Compositions for dissolving mercury sulphide and methods of use

Abstract

The invention provides a method of dissolving scale comprising mercury sulphide. The method comprises providing a composition comprising an alkali metal sulphide and contacting the scale with the composition.

Description

[0001] Compositions for Dissolving Mercury Sulphide and Methods of Use

[0002] The present invention relates to compositions and methods for dissolving deposits of mercury sulphide, in particular to compositions and methods for removing scale comprising mercury sulphide from hydrocarbon pipelines, pipeline equipment, processing equipment and / or storage vessels involved in oil and gas operations. Aspects of the invention relate to methods of treating decommissioned pipelines and pipeline equipment and cleaning or decommissioning storage vessels to remove scale comprising mercury sulphide.

[0003] Background to the invention

[0004] During oilfield operations and transportation of hydrocarbons in pipelines, mercury and mercury salts naturally present in geological formations may accumulate on surfaces of pipes and vessels over time. This can present a significant safety risk to workers that may be exposed to mercury through skin contact or inhalation of mercury vapours. As such, mercury is considered a hazardous substance and its safe removal and disposal is a safety priority.

[0005] There is also an environmental concern that following decommissioning of pipelines especially subsea pipelines, highly toxic mercury salt present inside a pipeline can be released into the environment as the mercury salt leaches into seawater. This poses a serious biohazard and threat to the health of living organisms in the surrounding environment.

[0006] At present the available methods for removing mercury are limited and unsatisfactory. Mercury is insoluble in most solvents and is sparingly soluble in concentrated acids, including nitric acid, sulfuric acid, and aqua regia, which creates challenges in the removal and transport of mercury contaminants. The use of nitric acid, sulfuric acid, and aqua regia to remove surface contamination in pipelines is not practical because these treatments corrode the steel surfaces of the pipeline leading to structural damage of the pipeline being treated. The use of nitric acid, sulfuric acid, and aqua regia also poses safety hazards to workers during decontamination procedures. Summary of the invention

[0007] It is an object of the present invention to obviate or at least mitigate the foregoing disadvantages of mercury removal methods.

[0008] It is another object of an aspect of the present invention to provide a composition which can remove hazardous mercury scale contaminating equipment involved in oil and gas operations including pipelines, pipeline equipment, processing equipment and storage vessels in a safe manner which may mitigate environmental threats posed by the contents of decommissioned oil and gas equipment.

[0009] It is another object of an aspect of the present invention to provide a method of removing scale deposits of mercury sulphide from a pipeline and / or pipeline equipment.

[0010] It is a further object of an aspect of the present invention to provide a treatment for pipelines which can work quickly and remotely to dissolve mercury sulphide scale deposits.

[0011] Further aims of the invention will become apparent from the following description.

[0012] According to a first aspect of the present invention there is provided a method of dissolving scale comprising mercury sulphide, the method comprising providing a composition comprising an alkali metal sulphide; and contacting the scale with the composition.

[0013] The alkali metal sulphide may be selected from the group comprising lithium sulphide, sodium sulphide, potassium sulphide, rubidium sulphide, caesium sulphide and / or francium sulphide. Preferably the alkali metal sulphide may be sodium sulphide.

[0014] The composition may be an alkaline composition. The composition may have a pH greater than 7. The composition may have a pH in the range of about pH 8 to about pH 14. The composition may have a pH in the range of about pH 8 to about pH 14. The composition may have a pH in the range of about pH 9 to about pH 14. The composition may have a pH in the range of about pH 10 to about pH 14. The composition may have a pH in the range of about pH 10 to about pH 13. The composition may have a pH in the range of about pH 10 to about pH 12. The composition may have a pH in the range of pH 10 to pH 14. The pH of the composition may be imparted by the type and amount of components forming the composition. Suitable buffer systems may be used for adjusting and / or stabilizing the pH. The composition may be a liquid composition. The composition may be a solution. The composition may be an aqueous composition. The composition may be an aqueous solution.

[0015] The composition may comprise from about 5% to about 50% alkali metal sulphide by weight. The composition may comprise at least 5% alkali metal sulphide by weight. The composition may comprise at least 10% alkali metal sulphide by weight. The composition may comprise at least 15% alkali metal sulphide by weight. The composition may comprise at least 20% alkali metal sulphide by weight. The composition may comprise from about 5% to about 30% alkali metal sulphide by weight. The composition may comprise from about 10% to about 40% alkali metal sulphide by weight. The composition may comprise from about 10% to about 30% alkali metal sulphide by weight. The composition may comprise from about 10% to about 20% alkali metal sulphide by weight.

[0016] The composition may comprise at least one organo-metallic compound. The composition may comprise at least one chelating agent. The at least one chelating agent may be selected from the group comprising ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), diethylenetriaminepentaacetic acid (DTPA), gluconic acid, poly amino polycarboxylic acids. The at least one chelating agent may be selected from at least one salt of any of ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), diethylenetriaminepentaacetic acid (DTPA), gluconic acid, and / or poly amino polycarboxylic acids. The at least one chelating agent may comprise sodium glucoheptonate. The at least one chelating agent may comprise potassium DTPA K5.

[0017] The composition may comprise at least 5% chelating agent by weight. The composition may comprise at least 7.5% chelating agent by weight. The composition may comprise at least 10% chelating agent by weight. The composition may comprise from about 5% to about 25% w / w chelating agent. The composition may comprise about 10% w / w chelating agent. The composition may comprise at least 5% sodium glucoheptonate by weight. The composition may comprise at least 7.5% sodium glucoheptonate by weight. The composition may comprise at least 10% sodium glucoheptonate by weight. The composition may comprise from about 5% to about 25% w / w sodium glucoheptonate. The composition may comprise about 10% w / w sodium glucoheptonate. The at least one chelating agent may maintain an aqueous phase equilibrium. The at least one chelating agent may keep the dissolved mercury ions in an aqueous solution. This may prevent or mitigate reprecipitation of mercury sulphide to a solid from the solution.

[0018] The composition may comprise a strong base. The strong base may be selected from the group comprising alkali hydroxide, potassium hydroxide, sodium hydroxide, lithium hydroxide, calcium hydroxide, barium hydroxide and / or strontium hydroxide.

[0019] The composition may comprise at least 5% w / w strong base. The composition may comprise at least 7.5% w / w strong base. The composition may comprise at least 10% w / w strong base. The composition may comprise at least 15% w / w strong base. The composition may comprise at least 20% w / w strong base. The composition may comprise from about 5% to about 25% w / w strong base. The composition may comprise from about 5% to about 20% w / w strong base. The composition may comprise from about 7.5% to about 15% w / w strong base. The composition may comprise about 10% w / w strong base. The composition may comprise 10% w / w strong base. Preferably the strong base is potassium hydroxide.

[0020] The composition may comprise at least 30% w / w water. The composition may comprise at least 40% w / w water. The composition may comprise at least 50% w / w water. The composition may comprise at least 60% w / w water. The composition may comprise at least 70% w / w water. The composition may comprise from about 30% to about 85% w / w water. The composition may comprise from about 40% to about 75% w / w water. The composition may comprise from about 50% to about 70% w / w water. The composition may comprise from about 65% to about 85% w / w water. The composition may comprise from about 70% to about 85% w / w water. The composition may comprise about 70% w / w water. The composition may comprise 70% w / w water.

