TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe giant Statoil-operated North Sea oil field Statfjord is currently far down its production decline curve. During 23 years of production 60% of the STOOIP has been recovered, and the remaining reserves are characterized by complex distributions of oil, water and gas.In order to obtain a cost-effective production of the remaining oil, an aggressive drilling and intervention programme is necessary. Future developments might also include a pressure blow down phase of the reservoir. Then large volumes of water will be produced to surface. So far the Statfjord Field has exhibited a fairly mild scale potential. Sulphate scale has been detected in several wells down hole, whereas carbonate scale when found is mainly above the down hole safety valve. Carbonate scale precipitation will be more severe in any future blow down phase with a lowering of the reservoir pressure. To improve carbonate scale prediction a "correct" down hole pH value is necessary. The prediction program will then be capable of performing a better tuning sequence and give more accurate predictions, refer references.Petrotech has developed a pH sensor system for downhole use. In 2002 Statfjord performed a field test of this system by running it in a well with a "single phase" fluid sampling chamber. The water sample captured down hole was used to get a lab measurement of the pH in the formation water at reservoir temperature and pressure. This sample was then flashed to standard conditions and a full water and gas analysis was performed. The results were used in the MultiScale software program to calculate a pH value. The results indicated good correspondence between the pH values obtained from the down hole sensor, the water sample and the pH value calculated with the scale prediction program.