It is important that the production of geothermal energy is sustainable so that energy production can be maintained as long as possible without lasting impact. Geothermal energy production has to be less than or equal to the maximum processing stage of the geothermal area for the production to be sustainable. The geothermal energy production in geothermal areas in Þeistareykir, Krafla and Bjarnarflag should be less or equal to EO which is the maximum processing stage of a geothermal area for making the production sustainable. The pressure has to be constant and mass removal less than or equal to the inflow into the geothermal system. 

The flow into the geothermal system increases with less pressure in the system. Therefore it is important to find balance between the extraction and recharge. When an acceptable balance has been found this can be considered sustainable or the sustainability of the area can be determined. The longest history of processing from one geothermal area in Iceland is from Bjarnarflag, more than 50 years. In that time, no drawdown has appeared to have caused a change in the surface activity or drainage from the area.  

This index will publish information on mass removal in kg/s (ktons/year), pressure in bars/time and depression in bars.

Mass: Mass removal from a geothermal area is a part of the production system of the power station where a specified amount of steam goes into turbines and an amount of separated liquid is pumped down into the geothermal system or released in other ways.  Graph 1 here below shows the mass- and energy removal from one of the production areas in Krafla where measuring trajectories for mass removal, heat content and deep injection are shown. The mass can be separated into steam and water. This can also be broken down more to show one year at a time. 

Graph 1. An example of yearly production from the lower half of Leirbotnar in Krafla.

Aside from using direct performance measurements from boreholes to evaluate the mass removal from geothermal areas they use gravimetry. They are built on comparison between measurements from one period to another. Base measurements (0-positions) are made before production starts and thereafter measurements are made every 3-5 years and based on those results an evaluation is made whether changes in mass have occurred beneath the measurement areas and how those changes seem to be.

Pressure: Operational pressure of each hole is monitored regularly (Po ~ top of the hole) and the measurement is bar-g. That way a regular assessment is made on the flow from the hole, based on a certain pressure at the top of the hole. There is also a reason to keep the monitored holes, ones that do not blow (in production). They show uninterrupted pressure and heat in the production in the geothermal system and it becomes easier to evaluate the effects the production has on the geothermal system with regular measurements. Graph 2 shows pressure measurements from hole KJ-18, in the southern hillside of Krafla at 1000-2000 metres, along with the initial pressure when the hole was drilled, and the depth to the water surface measured from the top of the hole. For comparison is the overall production in the area.

Graph 2. Overall production, pressure and water level in hole KJ-18 in Krafla.

Mass removal from a geothermal system is based on the extents of the energy production and the size of the geothermal system. Pressure/depression is evaluated in three ways:

1. Measurements in boreholes that are not being used but well connected to the area. Depression is measured where it is likely that the geothermal system is being impoverished. 
2. Measurements of the initial pressure and temperature in boreholes. When new holes are drilled the pressure is measured in those and compared to the pressure as it was in the holes while they were new. 
3. With recovery measurements in working holes.

At the beginning of operations, a depression happens that then gets balanced again after a certain amount of processing period. When balance has been reached the natural inflow is equal to the mass removal. If the depression is steady the mass removal is too high and can impoverish the system. 

"For each geothermal area and each production method there is a certain maximum processing load, EO, which means that with a lower processing load than EO it is possible to keep the energy generation from the system unchanged for at least 100 years. If the processing load is higher than EO it is not possible to keep the energy generation from the system unchanged for a 100 years. If the processing load is higher than EO the energy generation can not be kept unchanged for that long. Geothermal processing that is less than or equal to EO is defined as sustainable but processing in excess of EO is not sustainable." (Guðni Axelsson et al. 2001). 

Graph 3. Energy generation as fall of time. If the processing load is less than or equal to EO the production is sustainable. (Mannvit engineering 2010).

Frequency:

Frequency of measurements is determined by their purpose. The majority is operational but can also be used for monitoring. The pressure at the top of working holes is measured once per week and sometimes more often if necessary. Productivity of the holes is normally measured 1-2 times per year. They are taken out of the power-harnessing circle while that is done. This has been done for the last few decades but new measuring methods, that have increased in popularity in the last few years, make it possible to measure more frequently without taking the holes out of commission. The efficiency of motors in power plants is also an indirect measurement of mass removal from the geothermal area but recording measurements are done throughout the year.  

Temperature and pressure measurements in supervised boreholes have been done once per year (graph 2) in Krafla and Bjarnarflag. A similar arrangement is expected in Þeistareykir when the power station starts operating. In addition to measurements in boreholes gravimetry analysis should be done every 5 years. 

Area

Central area: Þeistareykir, Krafla and Bjarnarflag. 

Accountability 

The project manager is responsible for acquiring the data and publishing it according to the publishing schedule.

Source Material

Data will be collected from Landsvirkjun and the National Energy Authority.