Thermo-hydro-mechanical coupling and pipe flow modeling of fractured underground reservoirs for optimal operations in geostorage

Non-isothermal fluid injection in fractured media is vital for the analysis of aquifers and underground reservoirs in hydrogen geostorage applications. Fluid flow, rock deformation, fracture aperture, and heat transport processes are fundamental to analyzing fluid storage, stability, and tightness of underground storage structures, induced seismicity or land subsidence. Modeling all these phenomena in a coupled fashion requires high computational effort, especially if the fracture network is explicitly reproduced in the model geometry. Realistic boundary conditions are important too, since pressure, flow rates, or temperature values are usually known only at surface level. This involves taking into account the properties of the injected and host fluids as well as the hydraulic head losses during fluid flow.

In this study, we propose a fully coupled finite element scheme to simulate fluid pressure, temperature, fracture aperture, and rock deformation evolution in highly heterogeneous fractured media. We model fractures as lower-dimensional elements with inherent stiffness, permeability, and thermal properties. Fractures represent preferential flow channels within the reservoir, in contrast to the low-permeability rock matrix. We also simulate injection and production wells as pipes with a certain diameter and roughness. The non-isothermal fluid flow along pipes is coupled with the fractured reservoir, as fluid and heat exchanges are allowed between pipes and fractures.

We compare the performance of our numerical model with some multirate mass transfer models and theoretical formulations, finding excellent agreement between all approaches. For a certain surface pumping pressures, the injection/production flow rates are fundamentally determined by the head losses along wells and fractures permeability, for a certain operation pressure. Fractures permeability, in turn, depends on the thermo- and hydro-mechanical processes that modify the fractures aperture. Our results demonstrate that it is possible to replicate the expected expansion/contraction behavior of fractures through the injection/extraction of fluids with thermal contrast. We note that hydraulic head losses along pipes can be crucial to model performance, with flow rates that can vary up to an order of magnitude if they are ignored.

Our approach reduces the disadvantages associated with mesh refinement and property contrast in fractured areas. It provides an efficient way to simulate coupled heat transport, fluid flow, and rock deformation in fractured zones, also including the non-isothermal flow along the injection and production wells. This capability enables a realistic representation of subsoil fracturing to model subsurface processes such as underground hydrogen storage in deep rock formations.

Acknowledgements

This research was supported by the Spanish Agencia Estatal de Investigación and the Ministerio de Ciencia, Innovación y Universidades (10.13039/501100011033) and by “European Union NextGenerationEU/PRTR” through grant Green-HUGS (TED2021-129991B-C32 and C33).