Unravelling the role and working principle of the catalyst.

The inhibition of iron sulfide mineral scale formation in oil and gas production is notoriously difficult. High concentrations of scale inhibitors are always needed to provide limited inhibition performance. In addition, it is extremely difficult to test the iron sulfide formation and inhibition in the laboratory. In this paper, a novel iron sulfide test method has been developed to test the performance of iron sulfide inhibition. Compared with the traditional static jar test and dynamic loop test, this novel developed test method shows good reproducibility and provides a quick and effective way to evaluate the performance of iron sulfide scale inhibition. An environmentally friendly iron sulfide scale inhibitor has been developed based on this novel developed test method. It provided good performance on iron sulfide inhibition in synthetic field water. The mechanism of iron sulfide inhibition will be addressed. This paper will give a comprehensive study of iron sulfide formation and inhibition, including iron sulfide prediction, development of test method and development of inhibitors for field application. Introduction The formation of mineral scale is a persistent and expensive problem in oil and gas production. Scaling of metallic or insulating walls in contact with hard water may create technical problems including pipe or valve blockage, under-deposit corrosion, and even more importantly, unscheduled equipment shutdown [1]. Over the past few decades, great efforts have been made to understand the conventional oilfield scale formation and inhibition, such as CaCO3, BaSO4 and CaSO4. Compared to these conventional mineral scale deposits, iron sulfide has received less attentions. Three of the major reasons are:Some iron sulfide crystals often form a softer scale than calcium carbonate. This scale may not always block tubing to the same degree that other mineral scales do. Thus, it may be seen as less of a problem than other mineral scales [2];Iron sulfide is much more difficult to study in the laboratory than other common mineral scales, where keeping the system oxygen free and the reproducibility of the iron sulfide test are two major challenges.Iron sulfide is present in several crystalline forms that have different sulfur to iron ratios. These different crystalline forms have different solubility in mineral acids, which make the research complex [3]. These challenges may deter the progress of research on iron sulfide scale formation and inhibition. Iron sulfide scale is thought to be deposited by microbial enhanced corrosion or derived from the reaction of iron oxide from corrosion and hydrogen sulfide, a by-product of sulphate reducing bacteria (SRB) metabolism. Another unexpected source of hydrogen sulfide is acid treatment [3]. Aksnick and Engen [4] found iron sulfide scale in well tubular following acid treatments of deep sour gas wells. Recently, sulfide scales have been encountered in more sour reservoirs and deeper wells and more attention has been paid on the study of iron sulfide. In the work reported herein, a novel iron sulfide test method has been developed to test the performance of iron sulfide inhibition. Compared with the traditional static jar test and dynamic loop test, this novel developed test method shows good reproducibility and provides a quick and effective way to evaluate the performance of iron sulfide scale inhibition. An environmentally friendly iron sulfide scale inhibitor has been developed based on this novel developed test method. It showed good performance on iron sulfide inhibition in synthetic field water. The mechanism of iron sulfide inhibition will be addressed.

Calcium sulphate and barium sulphate are two major scales experienced in the oil and gas fields, especially when sea water breakthrough in the waterflood supported HTHP wells. Normally, studies have been focused on a single scale component. Seldom studies have focused on the co-deposition of calcium sulphate and barium sulphate. The importance of interference between calcium sulphate and barium sulphate deposition in the field, especially for the HTHP wells, has been ignored.In this paper, the interference between calcium sulphate and barium sulphate deposition has been studied based on a field case in the North Sea. The mechanisms of co-deposition have been addressed using both scale prediction and laboratory tests. Environmentally acceptable scale inhibitors have also been developed.The scaling tendency and mass deposition of calcium sulphate and barium sulphate have been predicted with sea water breakthrough at different levels. The difference between calcium sulphate and barium sulphate, and the consequences of both types of scale deposition are discussed.Dynamic scale loop tests have been carried out. It demonstrated that a small amount of barium sulphate deposit substantially accelerates the co-deposition of barium sulphate and calcium sulphate. Linked to the scale prediction, the mechanism of co-deposition of calcium sulphate and barium sulphate has been addressed.Several scale inhibitors, including phosphonate and polymer based inhibitors, along with an amine based polymer have been tested under the worst case scaling condition. Environmentally acceptable scale inhibitors have been developed and are suitable for squeeze application. This paper will give a comprehensive study of co-deposition of calcium sulphate and barium sulphate, including scale prediction, laboratory evaluation, mechanism discussion and inhibitor selection. It will contribute to understand calcium sulphate and barium sulphate scale deposition in HTHP wells and find effective inhibitors for field application.

Zinc sulfide (ZnS) is an exotic scale formed in the oil and gas fields, especially in HT/HP wells. It is relatively difficult to test ZnS formation and inhibition in the laboratory using traditional static jar and dynamic loop tests due to the oxidization during the test and its naturally ‘soft’ scale characteristic. Limited studies have been focused on ZnS and the detailed inhibition mechanisms are still unknown. In this paper, a newly developed stress test method has been applied to evaluate the performance and mechanisms of ZnS inhibition. Compared with the traditional test methods, it shows good reproducibility and provides a quick and effective way to evaluate the performance of inhibitors and information to understand the mechanisms of inhibition. More than 15 typical scale inhibitors, representing several different types, have been tested using this newly developed method. The ZnS scale inhibitors were classified as three types based on the inhibition mechanisms from this work: Type 1: Dispersion and nucleation inhibitors. These scale inhibitors showed nucleation and growth inhibition effect at low concentrations of sulfide and dispersion effect at high concentrations of sulfide. Type 2: Nucleation and growth scale inhibitors. These scale inhibitors inhibit nucleation and growth of ZnS formation, where the test can be stressed further. Type 3: Scale inhibitors with poor performance on ZnS inhibition. The turbidity and stress curve did not change obviously in the presence of scale inhibitors. This paper will give a comprehensive study of ZnS formation and inhibition, including scale prediction, development of test method and inhibitors, insight into the mechanism of ZnS inhibition and identification of environmentally acceptable inhibitors.

Static jar and dynamic loop tests are two major test methods used in the oilfield scale industry to evaluate the performance of scale inhibitors. The minimum inhibitor concentration (MIC) under the test conditions can be determined, but with a certain amount of assumptions and theoretical analysis of the mechanism of scale inhibition, especially for scale inhibitors with dispersive effects. Unlike these traditional test methods, a newly developed test method, using an ultrasonic technique along with turbidity measurement, has been used to evaluate scale inhibitors and understand the inhibition mechanisms. In comparison with traditional test methods, this test method provides a quick and effective way to understand the scale inhibitors, including the effect of scale inhibitors on the induction, nucleation and growth stages of scale formation, and also the dispersion functions of scale inhibitors. Several typical commercial scale inhibitors, including phosphonate and polymer based chemistries have been tested using this newly developed test method. Different scale inhibitors demonstrated different inhibition mechanisms and will be addressed in detail in this paper. Results showed a polymer based scale inhibitor has good inhibition on the induction time of scale formation and a phosphoric based scale inhibitor showed a major dispersive effect on scale inhibition. The combined formulation based on these two inhibitors has been evaluated and a synergistic effect was observed and later confirmed by both the traditional static jar test and dynamic loop test, where the mixture of 75% polymer based scale inhibitor and 25% phosphoric based scale inhibitor showed much better performance than both neat scale inhibitors. In this paper, this newly developed test method will be introduced in detail and the benefits and limitations will also be discussed. By understanding the mechanism of the scale inhibitors, this technique might provide a new and alternative route of developing and formulating scale inhibitors.