Barium sulfate scale deposits are serious impediments in the gas and oil production industry as they are tenaciously adhering on surfaces and they are hard to remove due to the independence of their solubility on pH. The practice most commonly applied is prevention of their formation using water soluble compounds acting as inhibitors. The ability of inhibitor molecules in blocking active crystal growth sites depends strongly on their molecular structure.In the present work we report on the investigation of the kinetics of crystal growth of Barium Sulfate in the presence of two different types of organophosphorus compounds. This type of compounds have been widely employed for antiscale treatment in the oil and gas industry. More specifically benzene 135-triyltris phosphonic acid (BTTP) and amino tris(methylenephosphonic) acid (ATMP) were tested. The former possessed the ability of zwitter-ionic behaviour making it possible to interact both with the Ba2+ cations and the SO42- anions of the BaSO4 crystal lattice. The investigation of the kinetics was done using the seeded crystal growth methodology monitoring changes in the specific conductivity of solutions supersaturated with respect to barium sulfate over a wide range of pH covering both the acid and the alkaline ranges. The kinetics parameters were measured with the highly accurate and reproducible method of constant supersaturation. The specific conductivity probe was used in combination with a double syringe automatic titrator to maintain supersaturation constant during the process of BaSO4 formation. The preparation of supersaturated solutions was done in a way which ensured the equilibration of the seed crystals with the inhibitor tested. The results of the kinetics analysis for the crystal growth of barium sulfate showed that BTTP was a more efficient inhibitor in comparison with ATMP. At concentration levels of just 10 ppm BTTP was quite efficient in inhibiting crystal growth of BaSO4. As the concentration increased the inhibition efficiency dropped apparently due the increase of the solution supersaturation concomitant with the increasing solubility of BaSO4 in the presence of the additives. It is interesting to note the pH effect: At pH 9.5 for both ATMP and BTTP inhibition was higher at the lower concentration levels. The solubility effect which is systematically ignored in the literature is a possible explanation for this behaviour. In acid pH 3.6 ATMP inhibition was higher with increasing concentration in comparison with the alkaline pH values. In this case the solubility of barium sulfate seed crystals in the presence of ATMP was lower than the respective value pH 9.5. The increase of BaSO4 solubility is most probably due to the formation of Ba-phosphonate complexes. Finally barium sulfate scale can be inhibited by ATMP and BTTP. The BTTP gave us better results than ATMPs at the same pH value. Furthermore ATMP can inhibit barium sulfate scale not only in alkaline pH values but and in acidic pH values. Further research should be conducted for higher concentrations of ATMP in acidic pH values and for the presence of more than one cations like as calcium zinc tin and more in the absence and presence of additives.ACKNOWLEGMENTThis research was partially funded by the European Union (European Social Fund-ESF) and Greek National Funds through the Operational program Education and Lifelong Learning’ under the action Aristeia II( Code No 4420).
