Abstract

Effective scale and corrosion control in production of hydrocarbon deposits is in many fields essential to the economic and safe production of these fluids. This paper presents field results and lessons learned from a North Sea field where a combined scale and corrosion inhibitor has been applied using a single formulation to control both the deposition of scale and associated corrosion. The use of a single product has facilitated a reduction in the number of umbilical lines to be used to provide flow assurance on the subsea flow lines and topside process of this facility. As part of the drive to reduce the environmental impact of production chemicals an improved performance formulation was developed. This formulation was required to contain no material on the current North Sea production chemical substitution list.

The laboratory methods used to select the product along with measurement methods used in the field are also discussed. The impact of current environmental legislation on raw material selection will also be outlined. The validation of the laboratory selection methods in the field required the development of a monitoring program to establish base line control and to assess the degree of protection from scale deposition and corrosion across the process.

The changing physical conditions within the flow lines and across the topside process will be described, as well as the changes in brine chemistry within the field, as injection water breakthrough occurred. This is vital for understanding how flow assurance was maintained and how treatment rates and products could be optimised.

This paper will outline in detail the particular issues associated with chemical injection to a subsea facility, many of which are currently being developed in the Gulf of Mexico and West Africa.

Introduction
Scale and Corrosion Control in Subsea Facilities

The development of subsea facilities to improve the economics of marginal oilfield developments has focused attention on the need to develop single and combination production chemicals that can function in subsea environments. The recent development of long (> 20km) subsea tie backs in the North Sea, Gulf of Mexico and West Africa has focused attention on long term product stability at seabed temperatures and the associated product specification required for this kind of service. The following short section outlines some of the technical challenges and current methods that can be utilized to eliminate or at least reduce the risk to flow assurance by using such combination chemicals.

Technical Issues
Fewer umbilical lines.

With fewer chemical injection lines being designed into subsea developments, the requirement for combination products has become increasingly important. In many cases these products involve the formulation of generic chemicals that would normally be considered to be incompatible or sparingly compatible when combined or mixed. To overcome this problem, multifunctional molecules (such as poly aspartic acid) have been developed which function as both a corrosion inhibitor and scale inhibitor. Conventional water based scale and corrosion inhibitors can also be formulated together with the aid of mutual solvent systems.

Hydrate formation within chemicals.

Most conventional production chemical formulations rely on water as the solvent. Cases have been reported in the North Sea where production gas has bled up subsea injection lines, due to the malfunctioning of non-return values. The result is the formation of a hydrate plug within the umbilical injection line.

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