Acid Corrosion Inhibitors - Are We Getting What We Need?
Authors: Smith C.F. , Dollarhide F.E. , Byth Nancy J. 
Published: December 2007

Acid corrosion-inhibitor test results are presented to demonstrate how dataon inhibitor effectiveness can be misleading and why an industry-approvedstandard method must be developed. The corrosion rate of an acid systemcan be halved simply by increasing the acid-volume/steel-area ratio. Othervariables affecting inhibitor performance are test pressure, time attemperature, chemical additives, test agitation, and type of steel.

Introduction

The introduction in 1932 of the arsenic acid corrosioninhibitor (ACI) primarily was responsible for thedevelopment of well acidizing. Adding a chemical inhibitorsuch as arsenic reduces the rate of acid reaction withsteel, but never completely stops the reaction undernormal treating conditions. Therefore, the type andconcentration of ACI needed to reduce the reaction rate to anacceptable level must be decided when planning an acidtreatment. The most common factors affecting ACIrequirements are bottom-hole temperature, exposure time, steel metallurgy, acid type and concentration, andsurfactant use.

Acid corrosion inhibitors normally are evaluated interms of metal loss resulting from exposure to a giventype of acid at varying concentrations, temperatures, andexposure times. The most effective ACI concentrationfor a given set of conditions normally is obtained fromthis data. Testing methods thus lead to problems. Manycritical acid treatments are decided on the basis ofcomparative inhibitor performance at high temperature.In less temperature-critical situations, treatment costmay be reduced by using a cheaper ACI or alower-concentration one to give minimum desired protection.Unless test data from different sources are obtained withstandard test procedures, a true comparison of ACIperformance is impossible. performance is impossible.In spite of organized efforts, no standard test procedureexists today. Typically, the term "acceptable corrosionrate" is arbitrary and varies among companies. Also, asthe standard changes so do temperature ranges. Table 1 isan example of this variation. Table 1 indicates that ACIperformance requirements are less rigid at higher performance requirements are less rigid at higher temperatures. The question then is "Why should corrosion bemore acceptable at higher temperatures?"

Actually, the entire method of comparing ACIperformance is vague and ambiguous. Data for inhibitors can be performance is vague and ambiguous. Data for inhibitors can be obtained only by designing special tests that fail tosimulate treating or down-hole conditions. Frequently, criticaldecisions are made using this data. This study points outthe effect of test conditions on inhibitor performance sothat acidizing treatment designers can understand realperformance limits on ACI systems better and can select performance limits on ACI systems better and can select the best inhibitor for existing well conditions. Some casesinvolve weighing performance claims with how the datawas obtained.

The comparative effectiveness of an ACI can bejudged only in terms of laboratory simulations.Simulation quality becomes the primary factor in judging ACIperformance. Thus, we must look at test conditions that performance. Thus, we must look at test conditions that generate data affecting inhibitor choice and point out theneed for better simulation of well conditions. Also, wemust develop standards and test procedures that areuniform throughout the industry so that ACI performancedata truly is comparative.

Several years ago, the API-NACE Subcommittee onCorrosion Testing began to develop a standard testmethod and equipment. The test procedure and cellspecifications are scheduled for release in 1978.

JPT

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