Development and Application of Accurate Detection and Assay Techniques for Oilfield Scale Inhibitors in Produced Water Samples

Abstract

Abstract In the application of chemical inhibitors in field squeeze treatments for the prevention of sulphate and carbonate mineral scale formation, it is very important that the chemical species involved can be accurately assayed. When the inhibitor Concentration drops below a predetermined threshold level for scale inhibition (Ct) then the well may need to be resqueezed. The accurate assay of scale inhibitors down to concentration levels of a few ppm in real field brines can be a difficult task. In this paper, we examine a number of interferences which often make assay techniques very difficult to apply in field produced brines. The inhibitors examined in this work include phosphonates (PH), polyacrylates (PAA) and phosphinopolycarboxylates (PPCA). The main objective of this work is to develop suitable pre-treatment/purification techniques which allow the standard wet chemical techniques to be applied effectively after appropriate modification. Successful techniques - all based on careful modification of existing methods - have been developed by which these common inhibitors can be assayed very accurately at ppm and sub-ppm levels in a variety of North Sea field produced waters. This paper examines some of the major problems and interferences associated with poor analysis and introduces modified methods which can be applied in the field without the use of expensive equipment. It is also shown that different detection methods can often be employed in order to avoid more extensive clean-up strategies. Finally, instrumental methods such as ICP analysis (commonly used for phosphonates) are examined and pre-treatment methods are developed which allow phosphino-polycarboxylic acid based inhibitors to be assayed very accurately by this method. The results from an independent assessment by a North Sea operator, using spiked field produced water, are also presented as an independent verification of the accuracy of the techniques which have been developed in this work. Introduction Oilfield scale has long been recognised as one of the major chemical problems in the oil production industry. The formation of mineral scales may result in greatly reduced well performance as the rock pores, tubulars and topside machinery become choked by a build up of insoluble inorganic precipitate. In many cases, the most effective measure in dealing with the problem is through the treatment of the near wellbore area of the producers with chemical scale inhibitors applied in a "squeeze" process. In a successful squeeze treatment, the injected inhibitor is required to return to the production tubing at levels above the threshold concentration, Ct, over long periods of time (typically 3 - 12 months). Since threshold concentrations are often in the region of 5ppm or less, it is important to be able to accurately monitor the returning inhibitor at these low levels in order to determine the approach of the end of the squeeze lifetime. Monitoring of the scaling ions themselves, for example [Ba2+], will indicate when re-treatment is essential, but this approach provides no lead time in order to prepare for the operation. Furthermore, if successful computer modelling is to be carried out in order to either optimise the conditions for future squeeze treatments or to predict the end of the current squeeze lifetime, accurate low concentration return analysis is essential. In this paper, we are concerned with the detection and accurate assay in field produced waters of a variety of different chemical inhibitors commonly applied using the squeeze process. P. 543