Hagen Bültemeier (DBI Gas- und Umwelttechnik GmbH) about Bio-UGS
Biological conversion of carbon dioxide and hydrogen to methane in pore underground gas storage facilities (UGS)
CO2WIN Connect: Dear Mr. Bültemeier, in the Bio-UGS project, among other things, various German pore underground storages (UGS) are to be analyzed with regard to their utilization potential for the microbial production and storage of methane, which is produced from industrial CO2 and green hydrogen. Can you and say if suitable reservoirs/UGS have already been identified or, if the statement that all reservoirs are basically usable is still true?
Hagen Bültemeier: We are working intensively on this very question in the Bio-UGS project. All pore underground gas storage facilities (UGS) currently in operation have a theoretical total potential of about 19 billion Nm3 total gas volume. Whether this can also be used for underground bio-methanation depends on a number of influencing factors. First of all, the occurrence of methanogenic microorganisms, which serve as catalysts for the reaction of hydrogen and CO2 to methane and water, as well as the pressure, temperature and saline conditions of the respective reservoirs.
Through the obtained sample material and corresponding laboratory experiments on methanation rates and microbial growth, the conditions beneficial for bio-methanation could be identified. It is found that the salinity of the reservoir water should be less than 50 g/L, and the temperature should be between 25 and 55 °C. Temperature correlates strongly with the depth of a reservoir, whereas salinity varies greatly. As a rule of thumb, it can be said that the pore UGS in northern Germany have a higher salinity than those in southern Germany. Hence, it is already foreseeable that only a part of the German UGS will be suitable. In addition, however, there are other sites with similar geological characteristics that are not currently developed / used as storage facilities. The analysis of the overall potential is the subject of extensive reservoir simulation work. For an insight into this I refer you to the publication "Development of a New Workflow to Underground Biomethanation Simulation Using CMG STARS", shown at the World Gas Conference 2022. The final results will then be presented at our final workshop on the project.
CO2WIN Connect: Would you please explain in a bit more detail what criteria make an attractive reservoir (in terms of the project)?
Hagen Bültemeier: In addition to the temperature range and salinity mentioned above, the presence of methanogenic microorganisms is critical. However, if sufficient concentrations of methanogenic microorganisms are not encountered in a geologically suitable formation, the artificial introduction of such could still be conceivable. This is currently still being investigated in laboratory tests.
From a geological and reservoir-mechanical point of view, a reservoir should also have good flow characteristics, so that the bio-methane formed can flow well to the production wells. Furthermore, a certain trap structure, e.g., anticlinal formation (example see figures 6 and 7 in the above-mentioned publication) of a reservoir formation over several meters depth is advantageous here, so that the water, which is also formed during methanation and is heavier than methane, remains in the deeper layers of the reservoir and flows as little as possible or not at all to the production well and thus displaces the methane.
CO2WIN Connect: Your DBI website we can read that you planned holistic assessment of facilitating and inhibiting influences on the UBM. We have already heard about the geology and petrophysics. What is it about abiotic reactions?
Hagen Bültemeier: Besides the bio-methanation, there are potentially other undesirable microbial side reaction, which are subject to our investigations. Most notable among these are the sulfate reduction and the acetogenesis, shown in the two equations below:
- SO42- + 5 H2 ---> H2S + 4 H2O
- 2 CO2 + 4 H2 ---> CH3COOH + 2 H2O
Both reactions consume the hydrogen, but do not lead to the formation of the desired methane. The first reaction in particular is problematic because it results in the formation of H2S, i.e., hydrogen sulfide. This H2S is toxic and has a corrosive effect already above a concentration of 1 vol.-% in the methane formed. Hence, appropriate protective equipment would become necessary. However, the CO2 injected as a starting material also places demands on the technical drilling equipment, since it is also slightly corrosive when dissolved in water.
CO2WIN Connect: Based on your findings on reaction rates, gas saturation distributions, mixing processes, pressure development, water drive, etc., realistic operating scenarios were intended be designed for the different sites / reservoir types. Can you tell us more about this?
Hagen Bültemeier: Yes, based on the range of geological, mineralogical, and petro-physical properties of the German UGS, several reservoir models have been created to represent the different reservoir types and the relevant mechanisms in each case (e.g., a strong water inflow in an aquifer or a high fraction of residual natural gas in a depleted reservoir). These reservoir simulation models have been coupled with microbial reaction kinetics, and analysis is now underway to determine how bio-methanation behaves in which reservoir and UGS (types), i.e., where a particularly large amount of methane is formed especially quickly, where flow processes are most beneficial, or how many storage cycles per year one can work with. Here, the work is still ongoing.
CO2WIN Connect: The methanogenic microbes in the UGS are a key element in the project. How sensitive are these microbes to the conditions prevailing in the UGS? That is, is the microbial population stable, or could fluctuations or run-away effects be expected?
Hagen Bültemeier: Fluctuations can be expected. Both the injected hydrogen and residual natural gas (which is a carbon source) serve as nutrient sources for the microorganisms. The microbial population, thus, depends on the one hand on the geological conditions in the reservoir (see above and also Figure 2 of the above-mentioned publication), but also on the available nutrient supply. Consequently, the potential bio-methanation is directly dependent on the microbial population. Thus, the microbial population fluctuates depending on the available nutrient supply. At times when there is a surplus of nutrients for the microbial population, the growth will be exponential - and vice versa.
Besides that, we must also consider the bio-methane production rates, which we have assessed in laboratory experiments under reservoir conditions. These rates range from 0.0003 to 6.5 mol per day and per cubic meter of microorganisms under reservoir conditions, which is a not insignificant range and illustrates that certain reservoirs will be more suitable than others. Thus, the amount of bio-methane that can be produced annually will also vary widely, affecting the possible storage cycles per year.
CO2WIN Connect: In order to take the next step, the project has been looking for interested UGS operators since the beginning. Have any partnerships or discussions been initiated during the course of the project?
Hagen Bültemeier: Yes. We have several UGS operators as official associated partners (Storengy, VGS, terranets BW, NAFTA), as well as others who support the project anonymously. Some companies were already associated with the project during the application phase, others joined during the project implementation. For the UGS operators, the primary question is, of course, whether underground bio-methanization can be a viable business model in the context of the energy transition.
CO2WIN Connect: The project is now nearing completion. What is the next step? In the industrial implementation, there will certainly be a need for I&C technology and sensors to measure, for example, the hydrogen and methane slip, as well as to record and control the conditions (temperature, pressure, gas concentrations). How far have concepts of implementation in this area matured?
Hagen Bültemeier: The development of suitable monitoring concepts and procedures is indeed a work package of the project. However, I would like to wait until the end of the project and the final workshop with our partners to answer this question, since industrial implementation certainly depends to a large extent on the economic viability, and that brings us back to the operating scenarios.
CO2WIN Connect: Dear Mr. Bültemeier, thank you for the insights into the project. We are looking forward to hearing about the final outcomes at the CO2WIN Final Conference in September 2023!
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