Effect of PPCA and DETPMP Inhibitor Blends on CaCO3 Scale Formation

Abstract

Abstract PPCA and DETPMP are two common commercial scale inhibitors used to control mineral scaling in the oil and gas industry. Normally, PPCA is regarded as a nucleation inhibitor and DETPMP as a growth inhibitor. In this paper, the effect of PPCA and DETPMP blends on the inhibition efficiency of calcium carbonate scale formation is presented. In this paper, calcareous scale formation was studied both in the bulk solution and on the metal surface in supersaturated scale formation solutions, which represent typical waters encountered in oil and gas production. The effect of inhibitor blends on scale formation is studied. It shows that the inhibitor has a different inhibiting effect on precipitation in the bulk solution and deposit formed on the metal surface and reasons for this are discussed. It also demonstrates that bulk precipitation and surface deposition are two different processes and both processes should be studied to completely understand an industrial scaling system. Introduction and Background The formation of mineral scale is a persistent and expensive problem in the oil and gas industry. Calcium carbonate, CaCO3, is one of the most common scale components found in oilfield production wells and surface facilities. Carbonate scale formation can impair production by blockage of tubing and flowlines, fouling of equipment and concealment of corrosion1,2. Calcium carbonate crystallisation consists of two stages: nucleation and growth. In the nucleation period, sub-micron particles form and the number of the particles increases very quickly. Growth subsequently takes place on the particles formed during the nucleation phase. Often the nucleation and growth periods are not easily distinguishable3,4. Polyphosphonocarboxylic acid (PPCA) and Diethylenetriaminepenta (methylenephosphonic acid) (DETPMP), two common commercial scale inhibitors used in the oil and gas industry, are investigated as mono species and as binary blends in this study. PPCA is a polymer formed by two poly-acrylic-acids connected by a phosphorous group, as shown in Figure 1. PPCA is often regarded as a nucleation inhibitor5. After initial nucleation, PPCA continues to retard crystal growth, but it does not block it entirely and becomes less effective with time. This is due to its incorporation in the crystal lattice6. DETPMP, the phosphonate species, has a chemical structure as shown in Figure 2. In contrast to PPCA, DETPMP is thought to retard the growth of the crystals5 and is less effective in preventing initial nucleation. Once nucleation has started, it is effective at stopping the further crystal growth by adsorbing at active growth sites on the scale crystal lattice. The relatively high level of inhibitor must be maintained in the solution in order to prevent further nucleation4. In this paper, the effect of PPCA and DETPMP blends, the mixture of supposed nucleation and growth inhibitor, on calcium carbonate scale formation is studied and the comparison of the inhibiting efficiency on calcium carbonate systems is given. Also, the effect of inhibitor blends on scale formation is studied both in the bulk solution and on the metal surface. It shows that the inhibitor and the blends have a different inhibiting effect on precipitation in the bulk solution and deposition on the metal surface and reasons for this are discussed. Experimental Procedures In this paper, the experiments were carried out in scale formation solutions with different concentrations of inhibitor and inhibitor blends. The composition of the scale formation solution is 6870 ppm Na+, 1440 ppm Ca2+, 11871 ppm Cl− and 2196 ppm HCO3−, which represents a typical scale formation solution encountered in the gas and oil industry. Calcium chloride (CaCl2·6H2O), sodium chloride (NaCl), sodium bicarbonate (NaHCO3) used are of analytical grade supplied by BDH Laboratory. The concentrations of inhibitor and inhibitor blends are 0.02 ppm, 0.04 ppm and 0.08 ppm respectively (by active weight). In the inhibitor blends, the mass ratio of PPCA and DETPMP is 1:1. PPCA and DETPMP are provided by Biolab. The molecular masses of PPCA and DETPMP are 3800g/mol and 573g/mol respectively.