Scale Formation And Prevention In The Presence Of Hydrate Inhibitors

Abstract

Abstract Hydrate formation can be a serious problem in many gas production systems. Large volumes of hydrate inhibitors (e.g., methanol, ethanol, ethylene glycol, and triethylene glycol as cosolvent) are often added to control hydrate formation. Such practice has an adverse effect on scale formation since the mineral salts are generally less soluble in the cosolvent. Due to production from reservoirs oilfield brines are often close to saturation as they enter a well, and even a small amount of added methanol, ethanol, etc., is often sufficient to induce various minerals to precipitate. The scaling tendency of sparingly soluble mineral salts, e.g., calcite and barite, in methanol/brine and ethanol/brine solutions is observed to be orders of magnitude larger than in the brine alone. Halite scaling is also severely affected in the presence of methanol or ethanol. Ethylene glycol and triethylene glycol have a lesser adverse effect on mineral salt scaling tendency. There is no accepted methodology to correlate the effects of hydrate inhibitors on scale formation as there is for electrolytes. Similarly, the effect of hydrate inhibitor on scale inhibition with common inhibitors is not well known. In this paper, a semi-empirical approach is proposed to correlate the effect of hydrate inhibitors on scale formation and inhibition from experimental solubility measurements of halite, barite, gypsum, calcite and carbonate equilibrium chemistry. The ion-cosolvent activity coefficients can be used directly in any solution speciation code to evaluate the effect of cosolvent on mineral scale formation. The validity of the equation has been tested between 4-50 °C and 1-6 M ionic strength. Working equations that can be used in gas and oil production to calculate the effect of cosolvents on scale formation will be presented. Such equations have been incorporated into ScaleSoftPitzer V.4.0. ScaleSoftPitzer™ is a Microsoft™ Excel™ based program to predict scale tendency, specifically written for the oil and gas production systems. Details of using ScaleSoftPitzer V.4.0 to predict hydrate inhibitor induced scale formation and case studies will be presented. Initial tests of scale inhibitor efficiency in methanol will also be presented.