Reservoir and Production Engineering Challenges for Geothermal Systems Hosted in Australian Sedimentary Basins
Authors: Ricard Ludovic P. , Huddlestone-Holmes Cameron R. , Pujol Martin 
Published: October 2016

The Australian geothermal energy sector has undergone a boom and bust cycle since the early 2000's. In contrast to the absence of large-scale electricity generation, the development of direct use resources, particularly in Perth, Western Australia, has proved to be quite successful, reaching a total of almost 15 MWth by end 2015 and a projection of 18–20 MWth by end 2016, and the once negligible small-scale off-grid power generation is getting momentum. The contrasting fate of the large-scale geothermal electricity and direct use sectors provides valuable insights into the importance of reservoir quality in the development of these resources.

An entire energy value chain review of a dozen operating direct-use geothermal projects in Perth, was conducted to identify success factors for direct use projects. The review has identified that: Extensive exploration and production of groundwater resources for water supply from depths of up to 1100 m contributed to a major decrease on reservoir quality risk,Operating and planned project requirements range from 0.5 to 3.3 MWth and target a relatively shallow resource (less than 1500 m deep),The geothermal resource has very good reservoir quality, so that performances are above world average (permeability thickness from 50 to over 1000 Dm and coefficient of performance in the order of 20 to 35)Injection difficulties which have been experienced in other similar overseas projects have been largely avoided and overcome in Perth,Perth geothermal projects are well established, economically viable, competitive with other energy sources and environmentally sustainable.

A forensic analysis of two geothermal wells drilled in the Cooper and Otway basins targeting high temperature electricity generation within sedimentary basins was performed to identify key issues associated with deeper targeting and large scale geothermal development. The forensic analysis highlights the importance of reservoir quality rather than high temperature targeting in sedimentary basins.

Finally, the results of a technico-economics analysis for large scale electricity generation enabled to identify key production parameters and limiting factors. The economics analysis shows that high well productivity (permeability thickness and flow rate above 50 Dm and 55,000 bblpd) is required at high temperatures (&gt;120 °C) for large scale electricity generation. Destruction of permeability through diagenesis at these temperatures such as observed in one of the unsuccessful well may limit the likelihood of finding such high productivity reservoirs. There are also potential issues of the high flow rates required in terms of fine migration, well completion and pump design.

Ultimately it is the technical performance that controls the economic viability of these projects. With the necessary increase of energy delivery, new projects will need to investigate smart ways to achieve high flow rates, while limiting potential negative impacts in terms of fine migration and pressure losses.