Over the summer of 2021, 1st Subsurface provided three University of Aberdeen students with MSc projects. Focussing on the Southern Permian Basin, and using TROVE KnowledgeBases to inform their research, these projects were a great success.
Over the next few weeks our three students (Alexzandra, Queen & Angeliki) will be sharing their findings. We'd like to thank all three of the students for their hard work throughout and wish them all the best for their careers.
This week, Alexzandra describes her project focussing on the Permian gas fields of onshore Netherlands.
If you would like to reach out to Alexzandra, you can do so here: alexzandrairvine@msn.com
The reign of the prolific Groningen Field within the Netherlands is soon to be coming to an end. The field, which has been predicted to hold enough gas to carry on production until 2080, is due to close in 2022. This is a consequence of subsidence and induced seismicity from the production of gas from the field.
During the summer of 2021, I undertook my summer project as a requirement of my MSc at The University of Aberdeen. Following extensive research, data mining and brainstorming ideas, I came to the topic of; ‘Analysing the Relationship Between Subsidence and Production with Consideration of Geological Factors Across the Permian Gas Fields Onshore Netherlands in Light of the Shutdown of the Groningen Field’.
So, based on the topic of my project, this post will briefly run through what I covered and the results I discovered. I looked at 60 Permian gas fields onshore Netherlands, where I found porosity, pressure, and production values. These were compared against one another and also subsidence measurements found within each field. The aim of this was to find the relationship between geological factors and production with subsidence.
Subsidence within the Netherlands
So, as most of us will be aware, the Netherlands are a naturally subsiding country, with 60% of the country lying below high tide water level. Therefore, it is not surprising the Dutch have been attempting to address this issue since the 1800’s.
The key issue the country faces is the unstable, unconsolidated surface Holocene sediments. These comprise of clay and peat deposits which allow slow deformation. Where there is a lack of infrastructure, the natural creep of the subsurface and periodic fill are the key causes of elevation loss and water table lowering. The Afsluitdijk, is the largest and most prominent defence mechanism which spans 32km along the North coast. However, the next step for the Dutch was ‘what measures can be taken to reduce further subsidence across the country?’ Their solution is to target gas production. This is due to extraction leading to an increase in subsidence and induced seismicity.
The Groningen Field
So, to give some context on the prolific Groningen field, it is situated in the province of Groningen in the North. It has a static GIIP of 2900Bcm and was discovered in 1959, with production beginning with the Slochteren 1 well in 1963. Currently there are 258 wells at 22 production locations which were expected to carry production until 2080. The Dutch Government decided that in 2030 the field was to be shut down due to increased seismicity and subsidence, however, a 3.4 magnitude earthquake in 2018 has led to the shutdown being brought forward to 2022. It has been stated by van Thienen-Visser et al. (2016), that that the field is predicted to have reached 60cm of subsidence in the centre by the original year of cessation (2080). Continued back-up production will be carried out until 2026 to aid the transition of gas demand in the winters.
The effects of Facies Type and Porosity on Subsidence
So, the aim of my report was to analyse the relationship between gas production and subsidence, however, this is not possible without first analysing the key influencing factors on production. The first factors analysed were facies type and porosity. It has been concluded that they do not have a direct relationship to subsidence, but they do have a direct relationship with production.
The most commonly targeted reservoirs are the Rotliegend aeolian sandstones. They are comprised of quartz which is able to stand degrees of compaction. This aided the reservoir’s ability to endure extents of mechanical compaction. This, alongside fluid emplacement within the pore space prior to diagenesis, and a high prior porosity is assumed to be the reasoning behind the relatively high porosity between 10-20% at depths between 2000-4000m.
The Zechstein carbonate reservoirs showed to have much lower porosity, and this is due to experiencing diagenesis which allowed the destruction of pores. Coevorden has the highest porosity of 9% and is also the higher producing Zechstein reservoir analysed and shows the highest amount of subsidence
This is a topic in itself which I found particularly interesting. With more time, I think this would have been very interesting to delve deeper into.
Reservoir pressure and its decline
Reservoir pressure has been determined as the crucial link between production and subsidence. When plotting the relationship between reservoir pressure and depth, a clear trend of increasing pressure with depth was seen. However, regarding production, the fields which hosted higher pressures showed to have a higher production value.
