Novel Studies on Precipitated Phosphate Ester Scale Inhibitors for Precipitation Squeeze Application

Abstract

Abstract Often, scale inhibitors (SIs) applied in oilfields are organic phosphorus compounds, either phosphonates, phosphorus containing polymers, or phosphate esters. The latter group of SIs, phosphate esters, have not been as extensively studied as the others and we focus on these in this paper. The current work is focused on the properties of precipitated calcium - phosphate ester (PE) complexes denoted as PE_Can, with the stoichiometry n. SIs, injected into a wellbore during squeeze treatments, can be retained in the formation rock via two main mechanisms, adsorption and precipitation. Precipitation squeeze treatments are often based on the reaction between a SI and divalent cations where the mixed complexes that form, denoted SI_Mn2+, usually havea much lower solubility than the free SI itself. In this work we show, that the phosphate ester precipitation process is quite different from that of SIs commonly used by industry, i.e.phosphonates and polymers. Unlike phosphonate-divalent complexes, the phosphate ester complexes stoichiometry is not sensitive to solution pH. However, it is found that elevated pH of PE/Ca solutions positively affects the activity of the precipitates which form; the higher the precipitation pH, the higher the inhibition efficiency (IE, to barite) of the resulting precipitate. The higher IE was determined for stock, precipitated and then re-dissolved phosphate ester and its supernatant solution. These IE results show that phosphate ester activity is very sensitive to temperature; the IE of all phosphate ester solutions decreases with increasing temperature. This phenomenon probably occurs due to hydrolysis reactions at the higher temperatures which changes the chemistry of the phosphate ester solutions. To check this assumption, Nuclear Magnetic Resonance (NMR) and Fourier Transform Infrared Spectroscopy (FTIR) were applied to monitor structural changes occurring in molecules of precipitated and stock phosphate ester solutions. The results obtained in this work are of practical significance for the effective design of phosphate ester precipitation squeeze treatments, since these products offer (i) a more environmentally acceptable alternative to phosphonates, and (ii) a chemical that is significantly easier to detect within produced brine than many polymers used by the industry, many of which are phosphorus free polymers. However, PE scale inhibitors application environment is at lower temperatures, <80°C, relative to phosphonates which can fuctionup to 150°C, and polymers over 200°C.