Abstract

A large independent operator in Mobile Bay, Gulf of Mexico was experiencing a buildup of common and exotic scales where the produced waters are commingled.The scale species encountered included barium sulfate, zinc sulfide, and lead sulfide.Traditionally, scale inhibitor chemistries have not been used in the past to inhibit these exotic sulfide scales in this region.This is due to some amount of belief within the industry that conventional inhibitors alone could not inhibit these scale species.

Prior to the application of the scale inhibitor in the field, the platform's overboard water line required cleanout every two months due to scale buildup.Not a single cleanout has been necessary since a polymeric scale inhibitor has been applied downhole via continuous capillary injection for over one year.

This paper presents the results of laboratory tests which were used to demonstrate how a polymeric scale inhibitor was identified and approved for inhibition of lead sulfide and iron sulfide, as well as the more common scales.The sulfide scale tests performed in an anaerobic dynamic tube-blocking test unit showed this inhibitor chemistry to be as effective as combined scale inhibitor/dispersant products. The paper then presents the field treatment results, which shows good agreement with the lab test results.

Introduction

Commingled brines present problems in the surface and subsurface due to ionic species forming insoluble mineral scale species.Barium sulfate (BaSO4) and calcium carbonate (CaCO[3]) are two of the most common mineral scales encountered in the oilfield and could be considered well-understood.However, zinc sulfide (ZnS) and lead sulfide (PbS) are less common, and require a slightly different approach to treatment.It has been suggested in the past that a special type of "novel" scale inhibitor is best for inhibiting zinc sulfide and lead sulfide scales[1].Additionally, there are a handful of commercial inhibitors which are sold as specialized sulfide scale inhibitors.The goal of this work was to compare the efficacy of one of these specialized sulfide scale inhibitors versus a proprietary product chemistry to determine the relative performance under the conditions of this Mobile Bay field.

Whereas most ZnS/PbS scale inhibitor evaluations have relied on static bottle tests or field test data to determine[1,2,3,4], the tests described in this paper were performed using an anaerobic dynamic tube-blocking apparatus.There are distinct advantages to be gained with the ability to compare product efficacies before progressing to a field trial stage.The anaerobic tube-blocking tests more accurately simulate the dynamic environment where the scale inhibitor products would be used than static bottle tests.The differential pressure transducer detects very minor increases in differential pressure (DP), which indicates when scale has deposited in the test apparatus. This allows nucleation inhibition properties and dispersant properties of the scale inhibitor products to be more closely evaluated than in static bottle tests.

Scale Inhibitor Products

The main focus of this study was to compare the performance of a commercial sulfide scale inhibitor against a proprietary polymeric scale inhibitor.Product A is a commercially-available sulfide scale inhibitor.Products B through E were polymeric scale inhibitor products provided by chemical suppliers, who indicated the chemistries might be effective as zinc sulfide and lead sulfide inhibitors.Product F is a blend of polymers.Product G is a proprietary polymeric scale inhibitor which was readily available commercially.Table 1 provides a quick reference to the generic chemistry of each product.

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