But with gas extraction comes the inevitable drop in reservoir pressure. This increases the fields subsidence rate. Resulting in increased effective stress within the matrix and in turn, inducing displacement and compaction of the reservoir.
Pressure decline within the reservoirs is a key issue which is faced and also encourages subsidence. For example, the Groningen field suffered a loss of 27.7MPa over 58 years of production. Currently, the Bedum, field has the highest average subsidence when compared to pressure difference. This field has experienced a pressure drop of 22.49MPa over 43 years.
The relationship between subsidence and Production
The two maps below represent the relationship between production and subsidence regionally. Figure 5 shows the fields without the influence of the Groningen field, and it really shows just how much the field produces in comparison to the smaller fields.
They highlight the Lauwersmere Trough which is located within the Friesland province. The map shows the bubbles to be overlapping and this shows their close proximity to each other.
The majority of fields do not exceed over 6cm of average subsidence. However, as the maps show, fields with higher production levels do present themselves within the high category.
Another reasoning to why fields have high subsidence but possibly not a high production rate is their location. For example, the Ameland-Oost field (highlighted by the star in Figure 6) is located on a Frisian Island which is more susceptible to subsidence due to the soft, unconsolidated sediments. Within Holland, all but two fields lie within the high subsidence category. This is due to the naturally high water-table within this province, meaning 26% of the region is below mean sea level.
Subsidence Prediction across Onshore Netherlands
A prediction of average subsidence across the fields was carried out using maximum production volumes and realised production provided by the government alongside the average subsidence found across each field. The Dutch government introduced production caps and therefore, the subsidence value prediction is based on if these caps are met.
A proxy of 1cm per year was used to predict the time scale of these increases. The table shows fields which exceed 10cm of subsidence and their overall subsidence prediction. For example, the Ameland-Oost field is predicted to subside an additional 16cm and therefore, by 2037 it will have reached a total of 19.37cm.
Next Steps for the Netherlands:
Over the next 5 years, the Dutch government and NAM, the owner of the Groningen field is expected to spend around €24 billion. This includes the predicted €7 billion decommissioning cost and the €1.2 billion which will be dedicated to boosting the local economy, increasing property value, and building reinforcements.
A gas conversion facility is also to be introduced, costing €500 million. This is to convert the low-calorific gas (L-gas) to high (H-gas). As around 5 million gas appliances and plants within the Netherlands run off L-gas, they will have to be converted to H-gas appliances by 2030.
In order to combat the 54Bcm loss from the Groningen field, smaller fields are expected to increase their production. This is alongside the importation of gas from Norway every year.
Recommendations
Finally, based on the findings within this report, a couple of recommendations can be made.
The first, and most important one is the proposed monitoring of the Groet, Tietjerksteradeel, Middelie, Coevorden and Ameland-Oost fields. These are the fields highlighted to subside more than 10cm over the next 10-16 years. This, alongside increased production from the smaller fields, should be closely monitored as increased production will lead to an increase in subsidence.
Regarding the decommissioning of the Groningen field, special care should be taken during pipe removal procedures, particularly within deviated wells. This is due to depleted reservoir pressure from production increasing stress on the surrounding reservoir. This can cause the lateral support to be lost and the casing to collapse.
I would like to give a special thanks to Ben Duncan and Malcolm Pye at 1st Subsurface for helping me throughout this project. Also, the whole team at 1st Subsurface for providing the topic of my project and help along the way. I have really enjoyed learning more about this very current and crucial topic.
References used within this article:
- Haasnoot, M., Kwadijk, J., van Alphen, J., Le Bars, D., van den Hurk, B., Diermanse, F., van der Spek, A., Essink, G., Delsman, J. and Mens, M., 2020. Adaptation to uncertain sea-level rise; how uncertainty in Antarctic mass-loss impacts the coastal adaptation strategy of the Netherlands. Environmental Research Letters, 15(3), p.034007.
- NLOG, n.d. map-boreholes. [online] NLOG Dutch Oil and Gas Portal. Available at: <https://www.nlog.nl/en/map-boreholes> [Accessed 20 July 2021].
- van Thienen-Visser, K., Sijacic, D., van Wees, J., Kraaijpoel, D. and Roholl, J., 2016. Groningen field 2013 to present Gas production and induced seismicity. Utrecht: TNO.
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