Oil-well Casing Corrosiont R H GOODNIGHT*AND J P BARRETT*
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ABSTRACT Information 1s presented to outl~nethe major causes of both ~nternaland external corroslon and the poss~ble damage that could occur as a result of thrs act1011 The use of a volat~lelnhlb~torto control ~nternalcorroslon 1s proposed In those cases where ~t 1s ~mpract~cal to use a packer to seal the tub~ng-cas~ng aiinulus The use of ~nternallycoated caslng may become effect~vewith the development of niore su~table coat~ng materials and methods of appl~cation.Control of external corros.on on well caslng caused by bacterial a c t ~ v ~ t yelectrolyt~c . effects, or a c ~ dwater attack, by cathod~cproteet~on1s proposcd Protect~on from external corroslon on new wells by the use of coat~ngsplus cathod~c~ ~ r o t e - t ~1s on suggested.
INTRODUCTION There have been widely separated views on the matter of corrosion prevention 3s l~ractlced by tlle pipeline and procluction companies Pipeline pzoi11e generally coat their lines, wrap them, and even give them extra mechanical protection in rocky country Cathocl~c-protect1011systems are installeel and frequent inspections are made to clctei.n~ineif corrosion is being controlled All of this is on lines which can be dug u p and repaired a t a relatively low cost On the other hand, production people have installeel thousands of nliles of bare vertical pipel ~ n e s No protection llas been supplied against external corrosion, and in illany cases, no measures are taken to prevent internal chen~ical attacli Casing has failed rapldly in illany areas a s a result of either internal 01 external corrosion Casing strings in a nunlber of fields have been perforated by internal attack in a s little a s six years, whereas in other fields, fallures have resulted fro111 external attack in a like p?rlod. Typically, a number of wells in the same fielcl are affected Repairs of caslng failures are extremely costly and tests for detesmlning corrosion of casing a r e not well-clevelopecl The 011 ~ndustry'sinvestlnent in oil-well casing is well over $5 billion The yearly increase in this \
*Pan Amerlcan Petroleum Carp. Tulsa, Okla ?Presented at the spring meetlng of the Southweztern D.str,et, D ~ v l s ~ oof n P r u d ~ . c t l o n . Fort Worth. Texas, March 1956
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investment is over $85 million The cost of corroslon-mitigation measures .becomes relatively cheap when one considers t h a t uncontrollecl casing corrosion results not only in the loss of the casing but also in tlle interruption of procluction, and map-result in loss of the well, or even llernlanent damage to the reservoir.
INTERNAL CORROSION Although internal corroslon of the casing surfaces can be caused by carbon dioxide, l~ydrogen sulfide, and organic acids known to be responsible for tubing corrosion, production techniques i~lodifythe opportunity for attack In general, the nlain cause of lnternal attack is hydrogen sulfide The general mechanism f o r this attack may be expressecl a s follows H,S + Fe + FeS\ + 2H ~Glro~en Sulfide + Iron (yields in the Iron Sulfides presence of and water) Hyclrogen As this react1011is primarily one of acid attack, carbon dioxide, when present, accelerates the reaction by increasing the total acid present. The iron sulfide forilled sets up a galvanic cell in which tlle steel pipe becomes the anode This reaction is generally assumed to be responsible for the deep irregular pitting observed in sulfide COrroSlo11 Fig 1shows a sectlon of casing renloved from a west Texas sour well after only six years' exposure It is significant t h a t although the Inside of the casing was severely corroded, tlle external surfaces of tlle tubing were virtually unattacked This specinlen was in the upper part of the string above the fluid level. The mechanisnz of attack by hydrogen sulfide is a s follows. In the absence of a paclter, the casing-tubing annulus is exposed to the reservoir gases, which in this case contain hydrogen sulfide. The gas is saturated with water a t reservoir conditioils As tlle gas diffuses up the hole, t h e water condenses on the surface of the casing a t
Causes