[0021] The composition may comprise at least one alkali metal sulphide, at least one chelating agent, at least one strong base and water. The composition may comprise an alkali metal sulphide, at least one chelating agent, a strong base and water. The composition may comprise sodium sulphide, sodium glucoheptonate, potassium hydroxide and water. The composition may comprise from about 5% to about 15% w / w sodium sulphide, about 5% to about 15 % w / w sodium glucoheptonate, about 5% to about 15% w / w potassium hydroxide and about 55% to about 85% w / w water. The composition may comprise from about 5% to about 10% w / w sodium sulphide, about 5% to about 10% w / w sodium glucoheptonate, about 5% to about 10% w / w potassium hydroxide and about 70% to about 85% w / w water. The composition may comprise about 10% w / w sodium sulphide, about 10% w / w sodium glucoheptonate, about 10% w / w potassium hydroxide and about 70% w / w water. The composition may comprise 10% w / w sodium sulphide, 10% w / w sodium glucoheptonate, 10% w / w potassium hydroxide and 70% w / w water.

[0022] The composition may have a dissolution capacity of up to 1kg mercury sulphide / litre of composition. The composition may have a dissolution capacity of up to 0.5kg mercury sulphide / litre of composition. The composition may have a dissolution capacity of about 0.1kg to about 1kg mercury sulphide / litre of composition. The composition may have a dissolution capacity of about 0.2kg to about 0.8kg mercury sulphide / litre of composition. The composition may have a dissolution capacity of about 0.2kg to about 0.5kg mercury sulphide / litre of composition. The composition may have a dissolution capacity of about 0.1kg to about 0.5kg mercury sulphide / litre of composition. The composition may have a dissolution capacity of about 0.1kg to about 0.4kg mercury sulphide / litre of composition. The composition may have a dissolution capacity of at least 0.5kg mercury sulphide / litre of composition. The composition may have a dissolution capacity of at least 0.4kg mercury sulphide / litre of composition. The composition may have a dissolution capacity of at least 0.3kg mercury sulphide / litre of composition. The composition may have a dissolution capacity of at least 0.2kg mercury sulphide / litre of composition. The composition may have a dissolution capacity of at least 0.1kg mercury sulphide / litre of composition.

[0023] The composition may be configured to be compatible with pipelines, pipeline joints, pipeline equipment, storage vessels, vessels and / or processing equipment involved in oil and gas operations such that the composition is not corrosive to pipelines, pipeline joints, pipeline equipment, vessels and / or processing equipment and does not damage pipelines and / or pipeline equipment such as joints or seals. The pipelines, pipeline joints, pipeline equipment, storage vessels, vessels and / or processing equipment may be made of metal or a metal alloy. The pipelines, pipeline joints, pipeline equipment, storage vessels, vessels and / or processing equipment may comprise ferrous material. The pipelines, pipeline joints, pipeline equipment, storage vessels, vessels and / or processing equipment may be made of steel.

[0024] The method may comprise contacting the mercury sulphide scale with the composition for up to 5 minutes. The method may comprise contacting the mercury sulphide scale with the composition for at least 5 minutes. The method may comprise contacting the mercury sulphide scale with the composition for a period of at least 30 minutes. The method may comprise contacting the mercury sulphide scale with the composition for a period of at least 1 hour. The method may comprise contacting the mercury sulphide scale with the composition for a period of between 10 minutes and 2 hours. The method may comprise contacting the mercury sulphide scale with the composition for a period of between 15 minutes and 3 hours. The method may comprise contacting the mercury sulphide scale with the composition for a period of between 1 hour and 10 hours to dissolve the mercury sulphide scale. The method may comprise contacting the mercury sulphide scale with the composition for up to 24 hours. The method may comprise contacting the mercury sulphide scale with the composition for at least 24 hours. The period of time of contact may be dependent on the amount of deposit present its size and / or its thickness.

[0025] The method may comprise contacting the mercury sulphide scale with the composition at ambient temperature. Ambient temperature may be the temperature of the air, fluids and / or a surface surrounding or in the location of the scale. Ambient temperature may be the temperature of the pipeline, pipeline equipment, processing equipment, and / or storage vessel at the location of the scale. The ambient temperature may be in the temperature range of about 0°C to 60°C. The ambient temperature may be in the temperature range of about 10°C to 50°C. The ambient temperature may be in the temperature range of about 20°C to 35°C. The method may comprise contacting the mercury sulphide scale with the composition at a temperature in the range of about 0°C to about 60°C. The method may comprise contacting the mercury sulphide scale with the composition at a temperature in the range of about 10°C to about 50°C. The method may comprise contacting the mercury sulphide scale with the composition at range of about 10°C to about 40°C. The method may comprise contacting the mercury sulphide scale with the composition at a temperature range of about 20°C to about 35°C. The method may comprise contacting the mercury sulphide scale with the composition at a temperature range of about 20°C to 30°C. A reaction that may result in the dissolution of Mercuric sulphide by sodium sulphide may be described by the equation:

[0026] Where mercuric sulphide reacts with sodium sulphide solution to form water soluble Sodium dithio mercurate (II).

[0027] The method may comprise recovering and / or collecting the composition after it has contacted the mercury sulphide scale. The composition solution after it has contacted the mercury sulphide scale may be a spent waste solution. The method may comprise recovering and / or collecting the spent waste solution. The method may comprise containing or storing the spent waste solution in a storage vessel or tank. The method may comprise containing or storing the spent waste solution in a treatment vessel or tank. The treatment vessel may be a frac tank. The method may comprise adding hydrogen peroxide to the spent waste solution. The method may comprise adding hydrogen peroxide to the spent waste solution gradually. The method may comprise adding hydrogen peroxide at about 2.5% solution to the spent waste solution. The method may comprise adding hydrogen peroxide at about 5% solution to the spent waste solution. The method may comprise adding hydrogen peroxide at about 7.5% solution to the spent waste solution. The method may comprise adding hydrogen peroxide at about 10% solution to the spent waste solution. The method may comprise stirring the spent waste solution and hydrogen peroxide. The method may comprise stirring the spent waste solution and hydrogen peroxide for at least 15 minutes. The method may comprise stirring the spent waste solution and hydrogen peroxide for at least 30 minutes. The method may comprise stirring the spent waste solution and hydrogen peroxide for at least 45 minutes. The method may comprise stirring the spent waste solution and hydrogen peroxide for about 1 hour. The method may comprise stirring the spent waste solution and hydrogen peroxide for a period of time between 15 minutes and 2 hours.

[0028] The method may comprise settling particles of mercury sulphide out of the liquid suspension. The method may comprise leaving the solution to facilitate the mercury sulphide particles separating or settling out of the liquid suspension. The method may comprise settling particles of mercury sulphide out of the liquid suspension. The method may comprise settling particles of mercury sulphide out of the liquid suspension for at least one hour, at least two hours, at least three hours, at least four hours, at least five hours or at least six hours. The method may comprise settling particles of mercury sulphide out of the liquid suspension for between one to six hours. The method may comprise separating the composition solution from the settled solid mercury sulphide. The method may comprise containing the separated composition solution in a storage vessel or tank. The method may comprise containing the separated composition solution in a treatment vessel or tank. The treatment vessel may be a frac tank. The method may comprise adjusting the pH of the separated composition solution. . The method may comprise adjusting the pH of the separated composition solution to about pH7. The method may comprise adjusting the pH of the separated composition using an acid. The acid may be hydrochloric acid. The method may comprise discharging the separated composition.

[0029] The settled solid mercury sulphide may be in the form of a filter cake. The method may comprise disposing of the solid mercury sulphide. The method may comprise disposing of the solid mercury sulphide in a drum.

[0030] According to a second aspect of the present invention there is provided a method of removing scale comprising mercury sulphide from a contaminated surface, the method comprising providing a composition comprising an alkali metal sulphide; and contacting the scale with the composition.

[0031] The method may comprise removing scale by substantially dissolving the scale using the composition. The alkali metal sulphide may be selected from the group comprising lithium sulphide, sodium sulphide, potassium sulphide, rubidium sulphide, caesium sulphide and / or francium sulphide. Preferably the alkali metal sulphide may be sodium sulphide.

