How Equivalent Are Equivalent Porous Media?

Geoenergy and geoengineering applications usually involve fluid injection into and production from fractured media. Accounting for fractures is important because of the strong poromechanical coupling that ties pore pressure changes and deformation. A possible approach to the problem uses equivalent porous media to reduce the computational cost and model complexity instead of explicitly including fractures in the models. We investigate the validity of this simplification by comparing these two approaches. Simulation results show that pore pressure distribution significantly differs between the two approaches even when both are calibrated to predict identical values at the injection and production wells. Additionally, changes in fracture stability are not well captured with the equivalent porous medium. We conclude that explicitly accounting for fractures in numerical models may be necessary under some circumstances to perform reliable coupled thermohydromechanical simulations, which could be used in conjunction with other tools for induced seismicity forecasting. Plain Language Summary The subsurface will play an important role in decarbonizing the economy. The transition to carbon neutrality can be accelerated by utilizing geothermal energy, returning carbon underground, and storing energy in the subsurface to offset the fluctuations in production of renewables. These low-carbon geoenergy technologies oftentimes deal with fractured rock masses. To minimize the risks related to geoenergy projects, numerical simulations are performed to predict the response of the subsurface to fluid injection and production. Given the complexity and high computational cost of simulating fractures, fractured rock is usually treated as an equivalent porous medium. Here we investigate the validity of this simplification by comparing these two approaches. We find that even though an equivalent porous medium can reproduce the fluid pressure changes at wells, the pressure distribution within the fractured media significantly differs between the two approaches. Equivalent porous media fail to reliably predict the rock behavior when fracture spacing is within the order of the reservoir size and computer simulations should explicitly include fractures to provide reliable forecasts and reduce the risks of induced seismicity. The latter is necessary to enable a successful deployment of geoenergies to mitigate the climate crisis. Key Points Equivalent porous media can reproduce pressure and temperature evolution at wells but not their distributions within the fractured media Consideration of explicit fractures affects stability and induced seismicity in a way that cannot be represented by equivalent media Simulating thermo-hydro-mechanical processes requires explicit inclusion of fractures whenever they add inhomogeneities at reservoir scale