Numerical modelling of thermal convection related to fracture permeability: Implications for geothermal exploration and basin modelling
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The presence of convective fluid flow can create areas of anomalously high temperatures at shallow depths, which can be exploited for geothermal energy. Previous thermal investigations of the Dutch subsurface revealed a thermal anomaly located at the Luttelgeest 01 Well (LTG-01) in Noordoostpolder, The Netherlands. At depths greater than 4000 m, there is a shift to higher temperatures. Subsequent studies by van Oversteeg (2013) show that the Dinantian carbonates encountered at Luttelgeest contain intervals of relatively high fracture permeability showing potential as a geothermal reservoir for electricity production. Van Oversteeg (2013) constrained the permeability of the 600 m interval of Dinantian carbonates encountered at 4500 m depth to be 60 mD (5.92x10-14 m-2). Rayleigh number analysis estimates the minimum permeability for convective onset in the order of 30 mD (2.96x10-14 m-2). The temperature measurements at LTG-01 indicate variations in subsurface temperature that could be indicative of convection. Horner corrected temperatures determined by van Oversteeg (2013) reveals a 12°C temperature change (191 – 203°C) between the top and bottom of the 600 m interval (4550 – 5150 m) and a temperature gradient of 20°C/km across the permeable carbonate layer. This study aims to reproduce the thermal gradient at LTG-01 through three-dimensional numerical models in order to better understand the interplay between natural fracture permeability and temperature patterns. Numerical models of thermal convection are used to illustrate the role of permeability on the timing of convection onset, convection cell structure development and resulting temperature patterns. Numerical simulations of convection in the Dinantian carbonate platform show that: (1) spacing of convective upwellings can be predicted from aquifer thickness, geothermal gradient, and permeability; (2) convective upwellings can create significant temperature enhancements relative to the conductive profile; (3) the best fit modelled temperature profile occur under the following conditions: a geothermal gradient of 39°C/km, aquifer thickness of 600 m and permeability of 80 mD (7.89x10-14 m-2); (4) a pseudo-conductivity, determined for the 600 m convecting interval, is 2.61 W/m/K.