Convective heat transfer process in fractured tight shale formations based on a modified double-medium model: Theoretical model development and numerical validation

Abstract

Besides the flow channeling between the matrix and fracture, the heat channeling is not considered in the traditional double-medium model. Therefore, the prediction accuracy through the traditional double-medium model is extremely poor during the in-situ conversion process of tight shale formations after reservoir stimulation. To address this, a modified double-porosity and double-permeability model is proposed in this study to describe the convective heat transfer process in fractured shale reservoirs. This model considers both flow and heat channeling between the matrix and fractures. The model's accuracy was verified through numerical simulations, illustrating the effect of matrix permeability, fracture spacing, and width on the heat exchange process between the matrix and fractures. Notably, the heat transfer speed is greatly increased when considering heat channeling between the matrix and fractures, compared to the traditional dual-medium model. For tight formations, over 95 % of heat is transferred from fractures to the matrix through heat channeling, while in high permeability reservoirs, more than 90 % of heat is transferred through flow channeling. The contact area between the matrix and fractures increases exponentially with decreasing fracture spacing, leading to a drastic increase in both flow and heat channeling. However, the heat channeling can be ignored when the fracture spacing exceeds 180 cm. Additionally, the conductivity of the fracture substantially increases with the fracture width, resulting in an increase in heat carried by flow channeling. Conversely, the heat channeling decreases with increasing fracture width. This is because the contact area between the matrix and fractures remains unchanged, and thus the fracture width hardly affects the shape factor of the heat channeling.