This research was done in the framework of the RVO project on Development of a well impairment model for predicting geothermal clogging (DIMOPREC) .
The importance of developing new energy sources with lower carbon emissions than conventional hydrocarbon based energy sources has been globally recognized (Andrews-Speed, 2016). Geothermal energy is a lower carbon energy source, which can be used for both electricity production and for direct heat use (Fridleifsson, 2001).
However, radioactive mineral scaling can accumulate in filters and tubing of the geothermal facilities, which can be an operational hurdle as this scale needs to be removed with necessary caution. The problem is not only the riskiness of being exposed to radioactive elements, but also the rise in pressure caused by scale accumulation. This occurs at the filters resulting in more process stops. Another problem with the scaling is that it causes increasing injection pressure. Since there is a regulatory limit to this pressure, an increase in injectivity is not preferable. Here, a case of a low-enthalpy geothermal project is discussed where very limited radioactive galena, PbS, is found.
This geothermal system is modelled in the geochemical software package PHREEQC. The PHREEQC model shows that a fraction (78 wt.%) of the collected galena is produced in solid phase from the reservoir, and a smaller fraction (22 wt.%) is formed after the heat exchanger. Gamma ray logs analyses and sedimentation history are presented to find potential sources of Pb and S ions. With the geological history and literature study it is found that the radioactive Pb could be originated from the Zechstein and Rotliegend where it attaches strong to the Copper shale formations. Scale and water analysis show that most of the captured galena is transported in a solid phase into the geothermal facility.
In the second part we discuss the development of a SKID for scaling determination during geothermal production. It is proposed to design a new SKID with additional measurement and monitoring options, which is able to provide the requested input parameters for the PFREEQC model. Requested data acquisition for long term monitoring includes fluid pressure, flowrate, temperature and pH values. The mobile function makes it possible to sample at several surface locations along the line of the geothermal facility. The obtained and stored data can be analyzed and compared to other locations in order to find out whether or not the brine and its composition change, and if so, how it changes
In order to reduce the amount of radioactive PbS in the filters of the geothermal system, it could be considered to acidify the brine to a level where the minerals are dissolved. In addition, increasing the facility pressure and/or increasing the minimum brine temperature after the heat exchanger, could reduce the amount of galena precipitation captured in the filters of the geothermal facility.

In a world transitioning towards renewable energies and lithium-battery powered cars, there has been an annual increase in demand of 6% for lithium and other rare metals between 2000 and 2008 (Stringfellow & Dobson, 2021a), and this increase in demand is forecast to continue through 2040 (Latnussa et al., 2020). Meanwhile, there is a concern that supply will not be able to keep up with demand. Most of the lithium in the European markets comes from other countries such as Chile and Australia, making the European lithium supply vulnerable to supply chain disruptions, as was observed during the COVID-19 pandemic (Latnussa et al., 2020). In addition, the economic value of lithium has increased more than threefold compared in 2022 compared to 2017 (Latnussa et al., 2022).
The geological information of Northern-Western Europe at the latest stage, gives an explanation to the occurrence of lithium in the Netherlands.
50 water samples of 13 fields are considered for the chemical composition and origin of their brine. All geothermal brines contain dissolved lithium and other metals from the reservoir formation and surroundings, allowing for metal ‘mining’ in an environmentally friendly manner. The highest lithium concentrations in aquifer brines are in the province of Drenthe and Limburg, specifically in the Akkrum-13 gas field (47-48 ppm) and the Californie Geothermie (20-28 ppm). The correlation is positive only for lithium against rubidium and lithium against the vicinity of volcanic intrusions, from mineral mobilization through groundwater flow. There are no specific formations with high lithium concentrations.
In Germany, construction has started on a project to extract lithium from geothermal process water, indicating that it is economically viable (Wedin, 2022). For the Netherlands there are 5 different surface options to extract the highest lithium concentrations. Reverse osmosis, nanofiltration, ion-exchange, sorbents and electrodialysis can be used as direct lithium extraction methodologies. One potential economical method to extract lithium from geothermal brines is by utilizing two centralized lithium extraction plants. A plant for the direct lithium extraction with ion-exchange resins, placed before injection wells, and one for the refinement of the material.

One proposed method to delay the onset of gravity segregation between water and gas in enhanced oil and gas projects and extend the period of effective macroscopic sweep in “SWAG” is by separating the injection wells into two parallel horizontal wells (Stone, 2004). In this “modified SWAG”, the water injection well is aligned at a distance above the gas injection well, and both water and gas are pumped simultaneously to displace the reservoir fluids. The significant density difference between the fluids and the ensuing counter-direction flow impede the segregation process.

Initially Rossen et al. (2007) investigated the effectiveness of the technique based on 2D modelling and found that it increased the fluid segregation length. Van der Bol (2007) and Jamshidnezhad et al. (2010) broadened the scope of study and observed a non-uniform gas injection profile and volumetric sweep in 3D. Injection instability occurred in most of the simulation cases, despite the assumption of an ideally homogenous reservoir in the model. Mahalle (2013) identified several factors that triggers instability such as gas saturation and relative permeability behaviors in adjacent grid blocks. Since the instability was found to originate in the near-wellbore region, local grid refinement was applied along the entire length of the horizontal well, with more uniform gas-injection profile observed. Using a different reservoir simulator, Ranjan (2015) extended the previous study by checking again the effect of local grid refinement. The result, however, contradicts with the preceding finding. Grid refinement near the injection well did not improve stability in Ranjan’s study. The author also checked the effect of gas injection rate on the non-uniformity. Again, contradictory results were observed. While Ranjan reported that doubling the gas injection rate promotes non-uniform behavior, the earlier study obtained the opposite outcome.

This thesis extends the previous studies by examining other parameters that may influence non-uniformity. We developed a method to quantify non-uniformity by calculating the coefficient of variation and max-min ratios for gas injection rate along the well. We looked at the effect of changes in well placement, reservoir properties, reservoir boundaries, reservoir fluids and operating constraints. We also fundamentally modified how the perturbation is applied along the gas-injection well by altering the skin factor while maintaining constant permeability. Results show that the type of perturbation significantly effects the non-uniformity of gas injection. We believe that perturbing permeability promotes the uniformity of gas-injection rate because of flow to neighboring grid blocks, and thereby more simulations are seen to be uniform compared the results with perturbation in skin factor. The results from this study suggests that non-uniformity is associated to the feedback between gas injection rate, water saturation and gas relative permeability, which is shown by the gas injection rate to vary more than proportionally to permeability or skin, even in relatively uniform cases. The effect of adjacent grid blocks also plays a crucial role, as we can see from the different results between the two types of perturbations. Finally, the mobility ratio of the fluids strongly influences the occurrence of the instability.