Subject: Comments to Draft Specifications for Application of UNFC-2009 to Geothermal Energy Resources: from Manfred P Hochstein, Auckland
30 July 2016
With reference to the draft example of UNFC Geothermal Specifications, I found that there are significant uncertainties in the G-axis part of the specifications, i.e. probability of Known versus Potential Sources, that still require discussion.
1. For prospects related to geothermal reservoirs hosted by volcanic rocks, the assessment of the potential of the inferred resource should introduce the geological setting and the natural heat loss of prospects in the G-axis discussion (examples where such discussion would be required are case studies 1,7, 8, and 12).
2. Most high T prospects in developing countries (S-hemisphere and Africa) appear to be associated with volcanic geothermal resources. In all these cases a discussion of the observed natural heat loss associated with manifestations should be introduced in the G-axis discussion.
3. The potential energy estimates for prospects associated with strato-volcanoes and caldera settings (and other volcanic geothermal prospects) tend to be poor estimates if the volume method is used and the areal extent of the resource is estimated by using resistivity data (mainly MT surveys). Such assessments usually assume that low resistivity structures reflect on-going thermal alterations of host rocks in a present day hot fluid T-field – the possible effect of resistivity structures caused by palaeo-fluids and decaying systems have to be considered (if deep drill hole information is not available).
4. Despite advances in the theory of MT interpretation, the problem of pseudo- resistivity anomalies caused by terrain effects requires attention and could be mentioned in the G-Axis discussion. The terrain effect in a setting such as case study 1 is small and can be reduced, the effect in steep volcanic terrain, however, (case study 7), can produce topography-controlled pseudo-anomalies.
5. For prospects with acid feeder structures (such as at Alto Peak) only a G4 type assessment might be justified since there are examples in the literature showing that casing corrosion in such a setting can lead to an early abandonment of exploitation (the old Dieng Field in Indonesia, for example). A (deep) fluid-quality (G-axis) information could be added to volcanic geothermal prospects.
6. Concealed outflow structures of volcanic geothermal systems occur and can be mis-identified as a primary reservoir (the Tiwi example) – a G4 type discussion of the regional hydrological setting would be helpful.
One can assume that a proper UNFC edited document will be used to raise equity by the developer of a geothermal resource – whether all such documents could or should be used for raising funding, if the G-axis discussion remains as it is in the draft, is a problem we should think about.
Comments from Manfred P Hochstein, Auckland, to Draft Specifications for Application of UNFC-2009 to Geothermal Energy Resources.
30 July 2016
With reference to the draft example of UNFC Geothermal Specifications, I found that there are significant uncertainties in the G-axis part of the specifications, i.e. probability of Known versus Potential Sources, that still require discussion.
1. For prospects related to geothermal reservoirs hosted by volcanic rocks, the assessment of the potential of the inferred resource should introduce the geological setting and the natural heat loss of prospects in the G-axis discussion (examples where such discussion would be required are case studies 1,7, 8, and 12).
2. Most high T prospects in developing countries (S-hemisphere and Africa) appear to be associated with volcanic geothermal resources. In all these cases a discussion of the observed natural heat loss associated with manifestations should be introduced in the G-axis discussion.
3. The potential energy estimates for prospects associated with strato-volcanoes and caldera settings (and other volcanic geothermal prospects) tend to be poor estimates if the volume method is used and the areal extent of the resource is estimated by using resistivity data (mainly MT surveys). Such assessments usually assume that low resistivity structures reflect on-going thermal alterations of host rocks in a present day hot fluid T-field – the possible effect of resistivity structures caused by palaeo-fluids and decaying systems have to be considered (if deep drill hole information is not available).
4. Despite advances in the theory of MT interpretation, the problem of pseudo- resistivity anomalies caused by terrain effects requires attention and could be mentioned in the G-Axis discussion. The terrain effect in a setting such as case study 1 is small and can be reduced, the effect in steep volcanic terrain, however, (case study 7), can produce topography-controlled pseudo-anomalies.
5. For prospects with acid feeder structures (such as at Alto Peak) only a G4 type assessment might be justified since there are examples in the literature showing that casing corrosion in such a setting can lead to an early abandonment of exploitation (the old Dieng Field in Indonesia, for example). A (deep) fluid-quality (G-axis) information could be added to volcanic geothermal prospects.
6. Concealed outflow structures of volcanic geothermal systems occur and can be mis-identified as a primary reservoir (the Tiwi example) – a G4 type discussion of the regional hydrological setting would be helpful.
One can assume that a proper UNFC edited document will be used to raise equity by the developer of a geothermal resource – whether all such documents could or should be used for raising funding, if the G-axis discussion remains as it is in the draft, is a problem we should think about.
Comments from Manfred P Hochstein, Auckland, to Draft Specifications for Application of UNFC-2009 to Geothermal Energy Resources.