[0032] The composition may comprise from about 5% to about 50% alkali metal sulphide by weight. The composition may comprise at least 5% alkali metal sulphide by weight. The composition may comprise at least 10% alkali metal sulphide by weight. The composition may comprise at least 15% alkali metal sulphide by weight. The composition may comprise at least 20% alkali metal sulphide by weight. The composition may comprise from about 5% to about 30% alkali metal sulphide by weight. The composition may comprise from about 10% to about 40% alkali metal sulphide by weight. The composition may comprise from about 10% to about 30% alkali metal sulphide by weight. The composition may comprise from about 10% to about 20% alkali metal sulphide by weight.

[0033] The composition may comprise at least one chelating agent. The incorporation of chelating agent in the composition may mitigate re-precipitation of dissolved mercury sulphide. The at least one chelating agent may be selected from the group comprising ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), diethylenetriaminepentaacetic acid (DTPA), gluconic acid, poly amino polycarboxylic acids, and / or any of their salts. The at least one chelating agent may comprise sodium glucoheptonate. The at least one chelating agent may comprise potassium DTPA K5. The composition may comprise at least 5% chelating agent by weight. The composition may comprise at least 7.5% chelating agent by weight. The composition may comprise at least 10% chelating agent by weight. The composition may comprise from about 5% to about 25% w / w chelating agent. The composition may comprise about 10% w / w chelating agent. The composition may comprise at least 5% sodium glucoheptonate by weight. The composition may comprise at least 7.5% sodium glucoheptonate by weight. The composition may comprise at least 10% sodium glucoheptonate by weight. The composition may comprise from about 5% to about 25% w / w sodium glucoheptonate. The composition may comprise about 10% w / w sodium glucoheptonate. The at least one chelating agent may maintain an aqueous phase equilibrium. The at least one chelating agent may keep the dissolved mercury ions in an aqueous solution. This may prevent or mitigate reprecipitation of mercury sulphide to a solid from the solution.

[0034] The composition may comprise a strong base. The strong base may be selected from the group comprising alkali hydroxide, potassium hydroxide, sodium hydroxide, lithium hydroxide, calcium hydroxide, barium hydroxide and / or strontium hydroxide. The composition may comprise at least 5% w / w strong base. The composition may comprise at least 7.5% w / w strong base. The composition may comprise at least 10% w / w strong base. The composition may comprise at least 15% w / w strong base. The composition may comprise at least 20% w / w strong base. The composition may comprise from about 5% to about 25% w / w strong base. The composition may comprise from about 5% to about 20% w / w strong base. The composition may comprise from about 7.5% to about 15% w / w strong base. The composition may comprise about 10% w / w strong base. The composition may comprise 10% w / w strong base. Preferably the strong base is potassium hydroxide. The composition may comprise at least 30% w / w water. The composition may comprise at least 40% w / w water. The composition may comprise at least 50% w / w water. The composition may comprise at least 60% w / w water. The composition may comprise at least 70% w / w water. The composition may comprise from about 30% to about 85% w / w water. The composition may comprise from about 40% to about 75% w / w water. The composition may comprise from about 50% to about 70% w / w water. The composition may comprise from about 65% to about 85% w / w water. The composition may comprise from about 70% to about 85% w / w water. The composition may comprise about 70% w / w water. The composition may comprise 70% w / w water.

[0035] The composition may comprise sodium sulphide, sodium glucoheptonate, potassium hydroxide and water. The composition may comprise from about 5% to about 15% w / w sodium sulphide, about 5% to about 15 % w / w sodium glucoheptonate, about 5% to about 15% w / w potassium hydroxide and about 55% to about 85% w / w water. The composition may comprise from about 5% to about 10% w / w sodium sulphide, about 5% to about 10% w / w sodium glucoheptonate, about 5% to about 10% w / w potassium hydroxide and about 70% to about 85% w / w water. The composition may comprise about 10% w / w sodium sulphide, about 10% w / w sodium glucoheptonate, about 10% w / w potassium hydroxide and about 70% w / w water. The composition may comprise 10% w / w sodium sulphide, 10% w / w sodium glucoheptonate, 10% w / w potassium hydroxide and 70% w / w water.

[0036] The contaminated surface may be a surface of a pipeline, pipeline equipment, processing equipment, and / or storage vessel previously or currently involved in oil and gas operations.

[0037] Pipeline equipment may comprise pipeline joints and / or seals. The composition may be configured to be compatible with pipelines, pipeline joints and / or pipeline equipment such that the composition is not corrosive to pipelines, pipeline joints and / or pipeline equipment and does not damage pipelines and / or pipeline equipment such as joints or seals. The pipelines, pipeline joints and / or pipeline equipment may be made of metal and / or or an alloy. The pipelines, pipeline joints and / or pipeline equipment may be made of steel.

[0038] The pipeline, pipeline equipment, processing equipment, and / or storage vessel may be active or decommissioned. The pipeline may be a hydrocarbon pipeline. The pipeline may be an oil or gas pipeline. The method may comprise removing scale from the pipeline, pipeline equipment, processing equipment and / or storage vessel as part of a maintenance or cleaning operation. The method may comprise removing scale from the pipeline, pipeline equipment, processing equipment and / or storage vessel as part of a decommissioning operation.

[0039] At least a part of an internal and / or external surface of a pipeline, pipeline equipment, processing equipment and / or storage vessel may be contaminated with scale comprising mercury sulphide. The method may comprise contacting at least a part of an internal and / or external surface of a pipeline, pipeline equipment, processing equipment and / or storage vessel with the composition.

[0040] The method may comprise contacting a surface of a pipeline, pipeline equipment, processing equipment and / or storage vessel contaminated with mercury sulphide scale with the composition by passing the composition in, through and / or over the pipeline, pipeline equipment, processing equipment and / or storage vessel . The method may comprise contacting the mercury sulphide scale with the composition for less than 5 minutes. The method may comprise contacting the mercury sulphide scale with the composition for at least 5 minutes. The method may comprise contacting the mercury sulphide scale with the composition for a period of at least 30 minutes. The method may comprise contacting the mercury sulphide scale with the composition for a period of at least 1 hour. The method may comprise contacting the mercury sulphide scale with the composition for a period of between 1 hour and 10 hours to dissolve the mercury sulphide scale. The method may comprise contacting the mercury sulphide scale with the composition for up to 24 hours. The method may comprise contacting the mercury sulphide scale with the composition for at least 24 hours The period of time of contact may be dependent on the amount of deposit present its size and / or its thickness.

[0041] The method may comprise contacting the mercury sulphide scale with the composition by pumping the composition into and / or through the pipeline, pipeline equipment, processing equipment and / or storage vessel. The method may comprise contacting the mercury sulphide scale with the composition at ambient temperature. Ambient temperature may be the temperature of the air, fluids and / or a surface surrounding or in the location of the scale. Ambient temperature may be the temperature of the pipeline, pipeline equipment, processing equipment, and / or storage vessel at the location of the scale.

[0042] The method may comprise contacting the mercury sulphide scale with the composition by spraying the composition into and / or onto at least one contaminated surface. The at least one contaminated surface may be an interior and / or an exterior surface. The method may comprise contacting the mercury sulphide scale with the composition by spraying the composition into and / or onto at least one surface of a pipeline, pipeline equipment, processing equipment and / or storage vessel. The method may comprise contacting the mercury sulphide scale with the composition by spraying the composition onto contaminated surfaces such as the walls of storage tanks. The method may comprise applying a layer or film of the composition to a surface to be treated. The method may comprise applying a layer or film of the composition into and / or onto at least one contaminated surface. By applying a layer or film of the composition to a surface to be treated the volume of chemical required to treat the contaminated surface may be reduced.

