Scope of TOUGH2 and open-DARTS for the simulation of transport in porous-fractured media: an application to enhanced geothermal systems

Modeling transport in fractured geologic media has historically been treated with two main approaches: continuum and discrete. Both are often based on the same physical principles, but differ in their ability to represent fracture-scale phenomena. In this work, we evaluate the scope and performance of two numerical simulation tools based on the finite volume method that have been designed for transport in porous-fractured media: TOUGH2, which implements a Multiple Continua approach, and the open-source software open-DARTS, which works under Discrete Fracture-Matrix models. We first benchmark the simulation tools with a 1D homogeneous reservoir in order to highlight methodological and performance differences between them. Next, an Enhanced Geothermal System is proposed and simulated for the geothermal area of Acoculco, Mexico. Two fracture configurations are tested in TOUGH2 and six in open-DARTS to evaluate heterogeneity and fracture density. The 1D benchmark yields consistent results once an optimal space resolution is input into open-DARTS. With regard to the fractured reservoir, ∼ 14 MW may be delivered over a period of 30 years. The inherent channeling flow associated with Discrete Fracture-Matrix leads to an accelerated production decline, which is explicitly simulated by open-DARTS, at the cost of a higher computing time. For dense and regular fracture networks, however, both tools produce comparable results with a much lower computing time for TOUGH2. The averaged nature of Multiple Continua makes it a computationally efficient method. It is therefore necessary to develop robust strategies for fracture network averaging. • Heat mining strategy for a Hot Dry Rock geothermal reservoir. • Regular and random fracture networks for geothermal production. • Performance comparison between MINC and DFM approaches.