Abstract

A significant problem occurring during oil and gas production is the precipitation and deposition of inorganic scales in the near wellbore area, and the formation of ‘fouling layers’ on the surfaces of pipework and equipment. This affects the safe and reliable operation of equipment which must be repaired or replaced at a high cost. The ability to accurately predict what minerals will precipitate as scale, and where, informs the strategy developed to prevent or mitigate the risk. Modeling of scale formation at pressure above 1000 bar and temperatures up to 200°C (HPHT) is not well understood and so a significant margin of uncertainty must be adopted in engineering choices–potentially adding unnecessary cost or complexity. To make scale prediction robust at HPHT conditions the thermodynamic basis of the models used must be extended; new empirical data are required describing the speciation and solubility of carbonate and sulfate minerals in equilibrium with fluids at deep well (HPHT) conditions. Mineral solubility and fluid speciation can be measured in situ by coupling a diamond anvil cell (DAC) with Raman spectroscopy. This non–invasive and non–destructive optical technique enables in situ characterization of molecular composition and structure of a material. Coupled with a DAC , this technique provides precise information on the nature and the content of chemical species released in the fluid during dissolution of minerals at HPHT conditions. In this study, we present studies of the stability of sulfate ions up to 4 GPa during equilibration of a natural single crystal of CaSO4 in equilibrium with brine solutions. CaSO4 solubility was deduced from Raman intensities of symmetric stretching mode of sulfate ions and calibration curves obtained from aqueous solutions of known concentration. During compression, atypical changes in intensity and frequency shifts of the symmetric stretching mode of sulfate ions were observed between 0.2 and 0.5 GPa and explained by the presence under those conditions of a liquid–liquid phase in the fluid from low density water to a high density water phase. Similar behaviour was also detected for carbonate ions during compression of Na2CO3 aqueous solutions of different concentrations at ambient temperature. Finally, using appropriate equations of state for aqueous fluids, Grüneisen parameters have also been calculated for the two liquid phases of H2O .