[0043] The method may comprise creating a spray of the composition to wet at least one surface contaminated with mercury sulphide scale. The method may comprise using a jetting nozzle such as a rotating head nozzle to create a spray. The method may comprise recirculating unreacted composition that passes over the at least one surface. The method may comprise collecting unreacted composition that passes over the at least one surface and reapplying it to the at least one surface. The method may comprise creating a recirculation loop. The recirculation loop may allow a relatively small volume of composition which has passed over, on, in or through at least a section of the surface to be collected in, on, or near the pipeline, pipeline equipment, processing equipment and / or storage vessel of the vessel. The collected volume of composition may be reapplied to a surface to be treated until the composition comprises a high concentration of spent composition.

[0044] The collected composition may be lifted by a pump such as a submersible pump. The collected composition may be applied or sprayed on, in or through the at least one surface to be treated. The composition may be configured to flow down the at least one surface to be treated dissolving or partially dissolving the mercury sulphide scale coating before being collecting in a pool of composition where it is sprayed again forming a recirculation loop.

[0045] The contaminated vessels and storage vessel / tanks may be present on oil and gas production platforms, floating production, storage and offloading vessels and refineries. The contaminated pipework, equipment, vessels and / or storage vessel / tanks may be required to be cleaned for continuation of processing or storage operations or may be required to be decontaminated for the purpose of decommissioning.

[0046] The method may comprise flushing out spent composition and / or dissolved mercury sulphide scale from the pipeline, pipeline equipment, processing equipment and / or storage vessel. The method may comprise pumping out the spent composition and / or dissolved mercury sulphide scale from the pipeline, pipeline equipment, processing equipment and / or storage vessel to dispose of the spent composition and dissolved mercury sulphide scale.

[0047] Embodiments of the second aspect of the invention may include one or more features of the first aspect of the invention or its embodiments, or vice versa.

[0048] According to a third aspect of the present invention there is provided a method of removing scale comprising mercury sulphide from decommissioned oil and gas equipment, the method comprising providing a composition comprising an alkali metal sulphide; and contacting the scale comprising mercury sulphide with the composition.

[0049] The method may comprise removing scale by substantially dissolving the scale using the composition. The method may comprise contacting at least a part of an internal and / or external surface of the decommissioned equipment with the composition. The method may comprise dissolving mercury sulphide with the composition.

[0050] The alkali metal sulphide may be selected from the group comprising lithium sulphide, sodium sulphide, potassium sulphide, rubidium sulphide, caesium sulphide and / or francium sulphide. Preferably the alkali metal sulphide may be sodium sulphide. The composition may comprise from about 5% to about 50% alkali metal sulphide by weight. The composition may comprise at least 5% alkali metal sulphide by weight. The composition may comprise at least 10% alkali metal sulphide by weight. The composition may comprise at least 15% alkali metal sulphide by weight. The composition may comprise at least 20% alkali metal sulphide by weight. The composition may comprise from about 5% to about 30% alkali metal sulphide by weight. The composition may comprise from about 10% to about 40% alkali metal sulphide by weight. The composition may comprise from about 10% to about 30% alkali metal sulphide by weight. The composition may comprise from about 10% to about 20% alkali metal sulphide by weight.

[0051] The composition may comprise at least one chelating agent. The at least one chelating agent may be selected from the group comprising ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), diethylenetriaminepentaacetic acid (DTPA), gluconic acid, poly amino polycarboxylic acid, and / or their salts. The at least one chelating agent may comprise sodium glucoheptonate. The composition may comprise at least 5% chelating agent by weight. The composition may comprise at least 7.5% chelating agent by weight. The composition may comprise at least 10% chelating agent by weight. The composition may comprise from about 5% to about 25% w / w chelating agent. The composition may comprise about 10% w / w chelating agent. The composition may comprise at least 5% sodium glucoheptonate by weight. The composition may comprise at least 7.5% sodium glucoheptonate by weight. The composition may comprise at least 10% sodium glucoheptonate by weight. The composition may comprise from about 5% to about 25% w / w sodium glucoheptonate. The composition may comprise about 10% w / w sodium glucoheptonate. The at least one chelating agent may maintain an aqueous phase equilibrium. The at least one chelating agent may keep the dissolved mercury ions in an aqueous solution. This may prevent or mitigate reprecipitation of mercury sulphide to a solid from the solution.

[0052] The composition may comprise a strong base. The strong base may be selected from the group comprising alkali hydroxide, potassium hydroxide, sodium hydroxide, lithium hydroxide, calcium hydroxide, barium hydroxide and / or strontium hydroxide.

[0053] The composition may comprise at least 5% w / w strong base. The composition may comprise at least 7.5% w / w strong base. The composition may comprise at least 10% w / w strong base. The composition may comprise at least 15% w / w strong base. The composition may comprise at least 20% w / w strong base. The composition may comprise from about 5% to about 25% w / w strong base. The composition may comprise from about 5% to about 20% w / w strong base. The composition may comprise from about 7.5% to about 15% w / w strong base. The composition may comprise about 10% w / w strong base. The composition may comprise 10% w / w strong base. Preferably the strong base is potassium hydroxide.

[0054] The composition may comprise at least 30% w / w water. The composition may comprise at least 40% w / w water. The composition may comprise at least 50% w / w water. The composition may comprise at least 60% w / w water. The composition may comprise at least 70% w / w water. The composition may comprise from about 30% to about 85% w / w water. The composition may comprise from about 40% to about 75% w / w water. The composition may comprise from about 50% to about 70% w / w water. The composition may comprise from about 65% to about 85% w / w water. The composition may comprise from about 70% to about 85% w / w water. The composition may comprise about 70% w / w water. The composition may comprise 70% w / w water.

[0055] The composition may comprise sodium sulphide, sodium glucoheptonate, potassium hydroxide and water. The composition may comprise from about 5% to about 15% w / w sodium sulphide, about 5% to about 15 % w / w sodium glucoheptonate, about 5% to about 15% w / w potassium hydroxide and about 55% to about 85% w / w water. The composition may comprise from about 5% to about 10% w / w sodium sulphide, about 5% to about 10% w / w sodium glucoheptonate, about 5% to about 10% w / w potassium hydroxide and about 70% to about 85% w / w water. The composition may comprise about 10% w / w sodium sulphide, about 10% w / w sodium glucoheptonate, about 10% w / w potassium hydroxide and about 70% w / w water. The composition may comprise 10% w / w sodium sulphide, 10% w / w sodium glucoheptonate, 10% w / w potassium hydroxide and 70% w / w water.

[0056] The method may comprise Contacting the mercury sulphide scale with the composition by pumping the composition in, over and / or through the decommissioned equipment. The method may comprise contacting the mercury sulphide scale with the composition at ambient temperature. The method may comprise contacting the mercury sulphide scale with the composition at a temperature range of between 0°C to 60°C. The method may comprise contacting the mercury sulphide scale with the composition at a temperature range of 10°C to 50°C. The method may comprise contacting the mercury sulphide scale with the composition at a temperature range of 10°C to 40°C. The method may comprise contacting the mercury sulphide scale with the composition at a temperature range of 20°C to 35°C. The method may comprise contacting the mercury sulphide scale with the composition at a temperature range of 20 to 30°C.

[0057] The method may comprise contacting the mercury sulphide scale with the composition by spraying the composition into and / or onto at least one contaminated surface. The at least one contaminated surface may be an interior and / or an exterior surface. The method may comprise contacting the mercury sulphide scale with the composition by spraying the composition into and / or onto at least one surface of a pipeline, pipeline equipment, processing equipment and / or storage vessel. The method may comprise contacting the mercury sulphide scale with the composition by spraying the composition onto contaminated surfaces such as the walls of storage tanks. The method may comprise applying a layer or film of the composition to a surface to be treated. The method may comprise applying a layer or film of the composition into and / or onto at least one contaminated surface. By applying a layer or film of the composition to a surface to be treated the volume of chemical required to treat the contaminated surface may be reduced.

