A dynamic multilevel method for fully-coupled simulation of flow and heat transfer in heterogeneous and fractured geothermal reservoirs is presented (FG-ADM). The FG-ADM develops an advanced simulation method which maintains its efficiency when scaled up to field-scale applications, at the same time, it remains accurate in presence of complex fluid physics and heterogeneous rock properties. The embedded discrete fracture model is employed to accurately represent fractures without the necessity of unstructured complex grids. On the fine-scale system, FG-ADM introduces a multi-resolution nested dynamic grid, based on the dynamic time-dependent solution of the heat and mass transport equations. The fully-coupled implicit simulation strategy, in addition to the multilevel multiscale framework, makes FG-ADM to be stable and efficient in presence of strong flow-heat coupling terms. Furthermore, its finite-volume formulation preserves local conservation for both mass and heat fluxes. Multi-level local basis functions for pressure and temperature are introduced, in order to accurately represent the heterogeneous fractured rocks. These basis functions are constructed at the beginning of the simulation, and are reused during the entire dynamic timedependent simulation. For several heterogeneous test cases with complex fracture networks we show that, by employing only a fraction of the fine-scale grid cells, FG-ADM can accurately represent the complex flow-heat solutions in the fractured subsurface formations.

<p>The outcrop areas of metamorphic rocks in continental setting cover significant regions and their extension zones are even more significant if we also consider the shallow and medium subsurface areas under the sedimentary cover. However, the metamorphic rocks are usually disregarded as potential geothermal reservoirs since they are considered as tight rocks with no or very limited porosity and permeability. Even if this statement is correct, the metamorphic units are frequently associated with a long and complex tectonic evolution and in particular with pre-, syn- and post-metamorphism fractures, which represent potential target zones for the development of geothermal reservoirs. Another limitation to assess the geothermal potentiality of the metamorphic units is the very limited number of deep exploration wells, especially in comparison to other well-investigated reservoirs, such as those located in sedimentary formations.</p><p>The anchizone and epizone metasedimentary rocks in Southern Belgium (Wallonia) cover more than 30% of the territory and probably more than 40% if we also consider the metamorphic rocks under the non-metamorphic formations. These statistics are based on the depth interval between 0 km (outcrop) and 6 km, which are reasonable depths for the development of geothermal projects. The encountered lithologies consist of a few km-thick quartzite and slate formations of Lower Palaeozoic and Lower Devonian ages. These formations are associated with different events with the most significant ones regarding this study being the fracturing events related to the formation of the Rhenohercynian basin during the Devonian and the Dinantian times followed  by the Variscan orogeny during Upper Carboniferous.</p><p>The Havelange borehole was drilled by the Geological Survey of Belgium between 1980-1985 as a gas exploration reaching a maximum depth of 5648 m (MD). The aim of this borehole was to investigate the presence/absence of a Carboniferous gas reservoir located under the main décollement level of the Ardenne Allochthon. Even if the borehole never reached any Carboniferous rocks, it allowed a better characterization of the transition between shallow Lower Famennian shale units and Lower Devonian meta-sedimentary formations, along with Middle Devonian rocks at intermediate depth. The study conducted in the framework of the H2020 MEET project (www.meet-h2020.com) for the Havelange study-site includes the re-investigation of cores, cuttings and log data acquired during the drilling. The laboratory study entails the mineralogical characterisation of the host rocks and fracture zones as well as the petrophysical and rock mechanical characterisation and this borehole material is completed with outcrop analyses and comparable measurements in analogue zones. The lab and field results are cross-matched with the drilling archives and in particular the drilling report indicating the depths of mud losses, representing intervals of great interest for the potential reconversion of this borehole into a geothermal well.</p>

Accurate numerical simulation of coupled fluid flow and heat transfer in heterogeneous geothermal reservoirs demand for high resolution computational grids. The resulting fine-scale discrete systems--though crucial for accurate predictions--are typically upscaled to lower resolution systems due to computational efficiency concerns. Therefore, advanced scalable methods which are efficient and accurate for real-field applications are more than ever on demand. To address this need, we present an algebraic dynamic multilevel method for flow and heat transfer in heterogeneous formations, which allows for different temperature values for fluid and rock. The fine-scale fully-implicit discrete system is mapped to a dynamic multilevel grid, the solution at which are connected through local basis functions. These dynamic grid cells are imposed such that the sub-domain of sharp gradients are resolved at fine-scale, while the rest of the domain remains at lower (coarser) resolutions. In order to guarantee the quality of the local (heat front) components, advanced multiscale basis functions are employed for global (fluid pressure and rock temperature) unknowns at coarser grids. Numerical test cases are presented for homogeneous and heterogeneous domains, where ADM employs only a small fraction of the finescale grids to find accurate complex nonlinear thermal flow solutions. As such, it develops a promising scalable framework for field-scale geothermal simulations.