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Creating geothermal heat flow maps at ‘prospect’ scale with heat needle appliance

Heatflow map, Hengill/ Iceland (A?rnason et al. (2010).
Alexander Richter 28 Feb 2018

Australian Hot Dry Rocks PL has developed Heat Needle, a tool that can help produce a surface heat flow map over an entire geothermal prospect for less than the cost of a single heat flow. The company is now looking for partners on pilot studies on geothermal sites.

Several years ago, Australian Hot Dry Rocks reported on its development of a heat needle appliance. The company now has announced that after more than 10 years of R&D on the appliance, the tool has now been successfully introduced.

In a LinkedIn post, Graeme Beardsmore, Technical Director of Hot Dry Rocks PL, highlights posts by Raymi Castilla on the value of heat flow maps and data for geothermal exploration at the regional scale (see herehere and here).

A geothermal energy resource is HEAT, after all, and surface heat flow data provide the most direct evidence of subsurface heat of any geophysical technique. Historically, though, surface heat flow has failed as a routine geothermal exploration method because of the prohibitive cost of obtaining targeted data in areas of real interest. Marine measurements aside, the vast majority of heat flow data recorded in the Global Heat Flow Database were measured in boreholes drilled for another purpose (mostly mineral or petroleum exploration.) While these provide valuable information about the thermal state of the continents in general, they are heavily biased towards areas of mineral and petroleum prospectivity. These are not necessarily areas of interest for geothermal exploration.

Furthermore, heat flow in hydrothermally active regions (volcanoes, extensional zones etc) can vary greatly over short distances, so interpolating heat flow between widely spaced data points can very easily miss prospective targets.

Surface heat flow maps at ‘prospect’ scale would be incredibly value for geothermal exploration. But collecting new heat flow data is expensive. Raymi pointed out that a heat flow ‘measurement’ is really the product of two separate measurements—thermal gradient and thermal conductivity. Thermal gradient in the shallow subsurface is heavily disturbed by solar and atmospheric heating cycles, so boreholes 100 m deep or more are usually required to obtain ‘undisturbed’ gradient values. Thermal conductivity over the same interval is best measured on core samples in a laboratory, another relatively expensive exercise. In the latter stages of exploration for a new convection dominated geothermal resource, more than a million dollars might be spent to drill a small handful of ‘heat flow wells’ as an intermediate step to justify deep appraisal drilling, but conductive heat flow is otherwise ignored as a tool.

But what if we could produce a surface heat flow map over an entire geothermal prospect for less than the cost of a single heat flow well? After 10 years of R&D, Hot Dry Rocks PL has demonstrated a tool that can do just that. We call it the Heat Needle.

The system works like this. We drill a one meter long rod into the ground with an off-the-shelf electric drill, and insert a set of highly sensitive and accurate thermal sensors. These sensors passively record the temperature in the top meter of the ground several times each hour for several months. When processed in the frequency domain, these records of ground temperature tell us the thermal diffusivity of the ground to a precision of ~±1%. We also correlate our site-specific records of surface temperature against more diffuse but long-term satellite-derived records of surface temperature. That lets us extend our own surface temperature record many years into the past.

Above: Recreated daily average surface temperature history. Red are Heat Needle records; Blue were derived from correlated satellite records.

The next step is to forward model the diffusion of the historical surface temperature signal into the ground, using our measured value of thermal diffusivity. We end up with a prediction of the near-surface temperature disturbance due to solar and atmospheric causes. Subtracting that predicted disturbance from our ground temperature observations reveals the subtle, but constant, subsurface thermal gradient.

Above: Reduced subsurface temperatures at 50cm (blue), 70cm (red), 90cm (green) and 110cm (purple) depth after removing the surface temperature signal. A clear gradient of about 1.9°C/m is defined after an initial equilibration period. The Heat Needle is sensitive to much lower gradients than this.

The final piece in the puzzle is that the Heat Needle also contains an active heating coil that allows us to conduct an in situ thermal conductivity test. Multiplying the inferred thermal gradient by the measured thermal conductivity gives us a surface heat flow value. Experience to date suggests an accuracy on the order of ±0.05 W/m2 (±50 mW/m2).

You can download a case study of a trial of six Heat Needles over the Los Azufres Geothermal Region in Mexico from here. It describes the process in much more detail and concludes with the plots of heat flow through time shown below. AZ8 was chosen as a ‘background’ site outside the known geothermal area. The results for AZ2 suggested a transient flux of groundwater during the survey.

The Heat Needle is ready for pilot deployments. Depending on the location, up to 15 units might be concurrently deployed across a region for three months to achieve geothermal exploration goals such as:

  • Quantify the total natural thermal recharge of geothermal systems (when added to convective loss from springs, fumaroles and steaming ground)
  • Delineate the boundaries of geothermal systems
  • Discover ‘blind’ geothermal systems (could be particularly useful in extensional settings)
  • Trace outflow fluid conduits back to their ‘source’ (understand the plumbing)
  • Rule out inappropriate ‘gradient well’ locations
  • Determine whether a resistivity anomaly coincides with a thermal anomaly (discriminate between current and ‘fossilised’ clay caps)
  • Locate ‘upflow’ zones, even though they may be several kilometres from surface manifestations

The ability to produce new conductive heat flow data and maps precise enough to characterise the thermal features of geothermal reservoirs at typical depths of 1–2 km using a relatively cheap and portable surface instrument represents a significant advancement for geothermal exploration. This has never been possible before now! What might a future ‘heat map’ look like…?

Above: A future surface heat flow map over a convection-dominated geothermal reservoir might look something like this resistivity map from A?rnason et al. (2010).

Source: Graeme Beardsmore via LinkedIn