Hydrogen sulphide scavenging method

Abstract

A process for scavenging hydrogen sulphide from a mixed phase system, in particular a hydrocarbon fluid having a liquid phase and a gaseous phase, is described. The hydrocarbon fluid contains a high level of hydrogen sulphide, which can be above 5000 ppm by weight in the gaseous phase of the fluid prior to treatment. Formaldehyde, preferably water, with is added to the fluid. The process is particularly useful for in-line scavenging of hydrogen sulphide from sour crude oil containing very high hydrogen sulphide levels. The hydrogen sulphide content of the crude oil that is delivered to the platform is reduced to safe and commercially acceptable levels and solid by-product formation is insignificant. This process thus advantageously provides a low cost manner of developing sour oil fields that would otherwise not be safe or economically viable to develop or advantageously provides a way of modifying existing processes of handling sour hydrocarbon fluids, for example, within oil fields already in production, such that the method according to the present invention replaces existing methods adopted.

1 . A method for reducing the amount of hydrogen sulphide in a multiphase hydrocarbon fluid produced fluid prior to phase separation and processing, the method comprising the step of adding formaldehyde to the produced fluid, which produced fluid has, prior to the addition of formaldehyde, a concentration of hydrogen sulphide of at least 250 ppm by weight of the fluid.  
     
     
         2 . A method according to  claim 1 , wherein the produced fluid comprises liquid and gaseous hydrocarbon phases with or without water.  
     
     
         3 . A method according to  claim 1  or  2 , wherein the formaldehyde is introduced as formalin in-line into the multiphase system.  
     
     
         4 . A method according to  claim 3 , wherein the formaldehyde is introduced to the hydrocarbon fluid at a ratio by weight of formaldehyde (expressed as 37% formalin) to hydrogen sulphide of from 2:1 to 8:1.  
     
     
         5 . A method according to any preceding claim, wherein the multiphase system flows through a sub-sea flowline.  
     
     
         6 . A method according to any one of  claims 1  to  4 , wherein the multiphase system flows through a multiphase system flows through an on-shore pipeline.  
     
     
         7 . A method according to  claim 5  or  6 , wherein the formaldehyde is added at a location which provides a contact time of at least 20 minutes.  
     
     
         8 . A method according to  claim 7 , wherein the location provides a contact time of from 30 to 60 minutes.  
     
     
         9 . A method according to any preceding claim, wherein the method comprises the additional step of adding water to the hydrocarbon fluid.  
     
     
         10 . A method according to  claim 9 , wherein the hydrocarbon fluid is substantially free of water prior to the water addition step.  
     
     
         11 . A method according to  claim 9  or  10 , wherein an amount of waver of at least 5% by volume and preferably not more than 10% by volume is added to the hydrocarbon fluid.  
     
     
         12 . A method according to any preceding claim wherein the water is added at substantially the same time as the formaldehyde.  
     
     
         13 . A method according to any of  claims 2  to  10 , which is applied to a said multiphase system which comprises carbon dioxide.  
     
     
         14 . A method according to any preceding claim, which is carried out at a temperature of 60-75° C. and/or at a pressure of at least 20 bar.