The focus of this thesis is to determine the actual costs caused by the water production in particular oil and gas fields and to identify the cost drivers. The first part of this work, which is declared as the theoretical part, includes both the technical background of water production as well as methods of business accounting -which will be applied in this thesis. The second part, which is declared as the practical part, starts by capturing the current situation of the system in detail. Excel is used to develop a tool which maps the complete water cycle system and includes information about amounts of water as well as their cause for each point of the system. This dynamic tool uses mass balances and allows on the one hand an ongoing control of the system and on the other the implementation of scenarios. The evaluation of costs for the entire period follows as next. The entire production water cycle is divided in the areas of: production, transport, treatment and injection. Costs caused by the water are analyzed for each well in all aforementioned areas. This allows the identification of cost drivers. Those serve as basis to derive possible optimizations and finally come out with specific recommendations for the company.