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Abstract Although iron sulphide (FeS) scale is not as common as carbonate and sulfate scales, it is difficult to inhibit, especially at high temperature conditions, due to its low solubility and fast precipitation kinetics. Moreover, the complexity of FeS solution and solid phase chemistry makes FeS deposition and related issues difficult to be solved. This study is to identify more efficient and effective dispersants and inhibitors for FeS scale. Polyacrylamide (PAM), polyvinyl pyrrolidone (PVP), polyoxazoline (OX) and carboxymethyl cellulose (CMC), which are frequently employed during oil and gas production activities for various purposes, successfully prevented FeS particles from settling. CMC was the most effective to disperse FeS particles in brines and it can disperse FeS particles under the conditions of as high as 4M of ionic strength. The size of FeS stabilized with polymers remained in nano-scale. Polymers did not work as threshold inhibitors, but prevented particle growth. Phosphonates and carboxylate chelating agents were also tested for FeS scale inhibition. Diethylenetriamine pentamethylene phosphonate (DTPMP), ethylenediaminetetraacetate (EDTA) and nitrilotriacetate (NTA) successfully inhibited FeS nucleation greater than 90% in a given reaction time of 2 hours at 70 °C, based on the measurement of Fe concentration in filtered solution with 0.22 μm syringe membrane. NTA showed the best inhibition performance at pH 5.0 and all three inhibitors stopped FeS nucleation at a substoichiometric concentration of inhibitors to iron(II). EDTA performed better than NTA and DTPMP at pH 6.7 at about 10% excess of EDTA molar concentration over iron(II). As pH and saturation index (SI) increased, greater concentrations of inhibitors were required to inhibit FeS scale.
Ko, Saebom (Rice University) | Wang, Xin (Rice University) | Zhao, Yue (Rice University) | Dai, Chong (Rice University) | Lu, Yi-Tsung (Rice University) | Deng, Guannan (Rice University) | Paudyal, Samridhdi (Rice University) | Mateen, Sana (Rice University) | Kan, Amy T. (Rice University) | Tomson, Mason B. (Rice University)
Abstract Scale formation in oil and gas wells commonly occurs, causing not only pipeline blockage, equipment failure, or formation damage during production, trasnsportation, and treatment, but also premature abandonment of wells in serious cases. Although types of mineral scale occurrence depend on the types of ions in water, sulfate and carbonate scales are the most commonly found scales in oil and gas fields. In this study, we investigated a single approach to prevent complex mineral scales from deposition using water-soluble polymer dispersant or the combination of water-soluble polymer dispersant of carboxymethyl cellulose (CMC) and phosphonate inhibitors of diethylene triamine penta(methylene phosphonic) acid (DTPMP) or hexamethylene diaminetetra (methylene phosphonic) acid (HDTMP) in highly saturated solution or high ionic strength (IS) brines. This study shows that CMC effectively prevents sulfate (barite and gypsum) and carbonate (calcite nd iron carbonate) scales from deposition. The particle size dispersed in the presence of CMC remains in nanosize ranges. When CMC was combined with phosphonate inhibitors of DTPMP or HDTMP, sulfate scales were even more effectively controlled, compare to CMC or phosphonate inhibitors by themselves. In the combination of CMC and DTPMP, the majority of barite (> 90%) remained in a size of smaller than 200 nm and the total mass of barite deposition on 316 stainless steel coupon was negligible, as low as 0.079% of total input mass. Gypsum formation was inhibited for at least 6 hours and gypsum particles remained in a size of smaller than 200 nm for 12 hours in the combination of CMC and HDTPM. For calcite, measured induction time was 134 minutes and calcite particles were dispersed for at least 15 hours with its average particle size of 396 nm in the presence of CMC. Iron carbonate particles were well dispersed for 2 hours in the presence of CMC.
