Abstract

The formation of calcium carbonate mineral scale is a persistent and expensive problem in oil and gas production. Scaling of metallic or insulating walls in contact with hard water may cause unscheduled equipment shutdown and loss of production. The aim of this paper is to further the understanding of scale formation and inhibition by in-situ probing of crystal growth by synchrotron radiation Wide Angle X-Ray Scattering (WAXS) in the absence and presence of diethylenetriaminepenta (methylenephosphonic acid) (DETPMP) scale inhibitor at elevated temperature and high pressure. This novel technique enables in-situ study of mineral scale formation and inhibition and as such, information on the nucleation and growth processes is accessible. This technique studies bulk precipitation and surface deposition in the same system and has great benefit to understand an industrial scaling system. It offers an exciting prospect for the study of scaling.

It has been shown that the nucleation and growth of various calcareous polymorphs and their individual crystal planes can be followed in real-time and from this the following conclusions are reached.

  • The process of scale deposited on the surface can be divided into an unstable phase and a stable phase. The initial phase of crystallization of calcium carbonate is characterized by instability with individual planes from various vaterite and aragonite polymorphs emerging and subsequently disappearing under the hydrodynamic conditions. After the initial unstable phase, various calcium carbonate crystal planes adhere on the surface and then grow on the surface.

  • DETPMP has a profound effect on the induction time of the surface deposit. It inhibits scale crystal adhesion onto the surface, although the bulk precipitate is observed in the scaling system. It displays different inhibition mechanisms for bulk precipitation and surface deposition inhibition.

  • DETPMP inhibits surface deposition. It suppresses calcite formation and results in the least stable vaterite crystal formation.

This paper will discuss how surface scale evolves in the absence and presence of the inhibitor - exploring the power of the synchrotron in-situ methodology.

Introduction

The formation of mineral scale is a persistent and expensive problem in the oil and gas industry. Calcium carbonate, CaCO3, is one of the most common scale components found in oilfield production wells and surface facilities. Carbonate scale formation can impair production by blockage of tubing and flowlines, fouling of equipment and concealment of corrosion [1, 2].

Over the past few decades, great efforts have been made to understand the mechanism of scale formation and inhibition. Traditionally, studies of scale formation have concentrated on bulk scale formation using laboratory beaker tests[3]; turbidity probes, pH measurement or bulk chemical analysis have been used to analyse kinetics of precipitation. The primary focus has been the assessment of the kinetics of homogenous and heterogeneous precipitation in the bulk solution[4]. It has been demonstrated that there are often wide anomalies between actual deposition and rates estimated by predictive models based on scaling indices and thermodynamics[4].

Few studies have considered the activity of deposit formation (nucleation and growth) at surfaces. The theoretical aspects of crystal nucleation and growth in inhibited solutions are only partially understood and information on such processes on component surfaces is especially sparse.

In the last few decades, much effort has been turned to the aspect of scaling on surfaces and has resulted in numerous studies reporting methods to detect and assess scale formation on metal surfaces. Some focus has been turned to this aspect of scaling to attempt to overcome some of the shortfalls of beaker tests.

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