[0058] The contaminated vessels and storage vessel / tanks may be present on oil and gas production platforms, floating production, storage and offloading vessels and refineries. The contaminated pipework, equipment, vessels and / or storage vessel / tanks may be required to be cleaned for continuation of processing or storage operations or may be required to be decontaminated for the purpose of decommissioning.

[0059] The method may comprise flushing out spent composition and / or dissolved mercury sulphide scale from the decommissioned equipment. The method may comprise pumping out spent composition and / or dissolved mercury sulphide scale from the decommissioned equipment. The decommissioned equipment may have previously been used in oil and gas operations.

[0060] The decommissioned equipment may be selected from pipeline, pipeline equipment, storage vessels and / or processing equipment. The pipeline may comprise surface and / or subsea pipeline. Embodiments of the third aspect of the invention may include one or more features of the first or second aspects of the invention or their embodiments, or vice versa.

[0061] According to a fourth aspect of the present invention there is provided a method of treating a contaminated surface to remove scale comprising mercury sulphide, the method comprising contacting scale with a composition comprising an alkali metal sulphide.

[0062] The method may comprise removing scale by substantially dissolving the scale using the composition. The method may comprise contacting scale on at least a part of a surface with the composition comprising an alkali metal sulphide. The contaminated surface may be a surface of a pipeline, pipeline equipment, processing equipment, and / or storage vessel previously or currently involved in oil and gas operations.

[0063] The method may comprise contacting at least a part of an internal and / or external surface of a pipeline with the composition. The method may comprise contacting at least a part of an internal and / or external surface of a piece of pipeline equipment with the composition.

[0064] The composition may comprise sodium sulphide, sodium glucoheptonate, potassium hydroxide and water. The composition may comprise from about 5% to about 15% w / w sodium sulphide, about 5% to about 15 % w / w sodium glucoheptonate, about 5% to about 15% w / w potassium hydroxide and about 55% to about 85% w / w water. The composition may comprise from about 5% to about 10% w / w sodium sulphide, about 5% to about 10% w / w sodium glucoheptonate, about 5% to about 10% w / w potassium hydroxide and about 70% to about 85% w / w water. The composition may comprise about 10% w / w sodium sulphide, about 10% w / w sodium glucoheptonate, about 10% w / w potassium hydroxide and about 70% w / w water. The composition may comprise 10% w / w sodium sulphide, 10% w / w sodium glucoheptonate, 10% w / w potassium hydroxide and 70% w / w water.

[0065] Embodiments of the fourth aspect of the invention may include one or more features of any of the first to third aspects of the invention or their embodiments, or vice versa.

[0066] According to a fifth aspect of the present invention there is provided a composition for the dissolution of mercury sulphide, the composition comprising an alkali metal sulphide. The alkali metal sulphide may be selected from the group comprising lithium sulphide, sodium sulphide, potassium sulphide, rubidium sulphide, caesium sulphide and / or francium sulphide. Preferably the alkali metal sulphide may be sodium sulphide.

[0067] The composition may comprise from about 5% to about 50% alkali metal sulphide by weight. The composition may comprise at least 5% alkali metal sulphide by weight. The composition may comprise at least 10% alkali metal sulphide by weight. The composition may comprise at least 15% alkali metal sulphide by weight. The composition may comprise at least 20% alkali metal sulphide by weight. The composition may comprise from about 5% to about 30% alkali metal sulphide by weight. The composition may comprise from about 10% to about 40% alkali metal sulphide by weight. The composition may comprise from about 10% to about 30% alkali metal sulphide by weight. The composition may comprise from about 10% to about 20% alkali metal sulphide by weight.

[0068] The composition may be an alkaline composition. The composition may have a pH greater than 7. The composition may have a pH in the range of about pH 8 to about pH 14. The composition may have a pH in the range of about pH 8 to about pH 14. The composition may have a pH in the range of about pH 9 to about pH 14. The composition may have a pH in the range of about pH 10 to about pH 14. The composition may have a pH in the range of about pH 10 to about pH 13. The composition may have a pH in the range of about pH 10 to about pH 12. The composition may have a pH in the range of pH 10 to pH 14. The pH of the composition may be imparted by the type and amount of components forming the composition. Suitable buffer systems may be used for adjusting and / or stabilizing the pH. The composition may be a liquid composition. The composition may be a solution. The composition may be an aqueous composition. The composition may be an aqueous solution.

[0069] The composition may comprise at least one chelating agent. The at least one chelating agent may be selected from the group comprising ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), diethylenetriaminepentaacetic acid (DTPA), gluconic acid, poly amino polycarboxylic acid, and / or their salts. The at least one chelating agent may comprise sodium glucoheptonate. The composition may comprise at least 5% chelating agent by weight. The composition may comprise at least 7.5% chelating agent by weight. The composition may comprise at least 10% chelating agent by weight. The composition may comprise from about 5% to about 25% w / w chelating agent. The composition may comprise about 10% w / w chelating agent. The composition may comprise at least 5% sodium glucoheptonate by weight. The composition may comprise at least 7.5% sodium glucoheptonate by weight. The composition may comprise at least 10% sodium glucoheptonate by weight. The composition may comprise from about 5% to about 25% w / w sodium glucoheptonate. The composition may comprise about 10% w / w sodium glucoheptonate. The at least one chelating agent may maintain an aqueous phase equilibrium. The at least one chelating agent may keep the dissolved mercury ions in an aqueous solution. This may prevent or mitigate reprecipitation of mercury sulphide to a solid from the solution.

[0070] The composition may comprise a strong base. The strong base may be selected from the group comprising alkali hydroxide, potassium hydroxide, sodium hydroxide, lithium hydroxide, calcium hydroxide, barium hydroxide and / or strontium hydroxide. The composition may comprise at least 5% w / w strong base. The composition may comprise at least 7.5% w / w strong base. The composition may comprise at least 10% w / w strong base. The composition may comprise at least 15% w / w strong base. The composition may comprise at least 20% w / w strong base. The composition may comprise from about 5% to about 25% w / w strong base. The composition may comprise from about 5% to about 20% w / w strong base. The composition may comprise from about 7.5% to about 15% w / w strong base. The composition may comprise about 10% w / w strong base. The composition may comprise 10% w / w strong base. Preferably the strong base is potassium hydroxide.

[0071] The composition may comprise at least 30% w / w water. The composition may comprise at least 40% w / w water. The composition may comprise at least 50% w / w water. The composition may comprise at least 60% w / w water. The composition may comprise at least 70% w / w water. The composition may comprise from about 30% to about 85% w / w water. The composition may comprise from about 40% to about 75% w / w water. The composition may comprise from about 50% to about 70% w / w water. The composition may comprise from about 65% to about 85% w / w water. The composition may comprise from about 70% to about 85% w / w water. The composition may comprise about 70% w / w water. The composition may comprise 70% w / w water. The composition may comprise at least one alkali metal sulphide, at least one chelating agent, at least one strong base and water. The composition may comprise sodium sulphide, sodium glucoheptonate, potassium hydroxide and water. The composition may comprise from about 5% to about 15% w / w sodium sulphide, about 5% to about 15 % w / w sodium glucoheptonate, about 5% to about 15% w / w potassium hydroxide and about 55% to about 85% w / w water. The composition may comprise from about 5% to about 10% w / w sodium sulphide, about 5% to about 10% w / w sodium glucoheptonate, about 5% to about 10% w / w potassium hydroxide and about 70% to about 85% w / w water. The composition may comprise about 10% w / w sodium sulphide, about 10% w / w sodium glucoheptonate, about 10% w / w potassium hydroxide and about 70% w / w water. The composition may comprise 10% w / w sodium sulphide, 10% w / w sodium glucoheptonate, 10% w / w potassium hydroxide and 70% w / w water

[0072] The composition may be configured to be compatible with metal pipelines, pipeline joints, pipeline equipment, processing equipment, and / or storage vessels such that the composition is not corrosive to pipelines, pipeline joints, pipeline equipment, processing equipment, and / or storage vessels and does not damage pipelines and / or pipeline equipment such as joints or seals.

