Fully Coupled Thermo-Hydro-Chemical (THC) Modelling in Advanced Reservoir Engineering

Thermo-hydro-geochemical modelling is of great economic and scientific importance for the implementation of geothermal projects, where understanding the effects of fluid injection and extraction on reservoir properties is crucial. From an operational point of view, changing the temperature of the geothermal reservoirs can intensify both biotic and abiotic water-rock interactions. The latter, including mineral dissolution and precipitation processes, alter the rock’s structure and, consequently, its hydraulic and transport properties such as porosity and permeability. These changes in permeability - controlled by the mineral composition of reservoir’s rock, reservoir fluid composition, temperature conditions, and utilization scenarios - all affect the overall system’s performance and sustainability. The complex nature of these subsurface interactions requires to rely on numerical methods to solve systems of partial differential equations for flow, transport, and chemical reactions. The nonlinearity of such systems translates in high computational costs, mainly due to the reactive chemistry component, which has hindered the applications of those numerical methods for field-scale applications in complex reservoirs.

In this contribution we demonstrate recent development of a robust simulation environment able to handle the intricate couplings of thermohydraulic, mechanical, and geochemical processes for subsurface applications. The open-source GOLEM simulator for THM modelling in fractured reservoir has been coupled with the reactive chemistry PHREEQC library. The goal is to seamlessly integrate GOLEM's capabilities in solving thermohydraulic processes within a finite element mesh, with PHREEQC's robust handling of reactive chemistry calculations. This integration allows for the simulation of 3D reactive transport processes while accounting for the spatial heterogeneities typical of natural geothermal systems, as well as the evaluation of a chemical reaction-based alteration of formation’s porosity and subsequently permeability. Our GOLEM-PHREEQC implementation exclusively relies on open-source software, enhancing the accessibility of multiphysics simulations across different sectors. Herein, we showcase details of the implementation and its validation against available benchmark tests, as well as preliminary results from a field-scale application within the framework of an Aquifer Thermal Energy Storage (ATES) project in the Berlin urban area.