Geothermal energyProject Reservoir Quality in Sedimentary Geothermal Resources
This report will take the lessons learnt throughout the rest of this study to develop a workflow for assessing geothermal reservoir quality ahead of drilling and protocols for HSA geothermal well drilling. These recommendations for future development of sedimentary geothermal resources in Australia will help to reduce the risks that have been highlighted by the two exploration projects that were the subject of this study.
This project has focused on sedimentary geothermal resources, which are within the Deep, Natural Reservoir category described above. These resources are targeting permeable formations within sedimentary basins. The conceptual model is that these high permeability formations will allow geothermal fluids to be produced at high flow rates without any significant engineering of the reservoir. Fracture permeability may contribute but is not the primary target. The generally sub-horizontal nature of the layering of formations in sedimentary basins means that if a highly permeable formation can be identified, there is a good chance that the resource would have good lateral extent. Intersecting a horizontal formation with a vertical drill hole would be straightforward and lower risk than targeting the sub vertical fractures typically found in convective geothermal resources.
The thermal regime in sedimentary basins is dominated by conduction. Regions with anomalously high geothermal gradients are desirable, but not required may. Higher than average geothermal gradients maybe present due to elevated heat flow from the basement or due to heat being trapped by sediments with low thermal conductivities. Anomalously high thermal gradients in these conductive settings are likely to be more than 50°C/km (compared with the crustal average of about 30°C/km).
A key challenge in developing sedimentary geothermal resources is in finding areas where the permeability is preserved at the depths required to reach the necessary temperatures for a geothermal resource. The effects of diagenesis, both through compaction and chemical changes, act to reduce the permeability and porosity as sediments are buried.