Decimeter-scale Laboratory Investigation of Fracture Thermal Energy Storage

Abstract

Fracture Thermal Energy Storage (FTES) stores and recovers heat in low-permeability rock by circulating fluid through hydraulically created fractures that act as transient heat exchangers. We report decimeter-scale laboratory FTES experiments on 25-cm cubic granite and gabbro blocks containing one or three fractures created by hydraulic fracturing. Thermal charge–discharge cycles are performed by circulating hot and cold water between a central injection well and production wells while varying inlet temperature and flow rate, and monitoring inlet/outlet temperatures and the block surface temperature fields. The early-time mean surface warming is approximately linear and scales with injected thermal power (constant flow rate at fixed inlet temperature). The hydraulic response evolves during heating: at constant flow rate, injection pressure increases as the block warms and is higher for higher inlet temperatures, consistent with a temperature-dependent reduction of effective fracture transmissivity. Long injections reach a quasi-steady regime where input thermal power is balanced by boundary heat losses to the laboratory environment; this regime enables estimation of loss power and cumulative lost energy. These measurements constrain key parameters and would enable calibration and validation of a hydrothermal model accounting for the conductive host-rock boundary conditions expected in a field-scale FTES system. It provides quantitative insight into how fracture transmissibility and connectivity affect FTES efficiency.