Experimental Study of Underground Heat Storage via Hydraulic Fractures

We present a laboratory setup to test the energy budget of underground heat storage in an otherwise impermeable rock through engineered fractures, a so-called Fracture Thermal Energy Storage (FTES) system. A hydraulic fracture is first created in a cubic specimen of Zimbabwe Gabbro. In the experiment, de-ionized water is circulated under high pressure through an injection steel tubing automatically heated to a target temperature. The fluid adjusts to the tubing temperature during its flow before entering a production well drilled into the 250 mm edge length block. The warm fluid then circulates through the previously created hydraulic fracture at mid-height of the block toward a production well drilled at the block periphery. The production well is equipped with a dedicated completion to allow fluid outflow. External fracture appearances are sealed using an epoxy resin. We demonstrated through the results of a preliminary heating experiment that we can transfer nearly all of the heat from the circulating fluid to the block. An efficient charging of the thermal battery is achieved. This first experiment is currently under further improvement and will ultimately help to better understand the engineering of mid- to large-scale field implementation of FTES systems. 1 INTRODUCTION Storing excess heat produced during summer or as a byproduct of industrial treatments is of widespread interest in the context of the energy transition. Typically, the peaks in energy production are not aligned with peak consumption. This mismatch is notably observed for inter-seasonal differences in demand and production. Underground thermal energy storage (UTES) systems could provide an efficient mechanism to overcome these limitations. Several types of UTES systems are being used as of today. The most used systems, not including any phase change of the temperature carrier, as of today are Aquifer Thermal Energy Storages (ATES) (Bloemendal et al., 2015). Other systems include the use of a series of boreholes (Borehole Thermal Energy Storage - BTES) or underground caverns (Cavern Thermal Energy Storage - CTES). All of these systems have regulatory, economic, and geological advantages and disadvantages.