Abstract

Antiscaling agents are widely used to prevent scale formation in the oil and gas industry because scale's presence can lead to unscheduled shutdown and deferred oil production. The main problems result from the formation of scale on equipment. It has been shown that the mechanism of scale formation differed according to the scale nucleation that occurred in the bulk solution or on a surface.

The scope of this paper is to show, from experimental data, the effects of three different antiscalants on the growth rate of CaCO3 formation on a metal surface. The choice of which inhibitors to study was based on a survey of the environmental regulations that can be encountered in different parts of the world. A conventional phosphorous inhibitor was used as a reference (polyphosphinocarboxylic acid, PPCA) and contrasted to the performance of two environmentally friendly inhibitors (polymaleic acid, PMA, and carboxymethyl inulin, CMI).

The use of an in situ flow cell coupled with a video assembly and a microscope permits the real-time visualization of crystal morphology. Studies included looking at the surface coverage of the calcareous deposit and the number of crystals of CaCO3 formed. The effects of the scale inhibitors on the growth of calcium carbonate were studied after nucleation had occurred, in order to determine the effects of the inhibitors on calcium carbonate crystals already formed. A previous study was focused on the assessment of the inhibitors on the nucleation and growth steps. This paper is focused on the effects of the scale inhibitors on growing crystals which have been formed on a surface under uninhibited conditions.

The morphology of the crystals was found to be modified by the three inhibitors studied but to different extents. The addition of 4 ppm of PPCA and PMA significantly decreased the growth rate of the calcium carbonate crystals but the CMI did not modify it at the concentration range tested. This technique demonstrates that the efficiencies of scale inhibitors can vary according to the step in which the antiscalants were added (nucleation/growth).

Introduction

In the oil and gas industry, salts deposition is a problem widely studied because the presence of scale can lead to a loss of production, to formation damage and to the weakening of the equipment installations. Salts precipitation occurs when a system is not at ionic equilibrium. The equilibrium can be lost because of the changes of the temperature and pressure or with the introduction of incompatible brine. The supersaturation ratio S, characterizes the thermodynamic conditions of a solution and the equilibrium is reached when S is equal to 1.

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