Abstract

Chemical inhibitors that prevent the formation of sulphate and carbonate mineral scale formation are used in field squeeze treatments. The demand for competent evaluation of all issues associated with mineral scaling in wells and pipelines is continuous throughout the lifetime of any producing asset and the cost implications are very large. It is therefore important that the chemical inhibitor species can be accurately assayed so that the end of the squeeze lifetime can be determined.

Previous work by others has reported the development of accurate detection and assay techniques for oilfield scale inhibitors1,2. For PVS and VS-Co type inhibitors, these methods involve pre-treatment/purification stages that allow the application of standard wet chemical techniques, in particular the separation of the inhibitor chemical from interfering brine salts. However the typical size of samples from experimental core floods are so small that they render these methods impractical.

In this paper we detail the development of a turbidimetric method without the need for pre-treatment/purification stages for the determination of a sulphonated-polyacrylate co-polymer using Hyamine 1622. The applicability of this technique is illustrated with coreflood samples having small volume size and with real field post-scale inhibitor squeeze produced water samples. It is also shown that the method developed allows the determination of scale inhibitors in produced waters with up to 55,000ppm total chloride concentration and 100ppm crude oil in water.

Finally we report the potential interferences caused by the inhibitor carrier, crude oil and displacement fluid which may arise as a result of both typical and atypical squeeze treatments.

Introduction

Poly-vinyl sulphonate (PVS) and sulphonated polyacylate co-polymer (also known as vinyl sulphonated co-polymer; VS-Co) scale inhibitors are used in several North Sea fields. They characteristically display good scale inhibition properties for both barium sulphate and calcium carbonate scales and in some cases their inhibitor properties will far exceed other inhibitor types. However, PVS and VS-Co inhibitors are notoriously difficult to assay and existing techniques are cumbersome, time consuming and prone to interference effects and operator error. The current industry standard procedure for analysing PVS and VS-Co scale inhibitors is using the standard Hyamine 1622 assay2. Certain VS-Co inhibitors also have rapid inhibitor release rates that also make their field application unattractive.

Samples from core-flood tests tend to be small (5 to 10mls) and are almost always smaller than the 25ml volume used in the standard Hyamine assay. The existing pre-treatment dialysis step involves considerable operator handling and it is likely that sample will be lost on the dialysis membrane with the consequent inaccurate determination of inhibitor concentration. For this reason the Hyamine assay was reviewed. This paper describes a new accurate assay for VS-Co scale inhibitors. This revised assay provides an easy route to accurately determine residual concentrations of VS-Co inhibitors in either field produced water samples or in smaller core-flood effluent samples. The new procedure modifies the current industry standard method to become a fast, accurate and reproducible analytical technique.

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