Formaldehyde as an Inhibitor of Corrosion Caused by Hydrogen Sulphide

Abstract

Abstract This paper discusses the results of an investigation made to develop a methodof combatting corrosion of subsurface oil-well equipment caused by brinescontaining hydrogen sulphide. Carbon monoxide, aldehydes, and certain cyanidecompounds were found by laboratory tests to effectively inhibit corrosion ofsteel by sulphide brines. Experiments were made for one year in eight wells inWest Texas fields using formaldehyde as an inhibitor. It was found that theinhibitor treatment successfully prevented the severe corrosion previouslyexperienced in these wells, and effected important net savings in operatingcosts. Introduction Corrosion of oil-field equipment has long been a problem in areas producingsour crudes. These areas include the major fields of Arkansas, Western Kansas, West Texas and New Mexico. Corrosion in these fields is caused primarily by thechemical action of hydrogen sulphide in the presence of water and by electriccurrents entering the wells through flow lines. Hydrogen sulphide isnon-corrosive to iron or steel in the absence of water but under the usualconditions prevailing in oil wells where more or less water is present, hydrogen sulphide is very corrosive. This type of corrosion occurs over the pHrange below 9, which includes all natural oil-field brines. If air is presentin addition to hydrogen sulphide and water, corrosion proceeds at an extremelyrapid rate. It was reported several years ago that a string of oil-well tubingwas literally dissolved within 50 days where an air lift was used on a wellproducing considerable water and hydrogen sulphide. Air is generally absent inwells producing sour crude. After hydrogen sulphide corrosion starts, it proceeds with increasing severityand its correction becomes progressively more difficult. This characteristic ofsulphide corrosion has been attributed to the formation of galvanic couplesbetween the iron and steel equipment on one hand and the products of corrosionon the other. Severe corrosion of subsurface oil-well equipment leads to a multitude oftroubles. In addition to destruction of costly equipment, producing sands oftenare damaged by flooding with water leaking through corroded casing; badmechanical conditions may develop in a well due to obstructions caused bycorroded equipment lost in the hole, or partly collapsed casing; pumpingoperations are interfered with, and severe emulsification problems may resultfrom the homogenizing action of corroded balls and seats. A typical case of severe corrosion caused by hydrogen sulphide was reported inHoward County, Texas, where it was necessary to pull tubing once a week torepair leaks after the tubing had been in use for only one year. On one well inthis area a string of tubing was pulled II times in 8 months; II joints oftubing being replaced because of leaks. T.P. 1970