Heat transfer and parameterization in local thermal non-equilibrium for dual porosity continua

Processes of coupled fluid flow and heat transport in fractured porous media are of interest in many different natural and industrial applications. Often it is difficult to represent all features of a system in an adequate parameterization. The dual porosity model allows parameterization for pore space and fractures separately, representing high permeable fractures and high porous host rock at the same time. Local thermal non-equilibrium models (LTNE) overcome the limiting assumption of instantaneous local thermal equilibrium between fluid and rock phase in heat transport, which has been shown to be preferential in several scenarios. In this work we derive a LTNE model for a dual porosity fluid flow model, considering heat transfer between rock and both fluid domains, pore space and fractures, as well as heat transport between the two fluid domains, linked to mass transfer. This enables a well-defined parameterization for fluid and heat transport and allows deeper insights into heat transport processes. We test our model for some standard scenarios in the context of geothermal application. Our results indicate in comparison to a standard fluid-rock model, that water stored in low permeable regions of the host rock need to be considered in heat transfer models, otherwise the heat stored is underestimated. Pore water acts as an additional heat reservoir for the host rock, generating higher production temperatures in the reservoir compared with an equivalent fluid flow model also using LTNE. Only in regions with high pressure gradient between pore and fracture fluid, heat transfer due to mass transfer is significant and it is the smallest heat transfer process under any scenario tested. Our model can be easily adopted to other simulation tools and be extended with more, also dynamic, constitutive relationships for a stronger coupling between heat transport and fluid flow separately for porous matrix and fractures. (C) 2016 Elsevier Ltd. All rights reserved.