbstractNotwithstanding their complexity, a good understanding of the heat-transport mechanisms and the associated temperature distributions is a critical success factor for the design and management of thermal projects exploiting underground oil resources. However, for mathematical convenience, it is often assumed that the reservoir exists in a state of local thermal equilibrium (LTE), which implies that the trio of injected heating fluid, in-situ fluid and the rock matrix in a particular location always exist at the same temperature. In essence, current treatments assume that the in-situ fluid and rock matrix reach the temperature of the injected fluid immediately they are contacted by the latter, tacitly neglecting possible time lag to accomplish heat exchange among these three elements.Applying the principles of energy conservation at a pore scale to each element of the trio, this paper develops and solves a one-dimensional heat-transport model to investigate the validity of this fundamental assumption in thermal floods. The approximate analytic solutions under isothermal boundary conditions are presented in both Laplace and time domains. Employing the resulting solutions and a set of realistic input data, the conditions and time scales of validity of the LTE assumption are probed for steam, which is a popular and efficient heating fluid for oil reservoirs.Results of the sensitivity studies suggest that the assumption of LTE remains credible for most realistic thermal floods. These findings notwithstanding, some conditions that threaten the validity of this fundamental assumption are highlighted.