Sulphide scales, namely iron sulphide (FeS), zinc sulphide (ZnS) and lead sulphide (PbS), are increasingly being encountered in gas/oil wells. These scales can present serious safety concerns, impair well productivity and limit access to downhole tools. There are many published research studies addressing sulphide scale removal and inhibition. However, there is an incomplete understanding of the governing processes of sulphide scale formation and prevention. Furthermore, there are contradictory results in the literature on issues such as the removal procedures and inhibition tests for sulphide scales. Therefore, the main objective of this paper is to critically review the published work on sulphide scale formation, removal and inhibition, to address the factors that control them and to discuss some of the apparent discrepancies in published experimental studies.
The review discusses the formation mechanisms of different sulphide scales in relation to the sources and levels of Fe, Zn, Pb and the sulphide species. The experimental procedures used by different researchers to evaluate sulphide scale dissolvers and inhibitors are described, along with the performance results for the chemistries tested to remove or prevent sulphide scales.
Hydrochloric acid has been shown to outperform rival chemistries for dissolving sulphide scales, however the associated high corrosion rate and H2S generation has necessitated the development of other dissolvers to overcome such drawbacks. Several dissolvers based on chelating agent chemistries combined with catalysts provided high dissolution rates, and the dissolution results and the reaction mechanisms will be discussed in some detail.
Multiple factors have been shown to play a significant role in the inhibition efficiency of sulphide scale inhibitors including pH, salinity, temperature, scale formation sequence and mechanism, and the initial concentrations of sulphide species and scaling metals. In addition, there is a developing understanding of the significance of scale inhibitor consumption in these systems.
Understanding the formation mechanism is essential for accurate interpretation of scale-related issues in the field and for providing the correct treatment strategy. A more complete knowledge of these issues will lead to the further development of reliable procedures for generating dissolution and inhibition results and consequently improving the scale dissolver and inhibitor chemistries themselves.
The scale problem related to the formation of iron sulfide is commonly found during oil and gas productions in sour environments using low corrosion resistant carbon steel equipment and pipes. When hydrogen sulfide gas is evolved, resulting from sulfate reducing bacteria or thermal decomposition of sulfate, in the presence of iron from various corrosion processes in downhole, iron sulfide can quickly precipitate. There have been commercially available dispersants for iron sulfide scale particles, but can transfer iron sulfide scale to the oil phase, causing another issue during oil and gas operational activities. In this study, stability of FeS particles produced in a strictly anoxic condition were examined with several different water-soluble polymers in various reaction conditions. Among tested water-soluble polymers, polyacrylamide, polyvinyl pyrrolidone, oxazoline and carboxymethyl cellulose successfully prevented FeS particles from settling in the conditions of Ca2+ concentrations as high as 200 mM at both pHs of 5.0 and 6.7 with the polymer concentrations as low as 20 mg/L. The size of FeS stabilized in polymers remained in nano-scale. Polymers did not work as threshold inhibitors, but prevented particle growth. Polymers changed FeS wettability from oil-wet to water-wet.