[0073] The composition may be a treatment fluid. The mercury sulphide may be in the form of a scale. The scale may comprise mercury sulphide. The composition may be provided as a treatment for pipelines, decommissioned pipelines, surface pipelines, subsea pipelines, pipeline equipment, processing equipment, and / or storage vessels to dissolve and / or remove scale deposits comprising mercury sulphide. The scale deposits comprising mercury sulphide may be present on at least an internal and / or external surface of at least part of the pipelines, decommissioned pipelines, surface pipelines, subsea pipelines, pipeline equipment, processing equipment, and / or storage vessels.

[0074] Embodiments of the fifth aspect of the invention may include one or more features of any of the first to fourth aspects of the invention or their embodiments, or vice versa.

[0075] According to a sixth aspect of the present invention there is provided a composition for use in the removal of scale comprising mercury sulphide from a surface contaminated with the scale; the composition comprising an alkali metal sulphide. The composition may be configured to remove scale by substantially dissolving the scale. The contaminated surface may be a surface of a pipeline, pipeline equipment, processing equipment, and / or storage vessel previously or currently involved in oil and gas operations.

[0076] The surface may be at least a part of an internal and / or external surface. The surface may be at least a part of an internal and / or external surface of a pipeline, pipeline equipment, processing equipment, and / or storage vessel.

[0077] The pipeline, pipeline equipment, processing equipment, and / or storage vessel may be an active pipeline, pipeline equipment, processing equipment, and / or storage vessel. The pipeline, pipeline equipment, processing equipment, and / or storage vessel may be decontaminated by removing scale as part of a maintenance or cleaning operation. The pipeline, pipeline equipment, processing equipment, and / or storage vessel may be decommissioned. The pipeline, pipeline equipment, processing equipment, and / or storage vessel may be decontaminated by removing scale as part of a decommissioning operation.

[0078] The alkali metal sulphide may be selected from the group comprising lithium sulphide, sodium sulphide, potassium sulphide, rubidium sulphide, caesium sulphide and / or francium sulphide. Preferably the alkali metal sulphide may be sodium sulphide.

[0079] The composition may comprise from about 5% to about 50% alkali metal sulphide by weight. The composition may comprise at least 5% alkali metal sulphide by weight. The composition may comprise at least 10% alkali metal sulphide by weight. The composition may comprise at least 15% alkali metal sulphide by weight. The composition may comprise at least 20% alkali metal sulphide by weight. The composition may comprise from about 5% to about 30% alkali metal sulphide by weight. The composition may comprise from about 10% to about 40% alkali metal sulphide by weight. The composition may comprise from about 10% to about 30% alkali metal sulphide by weight. The composition may comprise from about 10% to about 20% alkali metal sulphide by weight.

[0080] The composition may be an alkaline composition. The composition may have a pH greater than 7. The composition may have a pH in the range of about pH 8 to about pH 14. The composition may have a pH in the range of about pH 8 to about pH 14. The composition may have a pH in the range of about pH 9 to about pH 14. The composition may have a pH in the range of about pH 10 to about pH 14. The composition may have a pH in the range of about pH 10 to about pH 13. The composition may have a pH in the range of about pH 10 to about pH 12. The composition may have a pH in the range of pH 10 to pH 14. The pH of the composition may be imparted by the type and amount of components forming the composition. Suitable buffer systems may be used for adjusting and / or stabilizing the pH. The composition may be a liquid composition. The composition may be a solution. The composition may be an aqueous composition. The composition may be an aqueous solution.

[0081] The composition may comprise at least one chelating agent. The at least one chelating agent may be selected from the group comprising ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), diethylenetriaminepentaacetic acid (DTPA), gluconic acid, poly amino polycarboxylic acid, and / or their salts. The at least one chelating agent may comprise sodium glucoheptonate. The at least one chelating agent may comprise potassium DTPA K5. The composition may comprise at least 5% chelating agent by weight. The composition may comprise at least 7.5% chelating agent by weight. The composition may comprise at least 10% chelating agent by weight. The composition may comprise from about 5% to about 25% w / w chelating agent. The composition may comprise about 10% w / w chelating agent. The composition may comprise at least 5% sodium glucoheptonate by weight. The composition may comprise at least 7.5% sodium glucoheptonate by weight. The composition may comprise at least 10% sodium glucoheptonate by weight. The composition may comprise from about 5% to about 25% w / w sodium glucoheptonate. The composition may comprise about 10% w / w sodium glucoheptonate. The at least one chelating agent may maintain an aqueous phase equilibrium. The at least one chelating agent may keep the dissolved mercury ions in an aqueous solution. This may prevent or mitigate reprecipitation of mercury sulphide to a solid from the solution.

[0082] The composition may comprise a strong base. The strong base may be selected from the group comprising alkali hydroxide, potassium hydroxide, sodium hydroxide, lithium hydroxide, calcium hydroxide, barium hydroxide and / or strontium hydroxide. The composition may comprise at least 5% w / w strong base. The composition may comprise at least 7.5% w / w strong base. The composition may comprise at least 10% w / w strong base. The composition may comprise at least 15% w / w strong base. The composition may comprise at least 20% w / w strong base. The composition may comprise from about 5% to about 25% w / w strong base. The composition may comprise from about 5% to about 20% w / w strong base. The composition may comprise from about 7.5% to about 15% w / w strong base. The composition may comprise about 10% w / w strong base. The composition may comprise 10% w / w strong base. Preferably the strong base is potassium hydroxide.

[0083] The composition may comprise at least 30% w / w water. The composition may comprise at least 40% w / w water. The composition may comprise at least 50% w / w water. The composition may comprise at least 60% w / w water. The composition may comprise at least 70% w / w water. The composition may comprise from about 30% to about 85% w / w water. The composition may comprise from about 40% to about 75% w / w water. The composition may comprise from about 50% to about 70% w / w water. The composition may comprise from about 65% to about 85% w / w water. The composition may comprise from about 70% to about 85% w / w water. The composition may comprise about 70% w / w water. The composition may comprise 70% w / w water.

[0084] The composition may comprise at least one alkali metal sulphide, at least one chelating agent, at least one strong base and water. The composition may comprise sodium sulphide, sodium glucoheptonate, potassium hydroxide and water. The composition may comprise 10% w / w sodium sulphide, 10% w / w sodium glucoheptonate, 10% w / w potassium hydroxide and 70% w / w water.

[0085] The composition may be configured to be compatible with metal pipelines, pipeline joints .pipeline equipment, processing equipment, and / or storage vessels such that the composition is not corrosive to pipelines, pipeline joints, pipeline equipment processing equipment, and / or storage vessels and does not damage joints or seals.

[0086] The composition may be a treatment fluid. The composition may be provided as a treatment for pipelines, decommissioned pipelines, surface pipelines, subsea pipelines, pipeline equipment, processing equipment, and / or storage vessels to dissolve and / or remove scale deposits comprising mercury sulphide. The scale deposits comprising mercury sulphide may be present on at least an internal and / or external surface of the pipeline, pipeline equipment, processing equipment, and / or storage vessels. Embodiments of the sixth aspect of the invention may include one or more features of any of the first to fifth aspects of the invention or their embodiments, or vice versa.

[0087] Brief description of the drawing

[0088] There will now be described, by way of example only, an embodiment of the invention with reference to the drawing, of which:

[0089] Figure 1 is a dissolution curve of mercury sulphide (as Cinnabar) in contact with a composition according to an embodiment of the invention.

