Journal of the Geothermal Research Society of Japan

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In this study, thermal response tests (TRTs) with water injection into a ground heat exchanger (GHE) were conducted to evaluate how vertical water flow in the well influences the heat exchange performance. The GHE used for TRTs has slotted screen pipes with depths from 41 m to 71 m, and natural groundwater level is 14.5 m. Hydraulic conductivity of the reservoir is 6.35×10&sup-6; m/s. Thermal tracer tests were first carried out for understanding how deep the injected water in the well runs out into fractures. The results of thermal tracer tests showed that the injected water runs out into fractures with depths from 57 m to 64 m. Thus, injected water in the well generates vertical water flow in this well. TRTs were then conducted on 4 different patterns of water injection rate; no injection (TRT-0), 5 l/min (TRT-5), 10 l/min (TRT-10) and 20 l/min (TRT-20). Temperatures of injected water were stable through each TRT. Averaged temperature of injected water in the case of TRT-5, TRT-10 and TRT-20 was 20.9°C as against 15.5°C for temperature of ground. The results of TRTs showed that the averaged temperature of heat medium during the circulation of between 43 to 46 hours in the case of 20 l/min injection is 5°C lower than ones in the case of no injection. Therefore, it can be said that vertical water flow induced by water injection into the GHE can enhance heat exchange performance of the GHE drilled throughpermeable layers.

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Use of fractional Advection-Dispersion Equation (fADE) has been proposed to describe mass transport in a fractured reservoir. In this study, a finite discrete method to solve the fADE is developed and its accuracy is tested against analytical solutions.
Tracer simulation is carried out using a 3D simulation code for flow analysis (FRACSIM-3D). The fADE mathematical model is applied to fit the numerical tracer results, which show highly anomalous behaviors such as a long tail. The solution to the fADE including a spatial fractional derivative is shown to be in reasonable agreement with the tracer response produced by FRACSIM-3D. The fitting parameters of the fADE equation obtained for well interval of 50 m are used to make predictions of tracer responses for well intervals of 100 m - 500 m. It has been shown that the tracer responses predicted by the fADE model are reasonable close to those obtained by FRACSIM-3D irrespective of the different well intervals. This study demonstrates that the fADE offers a method for predicting tracer responses in a fractured reservoir.

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