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General solved and unsolved issues / Gas aspects in geothermal systems

Authors:
N Hartog and F Eichinger, Gas aspects in geothermal systems, in Operational issues in Geothermal Energy in
Europe—Status and overview, S Schreiber, et al., Editors. 2016, Geothermal ERA NET: Reykjavík. p. 78-79
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4.2 General, solved and unsolved issues / Gas aspects in geothermal systems
Niels Hartog1, Florian Eichinger2,
1KWR Watercycle Research Institute, Groningenhaven 7, Nieuwegein, NL, niels.hartog@kwrwater.nl
2Hydroisotop GmbH, Woelkestr. 9, 85301 Schweitenkirchen, D, fe@Hydroisotop.de
The total amount and composition of gas dissolved in geothermal waters are important conditional aspects to
consider in the operational and risk management of geothermal systems. The resulting total and partial gas
pressures vary strongly between different geothermal areas. With respect to corrosion risks, particularly the
height of partial gas pressures for H2S and CO2 determine the sensitivity. Therefore, material selection
should be made with these conditions in mind.
With respect to scaling risks, the main gas related aspect is the extent to which CO2 degassing occurs as this
triggers the precipitation of carbonate minerals (as described in the detail in the scaling chapter). Depending
on the pressure difference between the total gas pressure and the pressure maintained in the above ground
installation, degassing will occur. In the absence of over- or under pressured reservoir conditions, this total
pressure is equal to the hydrostatic pressure exerted by the height of the overlying water column. With
increasing depths, the hydrostatic pressure increases linearly and for reservoir depths over 2km, hydrostatic
and associated maximum total gas pressures would be over 200 atm. Operating the surface part of the
geothermal installation under excessive pressures to prevent degassing becomes unrealistic. However,
whether or not the dissolved gas pressure equals the hydrostatic pressure depends on whether or not
sufficient gas is available. In the presence of known near-by free gas occurrences, this might be possible.
Bottom hole bubble point determinations therefore provide crucial information for pressure management in
geothermal system.
Since typically, the above-ground geothermal system is operating under pressures that are at least multiple
times lower (e.g. 15 atm) than the hydrostatic pressure in the aquifer from which the geothermal water is
produced. In such cases, it is expected that degassing occurs as the produced geothermal water is pumped
upward along the pressure gradient. This is in keeping with the observations for multiple geothermal systems
in The Netherlands where the pressure in the production well had already dropped below the bubbling point
(GPC/KWR, 2014), i.e. the gas pressure already exceeded the hydrostatic pressure in the upper part of the
well and a free gas phase had already formed. Compositional analysis of the gas phase indicates that the for
the largest part the gas is composed of methane (Figure 31), although with significant amounts of CO2
present (up to 20%) . For the highest CO2 fraction in produced geothermal water in this figure a partial
pressure of over 10 bar is calculated (with 53 bar total gas pressure). However for other geothermal systems
elsewhere in Europe (e.g. Austria) CO2 can be the dominant fraction (>90%).
Finally, the presence of free gas phase in geothermal systems can itself cause clogging problems, as
reduction in water permeability occurs in the presence of free gas. To control the degree of degassing that is
allowed, adequate system pressure control for the above and below ground parts of the installation is key.
The most common control measure to get rid of excess gas is the use of a degasser, which will aid the
prevention of gas clogging by removing the amount of free gas that is produced by the time the produced
water reaches the surface operation. In the evaluation to what extent excess gas can be released to the
atmosphere the composition of main and minor components (e.g. H2S, Hg) in the gas need to be considered.
Pressure keeping to avoid degassing is an economic factor. The coupled application of inhibitors and lower
operation pressures can increase the rentability of geothermal systems and prevention of carbonate scalings.
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Figure 31 The relative proportions of methane (CH4) versus carbon dioxide (CO2) in the total amount of gas extracted in
produced water from geothermal systems in The Netherlands (white circles) from various studies (e.g. (Hartog, 2015)). For
reference, the grey plusses indicate gas compositions measured for Dutch oil and gas production sites (nlog.nl).
4.2.1 References
GPC/KWR, 2014: Assessment of Injectivity Problems in Geothermal Greenhouse Heating Wells.
Hartog, N., 2015: Geochemical Assessment of Injectivity Problems in Geothermal Wells - a Case Study for
Several Greenhouse Geothermal Systems in the Netherlands. KWR 2015.012, KWR Watercycle Research
Institute.
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Technical Report
Full-text available
The horticultural sector is making increasing use of geothermal heat as a renewable substitute for the burning of natural gas. However, so far many of the completed geothermal systems in the Netherlands do not function as planned. After heat extraction, the water is reinjected into the reservoir. A frequent problem with this is the poor injectivity of the injection wells. The causes of injectivity problems at several geothermal systems used by the greenhouse industry were investigated by analysing the geochemical aspects and processes involved. This study focuses on the causes of the poor injectivity. Within this research GPC (France) focussed on the measurements of bubble point pressure and suspended particle size distributions, while KWR Watercycle Research Institute focussed on the (geo)chemical processes that occur in the geothermal systems. KWR’s research is reported here. Particular focus was on the precipitation of minerals, as mineral scaling was suspected to be a major contributor to the injectivity problems. It is concluded that the main cause for accumulation of minerals on filters and as scaling is the degassing of CO2 during the rise in the production well and in subsequent parts of the geothermal system. The CO2 degassing results from the drop in the gas pressure at reservoir depth (hydrostatic pressure) to the relatively low pressure at which the geothermal system at the surface is operated. The loss of CO2 results in subsequent precipitation of carbonate minerals. Depending on the chemical composition of the geothermal water in the reservoir, Ca-rich, Fe-rich or Pb-rich carbonates were preferentially precipitated, as was confirmed by analysis of filter and scaling accumulates from various systems. Since degassing of CO2 pressure acts as the main driver for the carbonate precipitation observed, CO2 pressure control also provides a solution. Luckily, since CO2 appears to only represent a relatively small fraction of the total gas pressure in the geothermal reservoirs studied, only a limited level of CO2-dosing is required to prevent or re-dissolve carbonate precipitates. The required partial CO2 pressure seems well achievable within the operational pressures currently maintained in the geothermal systems. For sites with lead carbonate precipitation, an increased CO2 pressure is required to compensate for the decrease of lead carbonate solubility with lower temperatures.