Subject: Comments to Draft Specifications for Application of UNFC-2009 to Geothermal Energy Resources: from Michael Forrest, FIMMM, FAusIMM, C Geol, Mining Research Co., United Kingdom
25 July 2016
I have a number of comments regarding the classification of geothermal resources under UNFC. I accept that a method of classification is useful in evaluating geothermal resources in a similar manner to those of other energy and mineral deposits. UNECE Expert Group on Resource Classification gives a series stages in the knowledge and commercial viability of solid mineral and mineral energy deposits. These deposits are finite in the sense that the amount of material, the resource, has been measured and is fixed in place. All mineral and hydrocarbon resources with the exception of very minor gas resources are the product of past geological activity and therefore finite even if no accurate resource figure is known. Furthermore after quantification and even extraction the economic case for any mineral deposit is dependent on the price of the mineral production at any one time. Nevertheless the CRIRSCO resource estimates are converted to reserves by the application of modifying factors such as economic, social and technical.
I note from the 12 case histories a distinction between those operations/projects that are used for space heating and those for electrical power generation. They require different temperature and flow regimes and could be regarded as separate deposit types. Those geothermal resources exploited by GSHP are dependent on marginally elevated heat flow from the earth usually at around 100 m. This heat flow can be diminished by over extraction as noted in examples reducing the output for space heating. It can be restored by a reduction in heat extraction rate to below that of the geological heat flow. GSHP can also be used in horizontal pipeworks supplying heat from what is essentially solar rather than geothermal energy. In a number of cases over extraction can actually freeze the ground in both vertical and horizontal GSHP systems. Although it is correct to classify the energy source as renewable there is a time constraint that influences the resource. Although the case histories give time intervals for energy production eg 6MW over 25 years, that is the rate of extraction, a value in the classification of the maximum output of the geological resource before a reduction in output is noted would enable the resource to be a reserve akin to other mineral and hydrocarbon resource/reserves.
Those higher temperature geothermal resources that can be used for electrical power generation are based upon hot rocks at much lower crustal levels. As the heat flow is driven by volcanic or granitic (often with a radiogenic input) rocks the volume of rock and its link to deep crust or then asthenosphere the heat flow can be regarded as constant over human time scales. Although great advances have been made in determining rock characteristics at depth what actually exists between injection and production wells sometimes leads to loss of pumped fluids into the surrounding geology. A project in Cornwall UK failed because of leakage when pressurised. Values of rock porosity could be include in the classification, or at least highlighted if not known. All of these factors would help to bring the geothermal resource in alignment with mineral resources.
Both space heating and electrical generation geothermal resources are critically determined by the markets in a way that other mineral deposits are not. This is a reflectance of the local market for the recovered energy. In GSHP the energy distribution has a very limited geographic distribution directly related heat losses in transmission. As the North Rhine Westphalia example show buildings are required near the source for the energy to be exploited. District space heating is only applicable in colder climates. Electrical generation systems fare better, although recover less energy, as power lines can feed national grids. Like other relatively low value resources significant infrastructure costs are required. Those too far from significant markets are stranded deposits just like those in a similar environment, the South Australian example being a case in point requiring an atypical market for the power.
These differences in classifying geothermal resources relate to the fact that it is difficult to transport the product (energy) on a global scale. This does not apply to oil or minerals although natural gas has similar problems overcome by expensive pipelines or liquefaction. These factors are more important in geothermal products than they are in solid minerals or oil.
Comments from Michael Forest, FIMMM, FAusIMM, C Geol, Mining Research Co., United Kingdom, to Draft Specifications for Application of UNFC-2009 to Geothermal Energy Resources.
25 July 2016
I have a number of comments regarding the classification of geothermal resources under UNFC. I accept that a method of classification is useful in evaluating geothermal resources in a similar manner to those of other energy and mineral deposits. UNECE Expert Group on Resource Classification gives a series stages in the knowledge and commercial viability of solid mineral and mineral energy deposits. These deposits are finite in the sense that the amount of material, the resource, has been measured and is fixed in place. All mineral and hydrocarbon resources with the exception of very minor gas resources are the product of past geological activity and therefore finite even if no accurate resource figure is known. Furthermore after quantification and even extraction the economic case for any mineral deposit is dependent on the price of the mineral production at any one time. Nevertheless the CRIRSCO resource estimates are converted to reserves by the application of modifying factors such as economic, social and technical.
I note from the 12 case histories a distinction between those operations/projects that are used for space heating and those for electrical power generation. They require different temperature and flow regimes and could be regarded as separate deposit types. Those geothermal resources exploited by GSHP are dependent on marginally elevated heat flow from the earth usually at around 100 m. This heat flow can be diminished by over extraction as noted in examples reducing the output for space heating. It can be restored by a reduction in heat extraction rate to below that of the geological heat flow. GSHP can also be used in horizontal pipeworks supplying heat from what is essentially solar rather than geothermal energy. In a number of cases over extraction can actually freeze the ground in both vertical and horizontal GSHP systems. Although it is correct to classify the energy source as renewable there is a time constraint that influences the resource. Although the case histories give time intervals for energy production eg 6MW over 25 years, that is the rate of extraction, a value in the classification of the maximum output of the geological resource before a reduction in output is noted would enable the resource to be a reserve akin to other mineral and hydrocarbon resource/reserves.
Those higher temperature geothermal resources that can be used for electrical power generation are based upon hot rocks at much lower crustal levels. As the heat flow is driven by volcanic or granitic (often with a radiogenic input) rocks the volume of rock and its link to deep crust or then asthenosphere the heat flow can be regarded as constant over human time scales. Although great advances have been made in determining rock characteristics at depth what actually exists between injection and production wells sometimes leads to loss of pumped fluids into the surrounding geology. A project in Cornwall UK failed because of leakage when pressurised. Values of rock porosity could be include in the classification, or at least highlighted if not known. All of these factors would help to bring the geothermal resource in alignment with mineral resources.
Both space heating and electrical generation geothermal resources are critically determined by the markets in a way that other mineral deposits are not. This is a reflectance of the local market for the recovered energy. In GSHP the energy distribution has a very limited geographic distribution directly related heat losses in transmission. As the North Rhine Westphalia example show buildings are required near the source for the energy to be exploited. District space heating is only applicable in colder climates. Electrical generation systems fare better, although recover less energy, as power lines can feed national grids. Like other relatively low value resources significant infrastructure costs are required. Those too far from significant markets are stranded deposits just like those in a similar environment, the South Australian example being a case in point requiring an atypical market for the power.
These differences in classifying geothermal resources relate to the fact that it is difficult to transport the product (energy) on a global scale. This does not apply to oil or minerals although natural gas has similar problems overcome by expensive pipelines or liquefaction. These factors are more important in geothermal products than they are in solid minerals or oil.
Comments from Michael Forest, FIMMM, FAusIMM, C Geol, Mining Research Co., United Kingdom, to Draft Specifications for Application of UNFC-2009 to Geothermal Energy Resources.