Abstract
Multiphase mass and heat transfer are ubiquitous in the subsurface within manifold applications. The presence of fractures over several scales and complex geometry magnifies the uncertainty of the heat transfer phenomena, which will significantly impact, or even dominate, the dynamic transport process. Capturing the details of fluid and heat transport within the fractured system is beneficial to the subsurface operations. However, accurate modeling methodologies for thermal high-enthalpy multiphase flow within fractured reservoirs are quite limited. In this work, multiphase flow in fractured geothermal reservoirs is numerically investigated. A discrete-fracture model is utilized to describe the fractured system. To characterize the thermal transport process accurately and efficiently, the resolution of discretization is necessarily optimized. A synthetic fracture model is firstly selected to run on different levels of discretization with different initial thermodynamic conditions. A comprehensive analysis is conducted to compare the convergence and computational efficiency of simulations. The numerical scheme is implemented within the Delft Advanced Research Terra Simulator (DARTS), which can provide fast and robust simulation to energy applications in the subsurface. Based on the converged numerical solutions, a thermal Péclet number is defined to characterize the interplay between thermal convection and conduction, which are the two governing mechanisms in geothermal development. Different heat transfer stages are recognized on the Péclet curve in conjunction with production regimes of the synthetic fractured reservoir. A fracture network, sketched and scaled up from a digital map of a realistic outcrop, is then utilized to perform a sensitivity analysis of the key parameters influencing the heat and mass transfer. Thermal propagation and Péclet number are found to be sensitive to flow rate and thermal parameters (e.g., rock heat conductivity and heat capacity). This paper presents a numerical simulation framework for fractured geothermal reservoirs, which provides the necessary procedures for practical investigations regarding geothermal developments with uncertainties.
Highlights
Convective and diffusive flow is common in the subsurface and can greatly influence the mass and heat transport process
The synergy of thermal convection and conduction plays a critical role in the development of geothermal reservoirs, where the heat is extracted with continuous injection and circulation of the heat carrier (e.g., water or CO2 (Randolph and Saar, 2018)) in manifold ways (e.g., well doublet (Willems et al, 2017), borehole heat exchanger (Hein et al, 2016), etc.)
Fractures are explicitly depicted with the discrete-fracture model (DFM) and the mesh quality of the DFM discretization is improved through a pre-processing procedure
Summary
Convective and diffusive (or conductive) flow is common in the subsurface and can greatly influence the mass and heat transport process. The development of geothermal systems can be assisted by the inherent or induced fracture networks, especially for reservoirs with a low permeable matrix (Wang et al, 2019c) Due to their high conductivity, open fractures behave as preferential flow channels for the injected. To accurately simulate mass and heat transport in fractured geothermal systems, a suitable fracture model is critical to capture the reservoir response. Karimi-Fard et al (2004) proposed the DFM method, which is suitable for general-purpose reservoir simulators This approach captures the pressure response generated by flow in fractured networks in a robust and accurate manner (Flemisch et al, 2018; Berre et al, 2021; Glser et al, 2017; Nissen et al, 2018). A realistic model based on practical outcrop measurements is utilized to perform numerical experiments with different parameters, and different scenarios are discussed
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