Mass transfer of H2O between petroleum and water: Implications for oilfield water sample quality

Water mass transfer can occur between water and petroleum during changes in pressure and temperature. This process can result in the dilution or concentration of dissolved ions in the water phase of oilfield petroleum-water samples. In this study, PVT simulations were undertaken for 4 petroleum-water systems covering a range of reservoir conditions (80-185 °C; 300-1000 bar) and a range of water-petroleum mixtures (volume ratios of 1:1000-300:1000) to quantify the extent of H2O mass transfer as a result of pressure and temperature changes. Conditions were selected to be relevant to different types of oilfield water sample (i.e. surface, downhole and core samples). The main variables determining the extent of dilution and concentration were found to be: (a) reservoir pressure and temperature, (b) pressure and temperature of separation of water and petroleum, (c) petroleum composition, and (d) petroleum:water ratio (PWR). The results showed that significant dilution and concentration of water samples could occur, particularly at high PWR. It was not possible to establish simple guidelines for identifying good and poor quality samples due to the interplay of the above variables. Sample quality is best investigated using PVT software of the type used in this study.