Abstract

A combination of Laser Raman Spectroscopy, Secondary Ion Mass Spectrometry and Scanning Electron Microscopy has been used to investigate the influence of corrosion inhibitors on the mechanism of scale formation.

In a series of experiments mixed barium and strontium sulphate scales were precipitated onto a steel surface in the separate presence of three generically different scale inhibitors. In each, one of two contemporary corrosion inhibitors was also added and the rate of scale growth monitored. All experiments were performed at a constant temperature, pressure and solution pH of 25°C, 1 ATM and 4.5 respectively.

A significant influence on the rate of scale formation was observed in the presence of corrosion inhibitors. The corrosion inhibitors affected the way in which scale ions, as well as scale inhibitors, attached themselves to the surface. These data indicate the presence of subtle interactions in these multi-component systems which strongly influence the efficiency of individual production chemicals.

Introduction

Oilfield scale is a global problem leading to loss in oil production. A range of scales form during secondary oil recovery from offshore reservoirs, where pressure is maintained by injecting seawater into the reservoir. Precipitation of barite, celestite and gypsum occurs when the sulphate-rich seawater comes into contact with alkaline-earth metals found naturally in formation waters. These minerals impede flow in production tubing and equipment and reduce the porosity of the surrounding reservoir rocks.

A majority of producing wells in which scale formation is a problem are treated using chemical scale inhibitors. A range of chemicals is available for the prevention of scale, each with different physical and chemical properties. This is, in part, due to the large variation in physical and chemical conditions exhibited in oil wells, and such factors must be considered when selecting an appropriate inhibitor for field use. The presence of specific ions, variations in pH, temperature and pressure must all be taken into account together with the influence of oilfield chemistry. Chemicals are deployed not only for scale prevention, but also routinely for corrosion prevention, wax inhibition, deflocculation, hydrate inhibition etc. One such example is the formation of blockages in injection lines where neat corrosion inhibitor comes into contact with wax inhibitor, left in the line from previous workovers. These interactions are not well documented in the literature and at best are only partially understood.

Corrosion inhibitors are introduced to the oilfield system in two ways: batch treatment and continuous treatment. Batch treatments are used periodically to treat wells by depositing a thin film of corrosion inhibitor which remains on vulnerable surfaces until the next treatment. The generally preferred treatment method however involves the continuous injection of corrosion inhibitor into zones liable to attack. The mechanism of corrosion prevention is slightly different for the two types of treatment. Continuous injection inhibitors tend to form films only several molecular layers in thickness on steel surfaces (McMahon, 1991) whereas batch inhibitors form thicker ‘macrofilms’. These macrofilms are formed by the use of high molecular weight components weakly soluble in the carrier solvent. When the inhibitor comes into contact with production fluids, the carrier solvent is washed away and the solute precipitated as an insoluble layer on the metal surface (Harrop, 1990).

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