Abstract

This paper describes an integrated approach to inhibitor assessment and selection for sweet gas corrosion control. A laboratory evaluation programme is described based on criteria which programme is described based on criteria which include production system compatability (process/materials), critical inhibitor properties and corrosion control efficiency under normal operating conditions and those when protection nay be impaired.

Introduction

Effective corrosion control of carbon steel equipment is the principal objective in the evaluation and selection of corrosion inhibitors for sweet gas production. However, wider terms of reference are essential if the overall influence of corrosion inhibitors on gas production and processing facilities are to be taken into account. processing facilities are to be taken into account. These wider terms of reference include the influence of inhibitors on separator efficiency, on their interaction with other production chemicals and fabrication materials, including elastomeric seals downhole or in topsides, and other alloy components.

It should also be recognised that inhibitor evaluation in the laboratory will not reproduce the full range of service conditions in a producing field. Often a production "norm" is used as a basis for testing. Significant variations may arise in metal surface condition, in galvanic couples, in flow velocities and the associated turbulence, or in droplet or particle impact. An appreciation of the extent to which an inhibitor can cope with these variations, without losing effectiveness, is important since this information defines the operational window for the inhibitor.

THE PERFORMANCE SPECIFICATION

The performance specification for a corrosion inhibitor will depend on the environmental conditions in which it is to be used and the application method. For example, application may be by batch treatment or by continuous injection. This first step of accurately defining the operating conditions is essential both for inhibitor selection, or for inhibitor recommendations by the vendor, and for the definition of appropriate test conditions, for the evaluation of the selected, or recommended, inhibitor.

Once these preliminary steps have been completed the evaluation and selection programme for an inhibitor will follow a set course, for example as shown in Figure 1. For the purposes of this paper sample data will be presented to illustrate the steps in this schedule for two proprietary sweet gas inhibitors based on formulations which include quaternary amines.

DEFINITION OF OPERATING CONDITIONS

Reservoir characteristics normally provide the design baseline operating conditions such as temperature, pressure, and product composition including GOR, water cut, water chemistry, carbon dioxide and hydrogen sulphide content. Other factors are determined by the design of the production facilities which will determine flow production facilities which will determine flow rate, pressure drop, liquids drop out, location and severity of condensing conditions and the characteristics of process plant operation such as dwell time in the primary separators where foaming and emulsion stability are critical.

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