Rock salt

In the Netherlands, rock salt is produced from the deep subsurface by solution mining, which involves dissolving the salt in the subsurface by injecting hot water through an injection well into a rock salt layer. The saturated salt water (brine) is pumped back up to the surface and processed into salt products. This type of salt mining results in underground cavities (caverns). Depending on the depth and manner in which these cavities are closed, the natural stresses in the relatively plastic salt may cause the cavity to gradually collapse over time.

Presence of rock salt in the Netherlands

Rock salt layers in the subsurface occur mainly in strata of the Zechstein Group (laid down between approximately 251 and 260 million years ago) and the Röt Formation (laid down between approximately 238 and 244 million years ago). Salt originally precipitated in shallow, partly restricted salt lakes. When the sea water evaporated, salt layers precipitated at the bottom of the lake or lagoon. Typical evaporate cycles consist of (from bottom to top) limestone (precipitated at normal sea-water salinity), anhydrite and eventually (with a further increase in salinity) rock salt and potassium and magnesium salts. A new cycle started with every new large-scale influx of sea water. This resulted in four to five Zechstein evaporite cycles, which form several tens to several hundreds-of-metres-thick rock salt sequences in the subsurface of the Netherlands.

At present, rock salt occurs both in layered sequences and in salt pillows and salt plugs. The latter two develop after the salt has been deposited. After a salt layer has become buried by an increasingly thick overburden, the temperature increases because of the deep burial, and the salt starts to exhibit relatively plastic behaviour. If subsurface salt layers move only slowly, e.g. in the case of faults, salt may develop into a pillow-shaped body that can eventually grow into a salt diapir, depending on the irregular loading by the overburden. Salt pillows are smoothly shaped domes in salt layers, while salt pillars or diapirs are narrower, often steeply dipping to even mushroom-shaped structures that have pierced the overlying rock strata and can reach heights of over 2.5km.

Maps, data and information

Determining production potential

The following geological and technical characteristics determine the regional potential for salt production:

Depth to the top of a salt accumulation
Although depth does not directly restrain salt production, it does determine the mining technique and the opportunities for re-using caverns later, for instance as storage facilities. We therefore distinguish between two types of salt mines:

  • Deep mine cavities (>1500m) in layered or pillow-shaped salt deposits. The cavities, formed by dissolution, are not suitable for use as storage facilities because salt at that depth is so plastic that the caverns are unstable and will ultimately collapse.
  • Shallow mine cavities (<1500m) in salt pillars, layered or pillow-shaped deposits, in which the resultant caverns are stable enough to be kept open.

Aggregate thickness of rock salt strata within the main layer
The depth, thickness and purity of a rock salt layer determine its economic recoverability. Volume is also an important factor in determining a cavern’s safety and stability. The total salt volume has to be sufficiently large to leave enough salt so that the walls and roof of a cavern that has been dissolved are sufficiently strong.

Deep salt deposits less than 200m thick are considered to be ‘of limited economic viability’. This is also because the salt sequence may not be entirely pure with other rock types intercalated. To recover shallower salt deposits, it must be possible to create a sufficiently large cavern with a roof that is stable. That is why a salt thickness of 300m is considered suitable within the 0-1500m depth range for creating large cavities, and a thickness of 50m is suitable for small, low caverns.

Salt quality
Mining targets salt layers with a composition that is as pure as possible and contains as few insoluble parts as possible. Compositions and homogeneity of the salt are interpreted from well logs. Salt pillars can be graded from suitable to unsuitable on the basis of seismic data, which show where, for instance, large anhydrite banks within salt bodies are located.

Sizes of salt structure
When constructing caverns, a safe distance has to be maintained between the boundaries of a salt body and the cavern, and also between adjoining caverns. There should be sufficient rock left to form a stable roof. As a rule of thumb, caverns with a diameter of 90m require a distance of 300m between the central axes of the caverns. The distance between a cavern and flank of a salt pillar should be at least 150m. Some salt structures are not suitable for making caverns because of the pillars’ shape or limited volume.

Possible interference with other applications and re-use of salt caverns

Salt mining takes place in an isolated cavity that is separated from its surroundings by thick salt sequences. The risk of direct interference (pressure, gas or fluid communication) with adjoining functions is therefore very low to non-existent. The following aspects should be addressed when a mining location is constructed:

  • If the salt layer acts as a seal for an underlying activity, sufficient separation should be maintained.Subsidence caused by salt mining is additional to subsidence caused by other activities in the direct vicinity.
  • Subsidence should therefore be assessed in combination with any other activities.

Salt mining may create opportunities for future storage, provided the dimensions of the caverns meet the requirements of such storage facilities. This applies particularly to depth (because of the pressure) and volume. Salt caverns are especially attractive for storing natural gas, nitrogen, compressed air, hydrogen or diesel fuel. Carbon dioxide storage is technically possible, but is a less interesting option because depleted gas fields can accommodate much larger volumes of CO2. National research programmes are currently investigating the possible use of rock salt caverns for final storage of radioactive waste.