Numerical Investigation of Non-Darcy Flow Characteristics in Rough Fractures for Geothermal Reservoir Modelling

The rapid increase in greenhouse gas (GHG) emissions due to industrialization, urban
development, and extensive use of cooling and heating devices and appliances such as air
conditioners, refrigerators, and water heaters has become a major environmental concern
worldwide. These systems release harmful GHGs that significantly contribute to global warming
and environmental degradation. To mitigate these impacts, geothermal energy stands out as a
reliable and environmentally friendly source of heat, with the potential to provide long-term
energy security and reduced carbon emissions. Despite its environmental advantages, the
development of geothermal energy projects has been limited due to issues of sporadic
distributions, involvement of high initial investment and operational costs, etc. Therefore,
accurate pre-assessment of reservoir performance is critical to determine whether a geothermal
project can meet the required energy demand and remain economically viable. Numerical
modelling plays a crucial role in this assessment by predicting fluid flow and heat transport
behaviour within geothermal reservoirs. Traditionally, most geothermal reservoir models assume
linear Darcy flow to be valid for both the porous rock matrix and the fracture networks.
However, this assumption may lead to inaccurate predictions when fractures exhibit high
roughness. In such cases, fluid flow within fractures deviates from linear Darcy behaviour and
becomes nonlinear due to inertial effects, which are better described by the Forchheimer flow
regime. Neglecting this nonlinear flow behaviour can result in significant errors in estimating
pressure losses, flow distribution, and ultimately the production temperature over time.
In this study, a numerical model of a fractured geothermal reservoir is developed that
incorporates nonlinear flow behaviour within rough fractures while retaining Darcy flow in the
surrounding porous matrix. The model is applied to a fractured geothermal system, and the
results are systematically compared with those obtained using the conventional Darcy flow
assumption for fractures. The comparison demonstrates that Darcy-based fracture models tend
to overestimate fluid mobility, leading to an underestimation of pressure losses and premature
thermal breakthrough. Consequently, Darcy flow models overestimate the magnitude of the
production temperature decline and underestimate the production temperature. The findings
highlight the importance of considering nonlinear fracture flow in geothermal reservoir
simulations, particularly for systems with highly rough fractures. Incorporating realistic flow
physics improves the reliability of production forecasts and provides a more accurate basis for
decision-making in geothermal project development.

Keywords: Geothermal energy, fractured reservoirs, nonlinear flow, Forchheimer equation, heat
transport.