Salt Reserve Estimate for the Twenthe-Rijn concession

Abstract

AkzoNobel has been producing salt from solution-mined brine since 1919. Production in the Hengelo area started in 1933, and is still ongoing today. Because at some point in time the mineable salt will all be mined and preparations for extension of the concession will need to be made, it is important to know the remaining amount of salt in the Hengelo area. To date no singular method for estimation of these reserves has been adopted. This leaves room for the development of a local method. It was chosen to develop this method in accordance with international standards. Due to its worldwide influence and being recognised by the Amsterdam Stock Exchange that AkzoNobel is registered on, it was opted to use the code of the Joint Ore Reserves Committee (JORC) of the Australasian Institute of Mining and Metallurgy (AusIMM). The geology of the Hengelo area was modelled using a modified version of the ordinary Kriging algorithm to take into account the presence of faults. Kriging was used because it also provides an estimate of uncertainty in the horizons that are modelled. To factor in the uncertainty about the continuity of the salt body, the resources in three areas were calculated: the area was investigated as a whole, and two sub-areas were investigated separately: · Sub-area 1 is the area within 500m distance of any measurement in the area. · Sub-area 2 is the area for which the geology was previously studied in great detail by GeoWulf Laboratories[5]. This area is essentially the area in-between the different measurements. The modelled horizons and their uncertainty values were then used in a Monte Carlo simulation, yielding cumulative probability distributions for the estimated amount of salt in the three scenarios. The total amount of salt was also calculated deterministically, meaning all calculations were performed using expected values. This produced the same values as the expectations of the probabilistic approach. It was decided that the probabilistic approach of the total area produced an over-optimistic value of the total Resource, and to only use the expected values of the three areas to distinguish between Inferred, Indicated and Measured Resources. Reserves were estimated for three scenarios: 1. Salt has been produced in the past in this area using a no longer used leaching strategy that results in large caverns with multiple wells. What would be the Salt Produced if the same locations had been chosen to produce salt using the modern Single Completion Cavern design? And what would be the historical recovery? 2. Assuming no production had taken place in the past, what would be the total Salt Reserve if the field would be developed now? 3. What is the Salt Reserve left in the area? It was found that the recovery percentages per cavern as asked for in scenario 1 were so widespread that no conclusive numbers could be given. The cavern lay-out throughout the area in scenarios 2 and 3 was not optimized, but because the overall estimation process was conservative the numbers could still be used as minimal values in future evaluations of the area. The results of scenario 3 are given in the table below.