Study on the influence of a single fracture on the performance of backfill heat exchangers in underground stopes

Thermal hazards in deep mines can be mined synergistically with the deposit as associated geothermal energy resources. Backfill heat exchangers (BFHEs) as a combination of mine backfilling and ground heat exchanger technology are one of the effective methods to realize ore deposit-geothermal energy synergy mining. However, in the complex environment of deep mines, BFHEs will inevitably suffer from fracture damage during long-term heat storage/release cycle operation, which will affect their working performance. To address this problem, this paper established a scaled-down experimental setup for fractured BFHEs units, and used the experimental data to verify the accuracy of the established three-dimensional transient heat transfer mathematical model of the BFHEs unit. Numerical simulation methods were used to investigate the effects of the position, aperture and fractured depth of a single fracture on the heat extraction performance of the BFHEs unit under fully dry or fully saturated working conditions. Significant differences were found in the effect of different fracture positions on the heat extraction capacity of the BFHEs unit. The extraction of geothermal heat by the BFHEs unit was favored when it was in the middle of two tubes, while the opposite was true for other positions. The effect of fracture on the heat extraction capacity of the BFHEs unit increased linearly with fracture aperture. The effect of the fractured depth was relatively greater in the 1/2–3/4 H_b range. The presence of groundwater significantly reduced the effect of the fracture. The effect of the fracture on the heat extraction capacity of the BFHEs unit was reduced by more than 80 % by changing the operating conditions from fully dry to fully saturated condition. The overall effect of a fracture on the heat extraction capacity of the BFHEs unit was not significant, with a 5 mm wide fracture having an effect of no more than 2.5 %, even under the fully dry condition. However, there was a combined effect of multiple fractures, and for a deposit size of 900 × 7 × 80 m, ten 2 mm apertures fractures would results in a maximum reduction of geothermal extraction from BFHEs by 8527 GJ in 10 years, which was equivalent to the heat load of a 13,753 m² building for one winter in the cold climate of China.