Reserve reconciliation is widely applied in the petroleum and mining industry. By updating the reserves throughout the operation, the remaining reserves can be better quantified and the extraction scheduling optimized. The goal of this study is to develop a reserve reconciliation tool for the Hengelo Brine Field, operated by AkzoNobel Industrial Chemicals B.V. in the Netherlands, in order to improve the long-term development and production planning for the field. The study follows the principles of the LeanSixSigma methodology for problem solving in five phases: define, measure, analyze, improve and control (DMAIC cycle). The study consist of 4 parts. The first part provides background information on the production process, the Hengelo Leaching Technique and the geological situation. The second part measures and analyzes the mass balances in the field, magnitude of fluid losses, Solling Fm., sonar measurements and insoluble material. The report continues with the reserve reconciliation in which an accounting tool is implemented on an area with 23 caverns. In the fourth part the discussion, conclusion and recommendations are discussed. The aim of this study was to determine if it is feasible to develop a predictive methodology able to accurately quantify the properly and if this methodology can be utilized in the long-term planning of the production and development of the Hengelo Brine Field. The extraction process is affected by geological and mining related uncertainties. The most important uncertainties are: • Fluid losses from caverns • Cavern shape & geometry • Insoluble material & bulking factor The reserves can be calculated more precisely using the new data. A reconciliation tool was developed and validated on 23 caverns. The production controlled handling of the caverns results in a loss of reserves. This loss is caused by the outflow of fluids from a cavern. The average recovery factor for the first leaching phase is estimated to be 75%, versus the 80% which is used in the current reserve estimates. However, due to the conservative initial estimate of the reserves based on the insoluble content, the caverns still show a small surplus. The implementation of this tool for the entire field and incrementally improving it will assist future long term planning of the Hengelo Brine Field.