Completed at the request of Kalahari GeoEnergy, this study refines the resource estimate associated with the Bweengwa River geothermal resource in the Kafue Trough of Zambia. The productivity hosting fracture system present within the basement has been modelled using a discrete fracture network (DFN) with input derived from core review and the faults contained within a newly derived fault framework. Deterministic faults and stochastically generated fractures sets of the DFN were utilized to perform an integrated modelling workflow and deliver 3D models with predictive resource capability. The DFN fracture model was conditioned to well test transmissivities and calibrated to the thermal model and reservoir rock-type. The resulting parameters include directional hydraulic conductivity tensor and fracture porosity that were upscaled to provide input to Reservoir Fluid Flow Simulation. A review of the composition of sampled fluids, alongside isotope ratios and interpreted downhole conditions suggests that primarily the water is derived from a meteoric source which has percolated from surface through permeable and fracture mediated processes, towards the basement, where it is heated and subsequently driven upwards by convection. The integration of static parameters with the dynamic transmissivity enabled a confident initialization of the static model for simulation. Thermal equilibration runs showed stability of the thermal plume, maintaining a suitable balance of conductive and convective heat flow processes dominating in the Karoo (Permian) age sedimentary cap-rock and the resource hosting metamorphic basement respectively. Simulation of prediction cases with existing well doublets highlighted the risk of injection water breakthrough related to permeability anisotropy. Geothermal power output was maximined by injecting into the shallower basement layers whilst producing from the deeper, hotter basement layers. An optimised development scenario of 3 producers plus 3 injectors, with production targeting the deep, hot up-flow and using well rate constraints consistent with the well test derived flow rates of up to 25 kg/s, proved to deliver a stable thermal output of 22 MWt over a 20 year span. The study has provided a fundamental shift in quantification of the resource that may be developed by following a robust fracture-centric characterisation of the basement – by way of a discrete fracture network (DFN) sub-model – that was tied to the productivity proven in those wells tested to date. Further flow testing of wells and especially of deepened well penetrations that will help prove the character of the assumed hotter ( greater than 130C) resource that is expected deeper than some 1000m below ground level.

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