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 P. 737

Fluid loss agents for crude oil and for water have been studied in dynamic tests. A treatment using a spearhead with a fluid loss agent followed by plain fluid appears feasible in crude oil, but not in water. An equation for spearhead depletion shows that spurt loss relative to fracture width must be low, if the portion of spearhead fluid in the treatment is to be small.The presence of colloidal matter in crude oils aids the fluid loss agent. Unlike in kerosene, where flow limited the agent deposition, in crude oils the filter cake continually formed and leak-off declined. The volume-time relation varied somewhat for different crudes, but was best described by a square root of time function. Spurt loss was inversely proportional to agent concentration. After the fluid loss agent initiated the filter cake, the crude oil colloids built on it effectively. A 2-minute or a 5-minute spearhead with double the normal agent concentration gave the same fluid loss curve as the same concentration did for a 30-minute test.The agents tested in water gave fluid loss plots on which, for the first few minutes, volume was proportional to the square root of time, but later became proportional to time. For fracture area calculation the customary square root of time function is a satisfactory approximation. Leak-off rates and spurt losses were higher in water systems than in oils. The spurt loss tended to be inversely proportional to concentration. In spearhead tests, the filter cakes were not eroded by water flow. However, the rather high spurt loss values make spearhead treatments impractical for water-based fluids.

Conventional static tests of fluid-loss agents do not realistically simulate conditions in a fracturing treatment. The dynamic tests reported here show that fluid-loss volume is better represented as proportional to time, rather than as the square root of time. This leads to a different equation for fracture area. The leak-off rate increases with increasing shear rate at the fracture wall, but appears to approach a limiting value. Pressure effects are minor. Spurt loss ordinarily is not affected by the flow velocity in the fracture and is inversely proportional to concentration of agent. The filter cake, once it is well established, is resistant to damage by the flow of plain fracturing liquid (without fluid-loss agent). The latter two findings indicate that a treatment employing a high-concentration spearhead followed by plain fluid can offer a more economical treatment under suitable conditions.

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