Impact of fluvial meander-belt sedimentary heterogeneity on the efficiency of low-enthalpy geothermal doublets: heat-transport simulations of forward stratigraphic models

In the present study, different geocellular models of meander-belt stratigraphic architectures were produced that are representative of the sedimentary products of sand-bed meandering rivers and their petrophysical heterogeneity. The static models were created by combining a rule-based stratigraphic modelling approach with geostatistical modelling, and were applied in groundwater-flow and heat-transport simulations using MODFLOW-2005 and MT3D-USGS software. Overall, histories of injected cold-water plume propagation were examined considering: (i) three architectural frameworks as representative of different river morphodynamics; (ii) four scenarios of facies architecture; and (iii) alternative well layouts. The presence, size and spatial distribution of sedimentary heterogeneities related to river hydrodynamics, channel-form abandonment or modes of meander-transformation are seen to control the shape of the thermal plume, thereby affecting well-doublet performance. The considered scenarios of facies make-up for point-bar deposits have a modest influence on the temperature decline near the abstraction well. The presence of sandstone beds in the lower heterolithic parts of abandoned-channel fill does not facilitate significant thermal-plume expansion beyond the mud-plug. The effect of basal lags made of open-framework conglomerates on heat advection depends on their geometry but is effectively negligible when it makes up less than 1% of the deposits. Relatively thin mud drapes lying on point-bar accretion surfaces are seen to act as baffles to flow but their impact is minimal in view of their small number. The study provides useful and novel insights into the potential impact of sedimentary heterogeneity in fluvial reservoirs, which can be applied to the design of well doublets and to highlight areas that deserve further investigations.