Fracture aperture anisotropic effects on field scale enhanced geothermal system thermal performance

Abstract

The thermal performance of a field-scale Enhanced Geothermal System was investigated, considering anisotropy in fracture aperture distributions. The fracture aperture distribution was treated as an autocorrelated random field with different ranges in the x- and y-directions, defined by geometric anisotropic ratios between 1.5 and 4. To model realistic aperture distributions, constraints were applied to ensure the aperture distributions had Hurst exponents that are typical of fractures observed in nature.The thermal drawdown between the perpendicular and parallel flow configurations was compared, revealing a 60 % likelihood of improved heat transfer in the perpendicular flow configuration. When exploring the impact of geometric anisotropy ratios on thermal performance, aperture distributions with a geometric anisotropy ratio of 2 demonstrated about 70 % better thermal performance in favor of the perpendicular flow direction. This ratio also exhibited aperture distributions with Hurst exponents within the range found in natural faults and fractures, suggesting a 70 % likelihood of improved thermal performance in the perpendicular flow configuration for actual fracture behavior.The perpendicular flow configuration mainly exhibited tortuous flow paths, while the parallel flow direction mostly had near-horizontal flow paths. The tortuous flow paths increased contact between the fluid and fracture surface area, resulting in improved thermal performance.The findings suggest the importance of considering shear direction at the field scale when placing wells. Placing a well perpendicular to the direction of slip or shear may yield enhanced thermal performance compared to placing a well parallel to the direction of slip or shear.