Inhibiting corrosion

Patented Aug. 26, 1647

2,426,31$

I UNITED STATES PATENT OFFICE

2,426,318

INHIBITING CORROSION ^TU£^OMa ’ ^ass|8Bor to Stanolind Oil and Gas Company, Tulsa, Okla., a corporation of Delaware

No Drawing. Application November 15, 1945,

Serial No. 628,678

Claims.

This Invention relates to a method of inhibiting the corrosive action of natural gas and oil containing soluble sulfides. More particularly, the invention relates to means for inhibiting the rapid corrosion of well tubing and/or pump rods which has heretofore occurred in producing wells where the well fluid contains both brine and hydrogen sulfide.

Crude oil containing soluble sulfides is produced in the major producing areas of the United States, and corrosion of oil field equipment is especially severe in areas where brines are produced with sour crudes. These corrosive sulfide brines include alkali sulfides, alkaline earth metal sulfides, and/or acid sulfides such as hydrogen sulfide. Hydrogen sulfide is very soluble in water and brine, causing the brines to become very corrosive to iron and steel equipment due to the active formation of iron sulfide. Hydrogen sulfide corrosion will take place in both acidic sulfide brines and in alkaline sulfide brines with pH below 9. This range embraces virtually all natural brines, since most sulfide brines are acidic within the range of about pH 4.5 to pH 7, those of pH above 7 being uncommon.

In some areas, the production is so corrosive that, after the tubing has been in use for a relatively short time, it has been necessary to pull the tubing once a week to inspect for failure. In one case, it was necessary to replace eleven joints in one string of tubing within eight months because of leaks; in another, a new string of pump rods began parting after thirty-six days of service. It will be apparent, therefore, that the cost of replacing equipment made useless by corrosion and the cost of labor Involved are major factors m the future economic operation of many oil fields producing sour crude and brine.

One object of this invention, therefore, Is to provide a means for inhibiting the attack of ferrous metals by sour crudes containing brine. Another object is that of providing a class of materials for preventing chemical attack of metals by well fluids which is effective in both its liquid and vapor phases. A further object of this invention is to provide an effective inhibitor for use in treating produced well fluids before they enter the well tubing. Still another object is to provide a protective film on metal surfaces. The above and other objects of my invention will become apparent to those skilled in the art as the description thereof proceeds.

For many years generally ineffective efforts have been made by equipment manufacturers &nd corrosion experts to develop materials capa(Cl. 252—8.55) ble of withstanding hydrogen sulfide corrosion of the type to which this invention is directed. Various steel alloy tubings were tried under these corrosive conditions without apparent success, 5 since the additional cost of the alloy did not give a corresponding additional length of service. Likewise, coatings of paint and synthetic rubber have failed to prevent metal corrosion when the equipment was put in field use. Zinc plates have 10 been installed on pump rods and in treaters as a form of cathodic protection, but this has also failed to prevent damage from sulfide corrosion. Chemical treatment of the well fluids with sodium hydroxide, tannin, sodium silicate and the like 15 in the attempted neutralization of the hydrogen sulfide has been tried in wells without inhibiting this type of corrosion.

In carrying out the present invention, an inhibiting substance, as, for example, an aldehyde, 20 and preferably formaldehyde, is introduced into the producing well, for example between the casing and tubing. Various methods of introducing the inhibitor may be used, but in each case the inhibitor is supplied to the fluids being produced 25 and is carried upwardly through the tubing.

There are many examples of threshold treatments in which organic agents react to produce traces of new compounds, only a relatively minor proportion of the original agents reacting. This 30 appears to be the case with formaldehyde and hydrogen sulfide in brines since the major portion of the hydrogen sulfide content is unaffected. Without being limited to any theory of operation, it appears that by the slow reaction of formalde35 fr/de with a component of the well fluids, a new compound is produced which in some manner inhibits corrosion. Thus the formaldehyde apparently reacts with a portion of the hydrogen sulfide in the presence of the brine to produce a thin 40 protective film of a reaction product on the exposed metal surfaces. A protective film is thereby maintained on the exposed surface of the well equipment, and this film apparently acts to prevent the corrosion by the brine and hydrogen sul45 fide while the normally corrosive fluids are produced.