[0090] Figure 2 is a flow chart showing steps for the treatment of spent waste solution according to an embodiment of the invention.

[0091] Figure 3 is a dissolution curve of mercury sulphide in contact with four different compositions (Compositions A, B, C and D) according to an embodiment of the invention. embodiments

[0092] The invention will be further illustrated by the following examples. The invention will be illustrated by means of tests made by the inventors which demonstrate the effectiveness of the process of removing mercury sulphide. This example is included to demonstrate the use of the compositions in accordance with embodiments of the present invention. Various modifications to the described example may be made within the scope of the invention.

[0093] Mercury (II) Sulphide (HgS) can take two mineral forms namely Cinnabar and Metacinnabar. Cinnabar is a red-coloured mineral consisting of hexagonal crystals with a density of 8.1g / cm3, whilst Metacinnabar consists of black cubic crystals with a density of 7.73g / cm3. Both forms of mercury sulphide are insoluble in water. In this example a mercury sulphide dissolver composition comprises 10% w / w sodium sulphide, 10% w / w sodium glucoheptonate, 10% w / w potassium hydroxide and 70% w / w water.

[0094] A 0.5 gram sample of Mercury (II) Sulphide (Cinnabar- AR Grade Reagent) powder was added to 100 ml of mercury sulphide dissolver composition and incubated at 20°C for 15 minutes. The mixture was shaken once at the beginning of the reaction and at five minute intervals. The dissolution of mercury sulphide was plotted in dissolution curve as shown in Figure 1.

[0095] Figure 1 shows a dissolution curve for mercury sulphide (as Cinnabar) in contact with mercury sulphide dissolver composition. The dissolution curve shows an initial rapid rate of dissolution of mercury sulphide with approximately 90% of the mercury sulphide being dissolved within 5 minutes of contact with the dissolver composition, with complete dissolution of the mercury sulphide within 10 minutes of contact with the dissolver composition. This composition had a dissolution capacity of 366g Mercury Sulphide / Litre of composition. The mercury sulphide dissolver composition is capable of dissolving both forms of the mercury sulphide (Metacinnabar and Cinnabar) very quickly.

[0096] Typically scale deposits occurring on steel surfaces of a pipeline form a thin layer of hard mineral scale that is estimated to be less than 1mm thick and has a high mercury sulphide content representing about 10% of the deposit’s mass. In an example pipeline 500 kilometres in length, the majority of the internal steel surface is covered with a protective coating that prevents the adherence of sulphide deposits. However, the large number of uncoated joints in the pipeline, where the steel surface is exposed, can amount to 4km in length and accumulate approximately 500 to 1000kg of mercury sulphide which needs to be removed. The mercury sulphide dissolver composition may be pumped into the pipeline for a short soak (contact) period of between 15 minutes and 3 hours at ambient temperature to completely dissolve the mercury sulphide deposit. The amount of soak (contact) time depends on the size, the contactability and wettability of the mercury sulphide crystalline deposits. The spent composition and dissolved mercury sulphide can be removed by pumping from the pipeline, leaving the pipeline free of mercury sulphide. In one embodiment, the dissolver volume required to bring about dissolution of the mercury sulphide present may be applied between two displacement pigs that are then displaced over the length of the pipeline in order to contact the internal pipe wall surface with dissolver whilst minimizing the volume of chemical required to treat the entire pipeline.

[0097] The presence of alkali metal hydroxide in the sulphide dissolver composition especially potassium is beneficial in cutting through any hydrocarbon coating the sulphide deposit in the pipeline.

[0098] The alkali metal hydroxide may be selected to be compatible with the material of the pipelines, pipeline joints, pipeline equipment, storage vessels, vessels and / or processing equipment being treated.

[0099] In some cases, after treatment a very thin coating of iron oxide rust and iron sulphide deposits coating may be present on an internal pipe surface or joint areas. These ironbased residues can be considered as innocuous minerals that provide no environmental hazard.

[0100] Example 2

[0101] A further example of the invention involves the dissolution of mercuric sulphide scale adhering to vertical walls of vessels and storage tanks by means of applying a mercury sulphide dissolver composition by spraying it onto the contaminated walls. In this example the walls may be wetted with a fine spray that relies only on wetting the mercuric sulphide scale to bring about its dissolution rather than any physical removal caused by the impingement of a high or ultra-high pressure liquid jet.

[0102] In a laboratory test a mild steel coupon cut from a spool piece was contaminated with mercuric sulphide in the form of meta-Cinnabar. A portable XRF analyser was used to determine the initial contamination of the surface with Mercury in its sulphide form. The initial surface contamination levels were found to be 550pg Hg / cm2. In this example the dissolver composition comprises a blend of Potassium Hydroxide (10%w / w), Sodium Glucoheptonate (10% w / w), Sodium Sulphide 60% flake (10% w / w) and water (70%w / w). The composition was sprayed via a plastic rotating nozzle onto the vertically mounted coupon positioned inside a sealed glass tank. The contamination levels following a spray period of 15 minutes at an ambient temperature ranging between 25-30°C during the course of the experiment, were found to fall by 97% and a further period of 45 minutes resulted in residual mercury levels of only 0.2% of the original mercury surface contamination levels. Following a period of 90 minutes spraying, the levels of mercury present on the coupon’s steel surface were below the detection limits of the portable XRF analyser.

[0103] In practice after the mercury sulphide has been dissolved the spent composition may be disposed safely. In the event that a disposal well is available, dissolved mercuric sulphide present in the form of a clear solution of sodium dithio mercurate (II) may be reinjected into the reservoir formation from where it originated. In the event that the disposal of mercury needs to be in the form of a mercury solid, the preferred mercury salt for disposal is mercuric sulphide due to its extremely low solubility and due to the fact that it does not produce mercury vapour.

[0104] As shown in Figure 2 spent dissolver solution containing dissolved mercuric sulphide in the form of Sodium dithio mercurate (II) may be treated with hydrogen peroxide solution which results in the oxidation of sodium sulphide present both as part of the sodium dithio mercurate aqueous ion and in the dissolver blend as unreacted sodium sulphide to form sodium sulphate. As sodium sulphate is formed the dissolved mercuric sulphide falls out of solution as a black insoluble precipitate of mercuric sulphide (HgS) in the form of metacinnabar which can be isolated for disposal as a solid by filtration.

[0105] Further Examples

[0106] Four formulations (A to D) containing similar amounts of component in terms of weight percent were compared for their effectiveness in dissolving mercuric sulphide deposited as a thin layer (<0.5mm) of black meta-cinnabar on a steel spool piece. The composition of the four formulations A to D are shown in Table 1, below. The spool pieces were immersed in the different solutions at 20°C under static conditions and the mercury sulphide content of the surface analysed at one hour intervals using an XRF (X-ray fluorescence) analyser.

[0107]

[0108] Table 1 : Compositions A to D.

[0109] Figure 3 shows a dissolution curve for mercury sulphide in contact with each of the mercury sulphide dissolver compositions A to D.

[0110] Composition A was found to be the overall most effective formulation for dissolving (removing) the mercuric sulphide. The substitution of potassium hydroxide with sodium hydroxide in Composition B resulted in a slightly slower dissolution rate which may have resulted from the slightly oil wet mercuric sulphide scale that had some hydrocarbon material associated with it, the potassium hydroxide being more effective at getting through the hydrocarbon under the experimental conditions than sodium hydroxide. The substitution of sodium sulphide for potassium sulphide in Composition C didn’t appear to make a significant difference to the dissolution rate but potassium sulphide is expensive compared with sodium sulphide and not readily available in bulk quantities. In Composition D the chelating agent used was penta potassium DTPA (DTPAKs) instead of sodium glucoheptonate. The initial dissolution rate using DTPAKs was similar to Composition A however the stability constant of DTPA under highly alkaline conditions is not as high as sodium glucoheptonate especially for divalent transition metal ions in the presence of hydroxyl ions and not only is the rate of dissolution and sequestration of mercury ions lower but some mercuric sulphide was seen to reprecipitate after about 6 hours which was also observed when no chelating agent was present. A further advantage of using sodium glucoheptonate is that it is readily biodegradable.

