The risk of drilling into marine clays

While drilling a geothermal well in Trondheim in May 2020, high groundwater pressure was encountered under massive deposits of potentially unstable clay. To avoid damage, the drilling contractor must recognize and understand such scenarios and respond with due diligence.

The example demonstrates that basic knowledge of geology and groundwater is essential for establishing groundwater wells and safe energy drilling.

During geothermal well drilling at Singsaker in Trondheim in May 2020, Båsum Boring Trøndelag Inc. registered unusually challenging ground and groundwater conditions. After drilling through about 21 metres of marine clay, the drill contractor encountered a ca. 7 m thick layer of sand and gravel with good water flow capacity and high groundwater pressure. The well casing was drilled 3 metres into bedrock found at 28 m depth and then sealed before drilling to 150 metres deep.

Up until then, everything went according to plan for construction of a closed-loop geothermal heating system (link in Norwegian).

The Problem

Immediately after the borehole was completed, the drilling contractor discovered that a significant amount of water was flowing along the outside of the casing and up to the surface (Fig. 1A).  The drilling contractor assumed that vertical drainage (a leakage) had occurred along the well casing during drilling and that groundwater under high pressure in the gravel layer at 21-28 metres - so-called artesian conditions was forced up to the surface.

The contractor understood that the situation was critical and could result in settlement damage or trigger a landslide, in the worst case. Therefore, he contacted the Geological Survey of Norway (NGU) for professional assistance. In consultation with hydrogeologists at NGU, Trondheim municipality’s technical department was notified, and a plan was quickly established to seal the leak.

Execution

A secondary relief well was drilled to reduce the water pressure at the primary well, thereby enabling sealing of the leakage along the primary well casing. While drilling the relief well, a so-called ‘blow-out’ occurred at about 9 metres depth. As a result, air and water from the drilling head were suddenly pushed up along the outside of the well casing. According to the drilling contractor, this was probably caused by the ca. 2-metre soft clay layer found between ca. 7-9 metres. The soft clay caused a hydraulic short circuit between the energy well and the relief well, so groundwater was released through the well casing of the relief well instead of along the outside the energy well (Fig. 1B and photo above). In effect, the driller established a pressure relief from ca. 0-9 metres.

It was decided to inject sealants along the outside of the pipe sleeve from 0 to 9 metres. In addition, chemical sealants were used to ensure good waterproofing. While injecting the sealant, the mass was pushed down along the outside of the geothermal well casing, through the short circuit and into the relief well.

Both water leaks were thus sealed, building up a hydraulic barrier, and halting uncontrolled drainage of groundwater (Figs. 1C and 1D).

Figure 1 Sketch of the situation at the borehole and measures to stop the leakage. Illustration: Cecilie Bjerke.

Why does this occur, and can it occur elsewhere?

This type of event is unusual but not unique: It can occur when drilling in localities below the marine limit, where thick, dense clay covers more permeable layers, as illustrated in Figure 2 and 3.

This area is also where unstable clay, quick clay can be encountered and collapse during drilling. In addition, over time, drilling or other subsurface activity can release groundwater from sediments or bedrock that underly massive clay deposits. The release of groundwater can lead to pore pressure changes and, subsequently, clay compaction that could damage buildings and infrastructure.

Through 2013-2017, the City of Oslo carried out a major interdisciplinary and cross-departmental project to increase subsurface knowledge: “The Underground Project (link in Norwegian). The background of the project was that increased subsurface exploitation had resulted in an increasing number of conflict-of-interest cases and an increased risk of damage to buildings and infrastructure below and above the surface. For project participants, it was especially important to communicate the risk of damage to built structures after lowering groundwater levels and drilling geothermal wells.

An important part of Oslo’s Underground Project has been identifying measures that could reduce damage to buildings and facilities that result from groundwater level changes. We highlight some key recommendations from this project.

Figure 2 Regional overview of the area with massive clay deposits and possible high groundwater pressure that increase the risk of ground settlement or landslides. A cross-section is shown in Figure 3, below. Map: Skrednett/NVE.

Figure 3 A cross-section outlines the situation with artesian groundwater. Illustration: Cecilie Bjerke

Planning and regulations

The Planning and Building Act is an essential tool for land use management, both above and below ground. The Act gives requirements on all levels of planning, from general areal planning down to individual building projects. Oslo’s Underground Project recommends that the use of the subsurface and ground information is assessed throughout all planning phases. Groundwater and ground conditions should be taken into special consideration when there are challenging ground conditions, as illustrated by the Singsaker, Trondheim example.

The Underground Project proposed that decisions for how groundwater levels are managed should be secured in new municipal plans and that the planning concept using ‘zones of special consideration’ (normally used for noise, heritage etc.) is expanded to include areas that are particularly vulnerable to groundwater changes.

Drilling should be subject to application

Generally, all subsurface interventions require a formal application to the municipality if they are not covered by the exemption provisions in Norway’s Planning and Building Act or Norway’s Building Codes. However, in practice, many subsurface interventions are exempt from applying for approval, including drilling for geothermal and groundwater wells.

If applications would be made mandatory, public authorities would have been able to better manage sites, reduce damages to infrastructure or unwanted consequences resulting from changes in groundwater levels. Therefore, NGU supports the recommendation by the Underground Project to introduce mandatory applications for all groundwater and geothermal drillings.

The municipality is responsible for the disclosure of information

The municipality is responsible for ensuring that the Public Map Data (DOK) on underground conditions is up to date. When the municipality has evidence that natural hazards are present, such as subsidence or marine clay landslides that can be triggered by groundwater pressure changes, it is the site developer’s responsibility to document that the hazard does not exist or outline measures to mitigate risks.

Artesian groundwater

Groundwater from a closed aquifer where the water pressure is higher than the corresponding to water table. This can occur when an aquifer is located between two dense layers in lower-lying areas, at the same time as recharge takes place in higher-lying areas.