Abstract

Conventional scale inhibitor squeezes are widely used to prevent or delay scale formation. Although long established the process can be wasteful of chemical used. In common with the use of most production chemicals such treatments are ‘once through’ and non-recoverable with high operating costs. In addition in offshore operations such production chemicals are discharged into the sea. Such discharges are also subject to environmental legislation with associated costs. Increasingly there is a need to avoid such discharges. Modifications of the squeeze procedure have been introduced to extend squeeze lifetime but further improvements are still sought. This paper describes the results of a study to investigate the potential for membrane separation to capture a representative scale inhibitor from brine. Over a range of temperatures, scale inhibitor concentrations and brine chemistries more than 96% of the scale inhibitor was captured. This was obtained without any attempt to optimise operating conditions. The resulting captured material was compared to the original product using dynamic tube blocking tests and static barium sulphate tests. The static tests showed similar performance for recovered and initial material, while the dynamic tests showed a marginal increase in performance for the recovered material in some cases. This increase in performance correlates with the retention factor for each given test run. It is believed that this is due to a slight purification of the inhibitor during the separation process, as shown by Dionex Ion Chromatography. The effective performance of the recovered inhibitor suggests that it could be re-used in a similar manner to the original product.

Introduction

Squeeze treatment, where by scale inhibitors are squeezed into the near well bore area of a producer well, is a long established method for the prevention of scale formation1-4. However the release rates of the inhibitor from such treatments are often far from ideal. Frequently a large amount of inhibitor returns rapidly and is consequently ‘wasted’. Resulting operating costs (OPEX) are consequently higher than they would ideally be. Variations on conventional adsorption squeezes have also been introduced to extend application lifetime including precipitation treatments5-7, low molecular weight amphiphilic molecules8–9, and emulsion-based treatments10-11. Controlled release systems, for example from solid scale inhibitors, have also been deployed as an alternative to squeeze treatments.

In addition in offshore operations the scale inhibitor from squeeze treatments- both during the initial peak and there after - is discharg ed overboard into the sea. With an increased emphasis on minimising environmental impact of such operations there is a need to identify and apply benign products for such use or avoid discharge altogether (for example by re-injection of produced water). Environmental performance is a key business driver for operators and some have associated a cost of discharge in terms of $/tonne for each of the HOCNS categories. In such a categorisation category ‘E’ was attributed the lowest cost while category ‘A’ the highest one12.

The drive towards development of environmentally friendly scale inhibitors has been seen in recent years with the development and introduction of new chemicals with preferable biodegradation, bioaccummulation and toxicity properties than established chemicals used in the past13-18. However such new inhibitors do not necessarily out perform established products in terms of product efficiency and regardless still get discharged to the sea in offshore operations. Recent OSPAR decisions now means that in the UK it will become compulsory to register chemicals for offshore use. The UK implementation proposal is to rank chemicals using the HQ (hazard quotient) derived from the CHARM model within a mandatory permit. Operators may also calculate a RQ (risk quotient) which is platform specific.

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