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The Role of Convection in Basin Modelling: Lessons From the Perth Basin

Abstract

The thermal state of the basin is fundamental for petroleum exploration. The classical assumption is that in a sedimentary basin heat is transferred through conduction. Yet reservoirs are by nature zones of high permeability in between barriers of low permeability. Such conditions can also trigger a different mode of heat transfer through convection, leading to significant difference in the temperature regime. As such fluid convection cells modify greatly the geometry of the isotherms and their transient nature makes them a complicated phenomenon to identify from temperature logs only. Indications for fluid convection can often be found in areas where thick units of high permeability are encountered such as the massive pre-salt reservoirs in Brazil. The Western Australian Geothermal Centre of Excellence (WAGCoE, 2008–2011) focused some its research on the role of convection in the Perth Basin and we are presenting here the methodology followed as well as some results which highlight the impact of basin convection for the petroleum industry. Large scale models (basin scale) were constructed to study the existence of convection cells and constrain the parameter space for all values required to populate the most realistic 3D model. The numerical results from hydrothermal simulations at that scale were then used to generate boundary conditions at the lower scale (Perth metropolitan) and numerous models were run to understand the sensitivity of the main parameters identified in order to account for the large uncertainty around the geological data. Results show that convection can easily occur in various locations within the Perth Basin and that convective patterns create significant temperature changes compared to conductive scenarios. Convection cells are strongly pinned by the geological faults and a detailed 3D model is required to obtain realistic results. Numerical simulations proved very useful tool to estimate the spacing of convecting upwelling zones and also showed that convection homogenizes salinity within the aquifer, with convective flow rates similar to regional groundwater flow rates. The strong association of convection cells with faults and their persistent nature allow us to more reliably predict thermal regimes in areas that were previously returning large errors in classical conductive temperature inversions.