The phenomenon was studied in the laboratory, and when formaldehyde was added to acidic hydrogen sulfide brines there appeared to be vis50 ible signs of chemical reaction. The clear brine became opalescent and a very small amount of insoluble precipitate formed, and the reaction drifted slightly toward neutral. The formaldehyde and hydrogen sulfide reacted, but only a 55 small portion of the dissolved hydrogen sulfide,

14,000 1,600

500 3,000 26,000 1,000 1,000 6 .parts per million— ____________do— _____________do---_____________do---_____________do_— _____________do____ _____________do____ since the brine still showed a high content of hydrogen sulfide following the treatment. Chemical reactions are known which involve the combination of formaldehyde with hydrogen sulfide to form mercaptan-like compounds, but when preformed mercaptans, carbon bisulfide, and the like were added to hydrogen sulfide brines they did not act as inhibitors of the hydrogen sulfide corrosion. .

Panels of mild steel were immersed in sulfide brine of the North Cowden Field, Ector County, Texas, which showed a corrosion loss of 0.92 pound per square foot per year. Chemical ananlyses of several brine samples from this field showed it to average 40,000 to 60,000 P. P. M. total dissolved solids. A typical analysis showed the following: Sodium-----Calcium-----Magnesium— Sulfate------Chlorine----Bicarbonate— Sulfide-----pH value----The various samples of brine from this field were found to be acidic, having a range of between about pH 4.5 and about 6.8. The gas produced contains about 0.5% carbon dioxide and between about 5% and about 8% hydrogen sulfide, which indicates that the brine is nearly saturated with hydrogen sulfide at bottom hole conditions.

Panels of mild steel were cleaned of mill scale and smoothed with the finest grade of emery cloth. These panels were dried and weighed and then suspended in the sulfide brine on glass hooks. After immersion for a known time, the panel was removed from the brine, washed quickly in a 1% hydrochloric acid solution, wiped free of corrosion products with a soft cloth, washed with distilled water, dipped in alcohol, dried rapidly and reweighed. The panels that had been immersed in the formaldehyde-treated brine retained a tarnished-like appearance after being cleaned with dilute hydrochloric acid solution. The weight loss was assumed to be due to hydrogen sulfide corrosion.

In the above laboratory tests, it was established that when formaldehyde was added to the brine in concentrations of 125 P. P. M. and 250 P. P. M., the corrosion loss was found to be 0.15 and 0.11 pound per square foot per year, respectively.

The invention has been used extensively m field tests, 'and for these tests oil wells where the subsurface equipment had shown severe corrosion conditions were chosen in three different oil fields. Although the laboratory tests indicated excellent inhibition of sulfide corrosion, the field results were even better. Technical grade formaldehyde, sometime referred to as formalin which consists of a 40% aqueous solution of formaldehyde, was injected down the casing at a rate of between about 1 and about 2 quarts per day with the wells being produced through the tubing. These trials were continued over extended periods, after which the pump rods and tubing of several wells in each field were pulled and inspected.

Various methods were tried for injecting formaldehyde into the annulus and it was found that a lubricator or drip system was the most satisfactory of any of the methods tried. Thus,

2,436,318 for example, formaldehyde can be stored in a tank or drum of suitable capacity supported at a sufficient height to permit gravity drainage of the formaldehyde into a small measuring container, usually of 2-quart capacity. In the case of wells requiring relatively large amounts of formaldehyde the 55-gallon formaldehyde container may conveniently serve as the storage container, in which case piping connections are made to the bottom end of the drum. When a well produces a small volume of water, quite small treatments of formaldehyde are used, and it is sometimes desirable to dilute the formaldehyde with an equal volume or more of water to facilitate measuring and injecting the chemical. In making the treatment, the measuring container is filled with formaldehyde from the storage drum and then permitted to drain from the measuring container into the annulus between tubing and casing. The batch injection of formaldehyde daily does not adversely affect the efficiency of the treatment, particularly in wells where the fluid level is substantially above the pump suction, and generally offers advantages over continuous injection as regards cost and maintenance of injection equipment. However, when the well is subject to being pumped off, continuous introduction of chemical is preferred.

> Prior to these field tests, well Number 1 has shown severe corrosion with perforations of the tubing and corrosion of the pump, rods, balls, and seats. New tubing and rods were run in this well just prior to the test, and injection of formaldehyde down the casing was begun and continued for five months, after which the rods, pump, and tubing were pulled for inspection. The examination of the tubing showed no evidence of corrosion on the outside as far down as the upper perforations in the tail pipe below the pump. The interior of the tubing examined at the joints showed no evidence of corrosion; neither were the threads affected. The pump, balls and seats were found in good condition. It appears that the outside of the tubing was protected as far as the top perforations below the pump and the inside of the tubing and the pump rods were protected as the well was. produced. As the inhibitor did not fall below the top perforations, the lower part of the tail pipe was not protected and suffered severe corrosion. In fact, the perforated tail pipe below the pump was found to be corroded paper thin in extensive areas. This well produced an average of 30 barrels of sulfide brine per day and had received 150 quarts of formaldehyde during the five months’ test at a cost for formaldehyde of $12.00. Previously, it had been necessary to replace the balls and seats frequently and sections of the 2000-foot tubing after a year’s service. Furthermore, it had been necessary to pull the tubing periodically to determine whether or not replacement was yet necessary.