[0111] It will be appreciated that other alkali metal sulphide types may be used in place of sodium sulphide or potassium sulphide including lithium sulphide, rubidium sulphide, caesium sulphide and / or francium sulphide. These alkali metal sulphide compositions may all function as mercury sulphide dissolver compositions. It will be appreciated that other chelating agents may be used in place of penta potassium DTPA (DTPAKs) or sodium glucoheptonate. It will be appreciated that other strong base types may be used instead of potassium hydroxide or sodium hydroxide. It will be appreciated that the relative amounts of the components may be different in other formulations.

[0112] Other composition examples may omit the presence of the chelating agent. The presence of the chelating agent in the composition may keep the dissolved mercury ions in an aqueous solution and / or prevent dissolved mercury sulphide from reprecipitating from the solution. However, a composition without the presence of the chelating agent may function to dissolve the mercury sulphide.

[0113] The term “about” may modify a value disclosed or claimed in the present disclosure. In such instances, “about” may signify that the value it modifies is within ±0%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5% or 10% of the value. It is also to be understood that any ranges and subranges relied upon in describing various embodiments of the present invention may independently and collectively fall within the scope of the appended claims, and are understood to describe and contemplate all ranges including whole and / or fractional values therein, even if such values are not expressly written herein.

[0114] While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated.

[0115] An embodiment of the invention may provide a treatment for pipelines to remove scale deposits comprising mercury sulphide. The treatment may comprise contacting the mercury sulphide scale with a composition comprising alkali metal sulphide at ambient temperature to dissolve the mercury sulphide scale.

[0116] The composition according to an embodiment of the invention can quickly and efficiently dissolve mercury sulphide. As a result, the treatment composition can be used in field applications in decommissioned pipelines to remove harmful mercury sulphide scale. The composition has the advantage over physical removal techniques in that it can work remotely, safely and is able to access the smallest most restricted inaccessible joint areas of a pipeline and pipeline equipment.

[0117] The composition according to an embodiment of the invention has the advantage that it may be capable of quickly dissolving the mercury sulphide deposit under alkaline conditions and at ambient conditions of temperature quickly and without the generation of toxic by-products such as hydrogen sulphide gas. The highly alkaline dissolver prevents the liberation of hydrogen sulphide gas and water wets any oil-wet sulphide mineral deposits facilitating efficient and safe decontamination.

[0118] Throughout the specification, unless the context demands otherwise, the terms 'comprise' or 'include', or variations such as 'comprises' or 'comprising', 'includes' or 'including' will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.

[0119] The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The described embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilise the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, further modifications or improvements may be incorporated without departing from the scope of the invention.

Claims

Claims1. A method of dissolving scale wherein the scale comprises mercury sulphide, the method comprising providing a composition comprising an alkali metal sulphide; and contacting the scale with the composition.

2. The method according to claim 1 wherein the alkali metal sulphide is selected from the group comprising lithium sulphide, sodium sulphide, potassium sulphide, rubidium sulphide, caesium sulphide and / or francium sulphide.

3. The method according to claim 1 or claim 2 wherein the composition is an aqueous composition.

4. The method according to any preceding claim wherein the composition comprises from about 10% to about 20% alkali metal sulphide by weight.

5. The method according to any preceding claim wherein the composition comprises at least one chelating agent selected from the group comprising ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), diethylenetriaminepentaacetic acid (DTPA), gluconic acid, sodium glucoheptonate, poly amino polycarboxylic acids and / or any of their salts.

6. The method according to any preceding claim wherein the composition comprises a strong base selected from the group comprising alkali hydroxide, potassium hydroxide, sodium hydroxide, lithium hydroxide, calcium hydroxide, barium hydroxide and / or strontium hydroxide.

7. The method according to any preceding claim wherein the composition comprises 10% w / w sodium sulphide, 10% w / w sodium glucoheptonate, 10% w / w potassium hydroxide and 70% w / w water.

8. The method according to any preceding claim wherein the composition has a dissolution capacity of about 0.1kg to about 0.4kg mercury sulphide / litre of composition.

9. The method according to any preceding claim comprising contacting the mercury sulphide scale with the composition for a period of between 15 minutes and 3 hours.

10. The method according to any preceding claim wherein the composition is configured to treat pipelines, pipeline joints, pipeline equipment, vessels, storage vessels and / or processing equipment involved in oil and gas operations.

11. The method according to any preceding claim comprising removing the scale comprising mercury sulphide from a contaminated surface wherein the contaminated surface is a surface of a pipeline, pipeline equipment, processing equipment, and / or storage vessel previously or currently involved in oil and gas operations.

12. The method according to claim 11 comprising removing scale from the pipeline, pipeline equipment, processing equipment and / or storage vessel as part of a maintenance, cleaning or decommissioning operation.

13. The method according to claim 11 or claim 12 comprising contacting a surface of a pipeline, pipeline equipment, processing equipment and / or storage vessel contaminated with mercury sulphide scale with the composition by passing the composition in, through and / or over the pipeline, pipeline equipment, processing equipment and / or storage vessel.

14. The method according to any of claim 11 to 13 comprising contacting the mercury sulphide scale with the composition by pumping and / or spraying the composition into, onto and / or through at least one surface of the pipeline, pipeline equipment, processing equipment and / or storage vessel.

15. The method according to any of claim 11 to 14 comprising pumping out spent composition and dissolved mercury sulphide scale from the pipeline, pipeline equipment, processing equipment and / or storage vessel to dispose of the spent composition and dissolved mercury sulphide scale.

16. A method of treating a contaminated surface to remove scale comprising mercury sulphide, the method comprising contacting scale on at least a part of a surface with a composition comprising an alkali metal sulphide.

17. The method according to claim 16 wherein the contaminated surface is a surface of a pipeline, pipeline equipment, processing equipment, and / or storage vessel previously or currently involved in oil and gas operations.

18. The method according to claim 16 or 17 comprising contacting at least a part of an internal and / or external surface of a piece of pipeline equipment or pipeline with the composition.

19. A composition for the dissolution of scale wherein the scale comprises mercury sulphide, the composition comprising an alkali metal sulphide.

20. The composition according to claim 19 wherein the alkali metal sulphide is selected from the group comprising lithium sulphide, sodium sulphide, potassium sulphide, rubidium sulphide, caesium sulphide and / or francium sulphide.

21. The composition according to claim 19 or claim 20 comprising from about 10% to about 20% alkali metal sulphide by weight.

22. The composition according to any of claims 19 to 21 comprises at least one chelating agent selected from the group comprising ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), diethylenetriaminepentaacetic acid (DTPA), gluconic acid, sodium glucoheptonate, poly amino polycarboxylic acid, and / or their salts.

23. The composition according to any of claims 19 to 22 comprises a strong base selected from the group comprising alkali hydroxide, potassium hydroxide, sodium hydroxide, lithium hydroxide, calcium hydroxide, barium hydroxide and / or strontium hydroxide.

24. The composition according to any of claims 19 to 23 comprises 10% w / w sodium sulphide, 10% w / w sodium glucoheptonate, 10% w / w potassium hydroxide and 70% w / w water.

25. The composition according to any of claims 19 to 24 wherein the composition is a treatment for pipelines, decommissioned pipelines, surface pipelines, subsea pipelines, pipeline equipment, processing equipment, and / or storage vessels to dissolve and / or remove scale deposits comprising mercury sulphide.

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