Summary
Large volumes of hydrate inhibitors [e.g., methanol, ethanol, monoethylene
glycol (MEG), and triethylene glycol (TEG) as cosolvent] 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 5wt% 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 nucleation 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.
Introduction
Barium sulfate, BaSO4 scale, is commonly found in oil and gas wells and
various industrial water treatment systems. It is problematic because
BaSO4 is difficult to remove once formed. Furthermore, it is often
enriched with radium as the result of coprecipitation. Therefore, the
study of precipitation of BaSO4 from supersaturated solution is of both
scientific and practical importance.
© 2005. Society of Petroleum Engineers
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