Pulse Testing for Monitoring Aquifer Thermal Energy Storage

Geothermal energy systems often suffer the inefficiency resulting from the seasonally fluctuating demand between summer and winter. One possible solution is seasonal storage of heat in relatively shallow aquifers. However, the efficiency of such a system will depend on a number of factors that cannot all be known a priori. Thus the importance of monitoring. We have expanded our earlier work on harmonic pulse testing (HPT) to incorporate the effect of a temperature front moving into the reservoir due to injection of hot (or cold) water. To assess the feasibility of the technique for thermal front monitoring, we devised a synthetic field case where water at 90°C was stored in a 20°C aquifer. First, we employed a numerical reservoir simulator to determine the temperature distribution in a doublet system. Then, this distribution was imposed as initial condition for a pulse test simulated with the same numerical technique; in this way, synthetic data were created. The data was then analyzed using our new analytical relationships. The thermal front around the injector could indeed be characterized through the application of the proposed HPT interpretation methodology. A mobility change was detected around the injector, corresponding to an increased local temperature, approaching the injection value. Moreover, the estimated radius of the thermal front was in good agreement (within 10%) with the equivalent radius of the – not perfectly cylindrical – numerically calculated heated zone. Adding noise to the pressure data did not deteriorate the signal much, as long as the tests were carefully designed in terms of pulse duration and sampling rate.