Well Number 2 was subject to frequent rod breaks, there having been 33 rod breaks in the ; seventeen months preceding the test. Likewise, there had been severe ball and seat corrosion. The well produces 45 barrels of water per day. Just prior to the test, 500 feet of new rods were run in at between the 1100 and 1600 feet level, the section where the most frequent rod breaks had occurred. The injection of formaldehyde down the casing was then begun and continued for five months. At the end of the test, these rods were pulled, and when the 500-foot section of new rods, the ball, seat, and pump were inspected for

2,426,318 •j ® evidence of corrosion no evidence of corrosion could be detected. The monthly cost of protection during the test was only a small fraction of the monthly cost of the thirty-three pulling jobs and replacements during the seventeen months preceding the test.

A third well producing 10 barrels of water per day had prior to the use of the inhibitor required frequent pulling and the well had not operated ionger than eight weeks between pulling jobs when balls and seats and joints of tubing and rods were replaced. During the five months’ test one quart °f formaldehyde was injected daily through a half-meh tube extending to the fluid level in the annulus between the tubing and casing As a part of the test, inexpensive line pipe had been run for tubing and used pump rods installed. During the test the well did not require pulling and, in addition, a troublesome emulsion production had been overcome. The rods and pump were pulled at the end of the test, but they did not show any active corrosion.

These foregoing examples offer conclusive evidence that the injection of an aldehyde inhibits sulfide corrosion of subsurface equipment of oil wells and that there is great economic advantage in the treatment.

More than 10 P. P. M. of aldehyde* preferably more than about 50 P. p. M. of aldehyde based upon the brine produced, is ordinarily required , for protection. Ordinarily not more than 100 ' B J?^{e required}· Typically about one quart of U. S. P. formaldehyde (formalin) per 100 barrels of water produced gives adequate inhibitions. Further beneficial results are obtained in some cases by employing larger proportions initially and decreasing with time the proportion of inhibitor used in subsequent treatments.

It is contemplated also that the treatment may be applied to crude and product pipe lines, refining equipment, and storage tanks where attack can be expected. The sour hydrocarbons in pipe lines and storage tanks usually carry only a small Percentage of water and a proportionally small amount of inhibitor may be effective. In the case °i^St°^{rage tanks}’ the inhibitor may be introduced into the vapor space and, in general, a vaporizable material should be used. In some instances it may be beneficial to apply the chemical agent to the surface in a carrier-coating.

The foregoing description has been with particular reference to the use of formaldehyde which °^{ne exampIe of} the class of materials which come within this invention. The invention however, contemplates generally the use of any inhibitor which has the characteristic aldehyde ο K-C^ where R represents any hydrocarbon radical or materials producing aldehydes under conditions existing within a producing well, for example hexamethylenetetramine. It is to be understood however, that water-soluble aliphatic aldehydes S ^a class are preferred, and more particularly those aldehydes having four or less carbon atoms ° ± ^{1716 aUphatic} aldehydes possess unusual characteristics and are selected since they constitute a class of compounds having _a^b°^ilin.^{g points} that make it possible to select the chemical agent having the desired properties under the particular conditions of the treatment. In some instances the materials form lower g

^azi^eoiropes with the produced well fluids which aid m the distribution of the chemical treating agent throughout the well. Specifically tests show that under conditions in which formaldehyde is an effective inhibitor, acetaldehyde in an equimolecular concentration is 80% as effective HA_n^C„°¹??t^ttL^{ng at}tack of hydrogen sulfide. Be.nzaldehyde on the other hand, was found to be only about 25% as effective as formaldehyde.

It is contemplated that the aldehydes may exist ^{fOrm such as par}aformaldehyde, ^lnhibiting power of these polymerized aldehydes depending chiefly upon their solubility in toe bnne The inclusion of a small proportion of methanol with the formaldehyde stabiu^g it against polymerization and maintains the inhibiting effect of formaldehyde.

This application is a continuation-in-part of my copendmg application Serial Number 570,800, filed December 30, 1944.

The preferred embodiments of my invention described herein are for the purpose of illustration oifiy and are not intended to limit the scope ^deflned by ^th® appended claims, since it is apparent that the principles of my SleSt^V^{6 aPPUed generally by} those skilled in the art in view of the foregoing descrip-