Because of its small carbon footprint and ability to provide baseload power around the clock, interest in geothermal resources is growing throughout the world. Projects are being developed in the USA, Indonesia, Philippines and Australia.

The picture below shows the different uses for geothermal resources (Geoscience Australia).

There are two major targets for geothermal resources hot dry rocks or hot wet sedimentary aquifers.

Hot Dry Rocks (HDR)

HDR is usually deals with higher temperatures and deeper depths (> 4 kilometres) than hot sedimentary aquifers. The target is often located just above or inside a high heat producing granite with a thick insulating layer.

Hot Sedimentary Aquifers (HSA)

Knowledge of the distribution and hydrological characteristics of deep aquifers and aquitards is also important in the characterisation and exploitation of potential geothermal resources in two key ways.

First, geothermal explorers need to know depths and properties (thickness, extension, porosity, permeability, presence of water) of all porous and impermeable formations in order to assess fluid volumes and circulation paths.

Second, thermal conductivity values of formations (and thus the rock column) are controlled by the formation lithology and porosity, as well as the presence of water, gas or oil. Thermal conductivity is needed to compute temperature gradients and therefore temperature at depth.

For example, an impermeable layer of shale has a low thermal conductivity and behaves as a thermal blanket, trapping the heat that comes from underneath. The temperature gradient evolution with depth will be perturbed by the presence of this low conductivity impermeable layer.

Once the fluid flows and temperature at depth has been estimated, one can calculate for example the heat in place or perform modelling of geothermal plays that will determine:

• the type of geothermal exploitation (direct or power production);
• the choice of the best drilling locations; and
• the most reliable estimate of drilling and production costs.

In order to succesfully exploit a hot sedimentary aquifer, decision-makers need to know, the depth of aquifer, geothermal gradient, porosity, permeability, thickness of geothermal blanket, connection to other aquifers/reservoirs and to a lesser extent groundwater quality within the HSA (controls potential reuse and reinjection).

Scale of FrOG Tech products

Depending on needs, FrOG Tech can produce products at the international, national, basinal or project scale.

For example, the current Australian scale temperature map (5 kilometres depth - Austherm07) is the result of interpolating dispersed onshore temperature data. As such, it is not appropriate for regional analysis of onshore and offshore sedimentary basins. The geothermal potential of individual states has been calculated and there have been some small scale geothermal studies in localised areas of interest conducted by government geological surveys and consultants. But there very few geothermal studies available at the whole of basin scale.