Inhibition of Barite Scale in the Presence of Hydrate Inhibitors

Abstract

Summary Large volumes of hydrate inhibitors [e.g., methanol, ethanol, monoethylene glycol (MEG), and triethylene glycol (TEG) as co-solvent] are added to control hydrate formation. Such practice has an adverse effect on scale formation because the mineral salts are generally less soluble in the cosolvent. Because of production from reservoirs, oilfield brines are often close to saturation as they enter a well; even a small amount of added methanol, ethanol, and so on is often sufficient to induce various minerals to precipitate, particularly the sparingly soluble minerals (e.g., barite). For example, barite solubility is reduced by as much as 20-fold with 50 wt% methanol. In this paper, barite nucleation rates were studied over a wide range of concentrations, such as Ba2+ (0.5 to 1.8 mm) SO42− (0.5 to 1.8 mm) methanol (0 to 40 wt%), MEG (0 to 40 wt%) or TEG (40 wt%). Barite nucleation rate is significantly accelerated in as little as 5 wt% methanol. The barite nucleation rate can be modeled with an equation modified from the classical nucleation theory in this study. The inhibition of barite by two phosphonate inhibitors and a polymer inhibitor in the cosolvent/brine solution is more complex. At lower cosolvent concentrations (<30 wt%), the nucle-ation inhibition can be predicted with a previously derived semi-empirical model that mathematically separates the effect of added inhibitors from that of the uninhibited mineral phase. At high methanol (>30 wt%) concentration, barite nucleation may be difficult to inhibit by scale inhibitors because of high supersaturation and the tendency of phosphonate to be precipitated as metal salt.