Since 1918 AkzoNobel produces salt by means of solution mining from the bedded Triassic Röt Evaporite deposit in the eastern part of the Netherlands. In this technique water is pumped into the rock salt layer through a well and dissolves the halite in situ. Subsequently the brine is pumped to the purification plant at surface level where the NaCl is extracted from the brine after several impurities have been removed.
Near the city of Hengelo, the Hengelo Brine field has been exploited by AkzoNobel since 1933 and based on the technological progress during the years it can be divided into five phases:
1. 1933-1958, 53 wells, 42 caverns;
2.1959-1975, 226 wells, 94 caverns;
3.1976-2005, 213 wells, 72 caverns;
4.2006-2013, 37 wells, 36 caverns;
5.2014-present day, 31 wells, 31 caverns.

The area corresponding to the first phase comprises 42 caverns which have been developed from 50 production wells. The production wells have been drilled between 1933 and 1958 and during later stages 3 appraisal wells have been drilled.
Present-day AkzoNobel pursues stable caverns at the end of their production life. This is achieved by leaving a sufficient thickness of salt in the roof above the cavity. The result is a ductile boundary between the void and the overburden material. The directly overlain anhydrite layer is much stiffer and shows brittle behavior. When this anhydrite layer is not supported by a sufficient amount of rock salt underneath it can potentially collapse. This roof failure occurs when the span of the exposed anhydrite is large enough. The fragments of the collapsed roof layer will deposit on the bottom of the cavern leaving the next roof layer exposed. With each roof layer collapsing the cavern top and bottom attain a new depth. This process is called cavern migration, which continues until the cavern reaches a status where the roof layer is stable either by sufficient support or strength.
Most of the 42 caverns of the phase 1 area have migrated and in five cases this led to significant surface subsidence. Currently AkzoNobel prepares abandonment of the last open wells in the phase 1 area. In order to proof long term safety after closure, the caverns have been examined, simulated, reconstructed, and modelled. By analyzing the available data and scrutinizing a few caverns where extensive migration did occur, the cavern migration behavior and its influential parameters were identified. This report clarifies how the caverns have been reconstructed and how the maximum migration potential is determined with the model.
The phase 1 area comprises the oldest part of the Hengelo Brine field. The wells were drilled between 1933 and 1958. During this phase some of the wells were positioned close to each other, intentionally allowing interconnections. The motive was to complete these wells as doublets, while the rest were setup as single completion caverns. Despite these intentions, the leaching during this phase was performed in a less restrained manner than what is acceptable today. The majority of the caverns in the phase 1 area have established unintentional connections and were completed as series of caverns. Due to this lack of control overmining occurred at some of the caverns and as a result most of the caverns have migrated upwards through the overburden and some induced significant subsidence at the surface.
As the amount of data and especially measurements of these phase 1 caverns is scarce, the caverns needed to be reconstructed to enable a comprehensive analysis. In order to reconstruct the cavern development and final dimensions, the caverns have been simulated based on their historical production and on interpretations of the geology and logbooks. After cross-correlating the dimensions with the available data, the dimensions are used in an analytical model. Considering multiple influential parameters the post-production ‘residual’ volume and dimensions of the caverns are deduced. Thereafter the maximum potential migration is determined using a migration model.