| Abstract |
Pressure-propped fractures, defined as hydraulic fractures that are maintained open only by fluid pressure without the use of proppants, have been used by (1) the GeneSys Project in Germany (2003 – 2011) for geothermal energy extraction (for district heating); (2) Quidnet Energy (2021) for mechanical energy storage in shallow hydraulic fractures; and (3) Sage Geosystems (2023) for mechanical energy storage in deep hydraulic fractures. Although similar in form, the three systems have fractures that are created differently, and they employ variants of Huff-n-Puff techniques for their respective energy production. This paper analyzes reservoir stimulations for energy storage in deep hydraulic fractures. The systems in mind are where hydraulic fractures on a single wellbore are pressure-propped and only a fraction of the fluid stored in the created hydraulic fracture is cycled via Huff-n-Puff operations. The analysis is done in the more general setting of downward propagating fractures that are opposite to the well-studied buoyant (or upward) propagating fractures, so the language already established there is used here. Attention is given to the hydromechanical dynamics that occur when creating these deep hydraulic fractures; these dynamics are interpreted from the fracture geometries and characteristics resulting from specific stimulation. The open-source solver PyFrac is used for the simulations, and different configurations are considered here that are relevant for energy storage in deep hydraulic fractures as well as heat-extraction systems. |