Geothermal study, performed for GeoScience Victoria – Melbourne

This study was performed as part of a geothermal heat flow project across the state of Victoria, Australia in order of GeoScience Victoria, Department for Primary Industries, Melbourne. There are a number of historic deep petroleum wells (2000-3000 m) with bottomhole temperature data and geophysical logs. The objective of the study is to determine the downhole thermal conductivities of the formations drilled in these wells so that the heat flow can be calculated. Within these holes a limited number of thermal conductivity analyses of available cores were performed by Hot Dry Rocks Pty Ltd. The target of the work requested from Geophysica GmbH was to integrate these spot core data with logging data in order to calculate continuous thermal conductivity profiles for the wells. These profiles should allow the calculation of statistically representative thermal conductivity values for the stratigraphic units of the Gippsland Basin. 


For a total of 13 wells borehole geophysical data were analysed. The log data were checked for their overall quality and the occurrence of depth off-sets. Log data from different wells were compared for data integrity and possible off-sets between the wells. Finally, core data of thermal conductivity, porosity and density produced within the running project were implemented. Petrophysical analyses were mainly targeted to derive shale volume, porosity and water saturation from the borehole geophysical data. Geological information was integrated with log data to reveal relationship of rock components and petrophysical characteristics for the different stratigraphic units with it’s components of shale, sand, carbonates and coal. By literature and data analysis, matrix values for rock density and sonic velocity were defined for the rock types.


Then, standard methods of formation evaluation were checked for the applicability to the drilled strata. While shale volume prediction could follow standard methods, a specific procedure for porosity prediction was developed which combines density and sonic slowness data. The procedure was developed in two key wells for the other drillings. During this stage, core measurements were integrated with log data in order to calculate continuous thermal conductivity profiles. The computed data of the different contents of shale, shale-free matrix and free water were used to calculate thermal conductivity profiles. The log derived thermal conductivity profiles were calibrated with the laboratory measurements. Using these profiles, statistical values of effective thermal conductivity could be calculated for the stratigraphic units.

.




Results displayed for one well of the Gippsland Basin. The figure shows the log derived porosity and thermal conductivity profiles compared with the results form laboratory investigation.