Studies into Dutch Ultra-Deep Geothermal Energy (UDG)

For the Ultra-Deep Geothermal Energy (UDG) Green Deal, the studies listed below were carried out between 2018 and 2020. Within this collaboration, the participating parties investigated whether Ultra-Deep Geothermal Energy can be safely developed in the Netherlands and, if so, where and under what conditions.

The studies primarily focus on the Dinantian carbonate formations (for example, limestone), which were considered promising for the potential development of Ultra-Deep Geothermal Energy. The studies are also valuable for geothermal energy production at conventional depths.

Seismic interpretation/time-depth conversion

Based on seismic data, maps were created showing the distribution, depth and thickness of the Dinantian carbonate formations. As (high-quality) seismic data was not available everywhere in the Netherlands, the maps contain gaps. Since then, the availability of seismic data has improved significantly through seismic acquisition carried out under the SCAN programme, which could be used to update these maps. Publication available via NLOG.

Burial and structuration

This study reconstructed the burial history of the Dinantian carbonate formations in parts of the Netherlands. It provides insights into how the properties of these formations have changed over geological time due to temperature and chemical processes deep within the Earth. These insights help us better estimate formation properties, such as permeability, in other locations. Publication available via NLOG.

Gravity and magnetic interpretation

In addition to seismic surveys, other geophysical measurement methods can be used to map the subsurface. These datasets contain less detail but can support seismic interpretations, particularly in areas where little or no high-quality seismic data is available. Some of these non-seismic methods, such as gravity and magnetic surveys, are already widely used in geothermal exploration. Publication available via NLOG.

Electromagnetic geothermal exploration

Other non-seismic methods, such as Magnetotellurics (MT) and Controlled Source Electromagnetics (CSEM), are less commonly used and less established. A study was conducted to assess whether these techniques could be applied within the Dutch geothermal sector. Publication available via NLOG.

Fracture characterization

The Dinantian carbonate formations are generally characterised by low natural permeability. Small fractures are important to enable sufficient water production from these rocks or to inject water into them. This study investigates whether fracture density and fracture permeability can be estimated. Publication available via NLOG.

Diagenesis

The permeability of the Dinantian carbonate formations can increase through the natural dissolution of rock material, creating so-called karst features. Conversely, permeability may also decrease as minerals or metallic particles precipitate within the rock as a result of other chemical processes. This report examines which processes may have occurred in areas where no drilling has yet taken place. The analysis combines observations from previous wells and outcrops with the burial history study. Publication available, together with the Facies Distribution study, via NLOG.

Facies distribution

This study investigated the distribution of different rock types within the Dinantian carbonate formations in the Netherlands. Significant differences exist between carbonate formations deposited on platforms (comparable to coral reefs) and contemporaneous deposits laid down in deep marine environments (comparable to shale formations). These variations were examined using rock samples from previous wells and examples of similar rocks exposed at the surface in neighbouring countries. Publication available via NLOG.

Petrophysical evaluation

This study used measurements from previous wells targeting the Dinantian carbonate formations to determine rock properties. The results show that these carbonate formations are often characterised by low porosity and permeability, except where fractures and karst features are present. Measured temperatures, pressures and fluid compositions were also compiled and analysed. Publication available via NLOG.

Temperature model

The deeper you go, the warmer it gets. The extent of this increase depends on the composition of the subsurface, particularly in the case of the Dinantian carbonate formations. This study examines temperature variations within these formations using data from wells in the Netherlands and abroad. The result is an improved temperature model for the deep (>3 km) Dutch subsurface, enabling more accurate temperature predictions. Publication available via NLOG.

Stress field

Natural stresses are present in the subsurface. These stresses can vary in both orientation and magnitude. A better understanding of these stresses is required to more accurately assess the permeability and properties of the Dinantian carbonate formations. This report contains a large-scale inventory of stress orientations and magnitudes, an analysis of the impact of local geological factors on stress fields, and guidelines and best practices for stress field modelling. Publication available via NLOG.

Productivity and injectivity

The aim of this study is to improve estimates of the performance of potential UDG doublets. Because many aspects of the subsurface remain uncertain, including permeability, temperature and faulting, the results are subject to significant uncertainty. Publication available via NLOG.

UDG Hazard Register

Drilling ultra-deep wells cannot be compared to drilling conventional wells. Every section of the well must be assessed for high pressures, high temperatures and unexpected geological conditions. A group of experts from the oil and gas industry and the geothermal sector compiled a list of risks that may be encountered during UDG drilling operations. This UDG Hazard Register can be used to assess location-specific risks and determine how these risks can be mitigated. Publication available via our website.