Abstract

Precipitation of mineral scales causes many problems in oil and gas production operations: formation damage, production losses, increased workovers in both producers and injectors, poor injection water quality, and equipment failures due to under-deposit corrosion. The most common mineral scales are sulfate and carbonate-based minerals. However, scale problems are not limited to these minerals and there have recently been reports of unusual scale types such as zinc and lead sulfide. This paper focuses on zinc sulfide scale that has been has been found in several fields along the Gulf Coast of the USA and fields within the North Sea Basin. Scale deposition has caused significant pressure and rates reductions in high temperature, high rate gas, condensate and black oil wells. After acid washes to remove zinc sulfide scale (and other acid soluble solids), production rates and flowing tubing pressures returned to previous levels, but new scale deposits formed in many wells and re-treatments were required. Topside process equipment most noticeable low-pressure separators and hydrocyclone were observed to suffer reductions in performance owing to zinc sulfide scale deposition. In addition, there are significant risks associated with acid treatments in high temperature, high-pressure gas wells: corrosivity of the acid at high temperatures (general corrosion, sulfide stress cracking, and chloride stress cracking) and safety (hydrogen sulfide generation by acid dissolution of zinc sulfide plus high pressure pumping). One possible method for preventing production declines and reducing the need for high temperature, high-pressure acid jobs, is to use scale inhibitors or a chelating agents to prevent the formation of zinc sulfide scale. The relative effectiveness of eight scale inhibitor chemistries and two chelating agents in preventing formation of zinc sulfide scale has been determined. The required scale inhibitor concentrations are significantly higher than concentrations needed for common sulfate and carbonate scales. For chelating agents, it is possible to prevent the formation of zinc sulfide scale with the required concentrations being proportional to the Zn2+ ion concentration in the scaling brine. This paper outlines the testing methods utilized for chemical screening and for prediction so that assessment of the potential problem within fields during appraisal can be assessed prior to production commencing and a method of managing the risk made available.

Introduction

The most common scales encountered in oil field operations are sulfates such as calcium sulfate (anhydrite, gypsum) barium sulfate (barite), strontium sulfate (celestite) and carbonates (calcite). Numerous studies on scale inhibition have been published over the past few years with regard to controlling such scale within the reservoir and in production equipment (downhole and topside).1–8 Other less common scales have also been reported such as iron oxides, iron sulfides and iron carbonate. These scale types are most commonly associated with iron generation from corrosion products although iron carbonate scale has been reported forming from produced water drawn from formations where iron containing authigenic minerals are present within the formation.9,10,11 As with the sulfate and carbonate scale types described above even iron carbonate scale is able to be controlled by inhibitor molecules.12

Lead and zinc sulfide scale has recently become a concern in a number of North Sea oil and gas fields. These deposits have occurred within the production tubing and within the topside process facilities. Investigation of the literature lead to a number of references where such scale had been observed13,14,15,16,17 but very little information was available on their inhibition by chemical means. A recent review paper outlines formation mechanisms of both lead & zinc sulfides and also reviews the data from the literature before describing how a chemical inhibition program has been effectively deployed within a North Sea field18.

Potential sources of Lead and Zinc sulfide

Several sources of zinc/lead ions and sulfide ions are possible within produced fluids.

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