Monitoring Short-Term Corrosion Rates in Some Oilwell Cements
Authors: Harari Zaki Y. 
Published: December 2007

Summary
This paper discusses causes of oilwell external-casing corrosion and thecorrosion-inhibiting properties of Portland cement. API Class G and ASTM Type Icements were used in a laboratory experiment to assess their short-termcorrosion-inhibiting properties. Synthetic carbonate formation brine anddistilled water were used as the corrodents. The instantaneous and averagecorrosion rates of mild steel electrodes embedded in cement were measured asfunctions of time. The tests were conducted with different combinations ofcement type, corrodent type, and temperature. The results showed that Type 1cement is more effective than the more commonly used Class G oilwell cement forshort-term corrosion inhibition. Short-term corrosion inhibition may lead tobetter long-term protection. protection.

Introduction
One purpose of a cementing job is to protect external casing from theadverse effects protect external casing from the adverse effects of corrosiveformation brines. Despite exhaustive measures for combating corrosion, localized casing leaks continue to occur at an alarming rate, especially inzones adjacent to formations containing corrosive waters. The cement may failto protect the casing from corrosive waters for any one or a combination ofreasons: insufficient or no cement behind the casing because of anunsatisfactory cementing job or lost circulation, localized dilution of cementslurry by the influx of formation waters, poor bonding of cement to analready-corroding casing, deterioration of hardened cement by sulfates andother salts contained in formation waters, and inadequate corrosion inhibitionand excessive chloride diffusivity characteristics of cement. The first threefactors are related to the primary placement of the cement; the last two aredetermined by the chemical and physical properties of the cement material. Efforts to improve these factors are usually augmented by alternative measuresof combating casing corrosion, including cathodic protection and the use ofcorrosion-resistant alloys in casing pipes. However, the correct choice ofcement type and casing metallurgy supported by a good cementing job are themost fundamental and effective anticorrosion measures. Of interest in this workis the concept of combating corrosion by using cement to provide a chemical andphysical shield for the provide a chemical and physical shield for the casingagainst the corrosive action of formation waters. A successful design based onthis concept requires an understanding of the factors that are likely to affectthe cement's protective efficiency. As a result, the right cement for theconditions at hand can be selected. API designated the various types ofPortland cement used in the petroleum industry Portland cement used in thepetroleum industry as Classes A through H. Those cements designated by theAmerican Soc. for Testing and Materials (ASTM), referred to as Types I throughV, are used mainly by civil engineers for construction purposes. The mainconstituents of unhydrated Portland cement are oxides of calcium, silicon, Portland cement are oxides of calcium, silicon, aluminum, and iron. These mainoxides commonly are regrouped into four individual compounds: dicalciumsilicate (2CaO SiO2), or C2S; tricalcium silicate (3CaO SiO2), or C3S;tricalcium aluminate (3CAO A12O3), or C3A; and tetracalcium aluminoferrite(4CaOAI2O3Fe2O3), or C4AF. Table 1 lists the compound compositions of the API-and ASTM-designated cements. When water is added to cement powder, a slurry isformed, and a complex chemical reaction called hydration begins immediately. Ashydration proceeds, the slurry gradually gels and eventually forms a hardenedcement. As long as abundant moisture is available, the hardening processcontinues, albeit at a decreasing rate, for many months and perhaps years. Forpractical purposes, most cements are considered adequately cured in 28 days, when they attain about 90% of their final long-term strength. Thecorrosion-inhibiting property of hardened cement is derived from the alkalinityof the calcium hydroxide liberated during hydration. The high alkalinity (pH ofabout 13) causes a thin oxide layer to form on the surface of the steel casing. This oxide layer prevents corrosive ions from attacking the steel. Thus, thesteel is in a passive state as long as the pH of the cement passive state aslong as the pH of the cement is sufficiently alkaline to maintain the oxidelayer. Under such conditions, the hardened cement will continue to provide achemical protection to the casing. protection to the casing. The oxide film ona passivating steel can readily be attacked by carbonate and chloride ions. Under the in-situ conditions prevalent in an oil well, a cemented casing pipeprevalent in an oil well, a cemented casing pipe is likely to be exposed toformation brines containing abundant quantities of chloride and sulfate salts. Under these conditions, the chloride ions threaten to induce corrosion whenthey adsorb onto and penetrate the oxide film, resulting in a large increase inconductivity of the film.
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