The scale problem related to the formation of iron sulfide is commonly found during oil and gas productions in sour environments using low corrosion resistant carbon steel equipment and pipes.1-3 When hydrogen sulfide gas is evolved, resulting from sulfate reducing bacteria or thermal decomposition of sulfate, in the presence of iron from various corrosion processes in downhole, iron sulfide can quickly precipitate.4 Although iron sulfide scale is not as common as carbonate and sulfate scales, it is difficult to inhibit, especially at high temperature conditions, due to its low solubility and fast precipitation kinetics.5 Because efficient inhibitors for iron sulfide scale have not been found, the application of iron sulfide dispersants could be an alternatively feasible method to control iron sulfide scale problems. There have been commercially available dispersants for iron sulfide scale particles, but can transfer iron sulfide scale to the oil phase,6 causing another issues during oil and gas operational activities.4, 7
Summary In oil and gas production operations, precipitation of mineral scales causes many problems, such as formation damage, production losses, increased workovers in both producers and injectors, poor injection-water quality, and equipment failures caused by underdeposit corrosion. The most common mineral scales are sulfate- and carbonate-based minerals. However, scale problems are not limited to these minerals, and there recently have been reports of unusual scale types, such as zinc and lead sulfide. This paper focuses on zinc sulfide scale that has been found in several fields along the Gulf Coast of the U.S.A. and in fields within the North Sea Basin. Scale deposition has caused significant pressure and rate reductions in high-temperature and 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 retreatments were required. Topside process equipment, most noticeably low-pressure separators and hydrocyclones, 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 (HT/HP) gas wells in corrosivity of the acid at high temperatures (general corrosion, sulfide stress cracking, and chloride stress cracking) and in 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 HT/HP acid jobs is to use scale inhibitors or 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 those needed for common sulfate and carbonate scales. For chelating agents, it is possible to prevent the formation of zinc sulfide scale when the required concentrations are proportional to the zinc ion concentration in the scaling brine. This paper outlines the testing methods used for chemical screening and prediction so that assessment of the potential problem within fields can be assessed during appraisal, before production commences, making a method of managing the risk available. Introduction The most common scales encountered in oilfield operations are sulfates, such as calcium sulfate (anhydrite and gypsum), barium sulfate (barite), strontium sulfate (celestite), and carbonates (calcite). Numerous studies on scale inhibition with regard to controlling such scale within the reservoir and in production equipment (downhole and topside) have been published in the past few years. Other less common scales, such as iron oxides, iron sulfides, and iron carbonate, have also been reported. These scale types are most commonly associated with iron generation from corrosion products, although iron carbonate scale has been reported to form from produced water drawn from formations where iron-containing authigenic minerals are present within the formation. Similar to the sulfate and carbonate scale types described previously, even iron carbonate scale can be controlled by inhibitor molecules. Lead and zinc sulfide scales have recently become a concern in a number of North Sea oil and gas fields. These deposits have occurred within the production tubing and topside process facilities. Investigation of the literature leads to a number of references in which such scale had been observed, but very little information was available on their inhibition by chemical means. A recent review paper outlines the formation mechanisms of both lead and 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 field. Potential Sources of Lead and Zinc Sulfide Several sources of zinc/lead and sulfide ions are possible within produced fluids. Sources of Zinc and Lead Ions. Reaction products of formation minerals (sphalerite zinc sulfide and galena lead sulfide) during connate and aquifer water contact during many millions of years could result in partial mineral dissolution. Zinc ion concentration within HP/HT fields within the Gulf Coast of Mexico were reported to be as high as 70 ppm Pb and 245 ppm Zn. Reaction of injected water used for pressure support into the aquifer or the oil leg can result in the fresh or seawater reacting with minerals within the formation, which can become enriched in heavy metal ions. Zinc ions can come from heavy-brine completion fluids lost into the formation during drilling and well workover operations (zinc bromide). Biggs reported that a loss of 500 bbl of 17.2 lb/gal zinc bromide completion fluid within a reservoir resulted in significant zinc sulfide scale formation with the presence of 2 ppm of hydrogen sulfide from the reservoir. In an oil field operated in the North Sea, U.K. sector, the presence of zinc sulfide on downhole gauges and logging tools was reported within a well where zinc bromide brines had been lost during completion operations. During initial water breakthrough, zinc levels within the produced fluids were in the range of 10 to 50 ppm for several months. Sources of Sulfide Ions. Hydrogen sulfide (H2S) gas is the most likely source of sulfide ions that allow the formation of lead/zinc sulfide scale. Low concentrations (in the tens of ppm levels) of H2S have been reported in produced gas from wells where lead and zinc sulfide scale problems have been reported. Decomposition of the corrosion inhibitor and drilling compounds can also produce sulfide ions when tested in autoclave equipment at high temperatures but are very unlikely to be the source of sufficient sulfide ions to give scale deposition during many years of production. The most likely source of sulfide ions is reservoir hydrogen sulfide gas.