Abstract

Effective thermal conductivity (ETC), as a necessary parameter in the thermal properties of rock, is affected by the pore structure and the thermal conduction conditions. To evaluate the effect of fractures and saturated fluids on sandstone’s thermal conductivity, we simulated thermal conduction along three orthogonal (X, Y, and Z) directions under air- and water-saturated conditions on reconstructed digital rocks with different fractures. The results show that the temperature distribution is separated by the fracture. The significant difference between the thermal conductivities of solid and fluid is the primary factor influencing the temperature distribution, and the thermal conduction mainly depends on the solid phase. A nonlinear reduction of ETC is observed with increasing fracture length and angle. Only when the values of the fracture length and angle are large, a negative effect of fracture aperture on the ETC is apparent. Based on the partial least squares (PLS) regression method, the fluid thermal conductivity shows the greatest positive influence on the ETC value. The fracture length and angle are two other factors significantly influencing the ETC, while the impact of fracture aperture may be ignored. We obtained a predictive equation of ETC which considers the related parameters of digital rocks, including the fracture length, fracture aperture, angle between the fracture and the heat flux direction, porosity, and the thermal conductivity of saturated fluid.

Highlights

  • The thermal properties and temperature-dependent petrophysical properties of rock, such as heat capacity, thermal conductivity, compressive strength, permeability, and porosity, are of great importance in many fields of applied geosciences

  • We used partial least squares (PLS) regression analysis to evaluate the relationship between effective thermal conductivity (ETC) and the related parameters, including fracture length and aperture, angle between the fracture and the heat flux direction, porosity, and fluid thermal conductivity

  • For the angle effect, when the heat flux is parallel to the fracture direction (Figure 7a), at the same slight reduction in ETC, we note that the obstructive effect of the fracture can almost be ignored when fracture length, the effect degree of different fracture apertures is almost consistent

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Summary

Introduction

The thermal properties and temperature-dependent petrophysical properties of rock, such as heat capacity, thermal conductivity, compressive strength, permeability, and porosity, are of great importance in many fields of applied geosciences. Energies 2019, 12, 2768 analytical studies have been conducted to evaluate different factors that affect thermal conduction, the leading factor of which is the rock structure (size, shape, and distribution of pores). In addition to the pore structure, the conditions of thermal conduction, such as temperature, pressure, and saturating fluids, affect ETC. Directional development of the contact area and pore structure under anisotropic stress conditions resulted in anisotropy of the thermal conductivity [22]. We carried out thermal conduction simulations along three orthogonal (X, Y, and Z) directions based on water- and air-saturated digital rocks with different fractures. We used partial least squares (PLS) regression analysis to evaluate the relationship between ETC and the related parameters, including fracture length and aperture, angle between the fracture and the heat flux direction, porosity, and fluid thermal conductivity

Thermal Conduction Model
Reconstructed Digital Rocks with Different Fractures
Temperature Distribution in Digital Rocks with Different Fractures
ETC with variable saturated fluid and fracture parameters
Conclusions
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