Laboratory Methods for Evaluation Of Inhibitors for Use in Oil and Gas Wells★
Authors: GRECO E. C. , SPALDING J. C. 
Published: July 2014

The need for a compilation of methods for laboratory evaluation of oil and gas well corrosion inhibitors was recognized at the national meeting of the NACE in 1952. At that time, the chairman of TP-1 appointed the authors as a committee to contact those companies, both producing and chemical manufacturing companies, known to be interested in the problem of oil and gas well inhibitors. Subsequently, inquiries were sent to all major producing companies that were members of NACE, and several of the chemical manufacturing companies that were known to be working on the development of oil and gas well corrosion inhibitors. As a result, methods used by some ten major producing companies and four chemical manufacturing companies along with methods used in one of the research departments of a university were gathered. The general classification of tests used in the laboratory are presented along with some meager information as to the correlation of such tests with field experience.

The short range purpose of this article is to present to all interested in the problem the possible methods that may be used in the laboratory for such evaluation work and at the same time to solicit from all those receiving this report information that may lead TP-1 to the adoption of or, at least, recommendation of standardized laboratory procedure. The latter action, of course, is the long range purpose of this work.

Data gathered from companies canvassed indicate the majority working in this field still rely upon corroding systems as set up in the laboratory (whether reflecting field conditions or entirely synthetic) to produce weight loss upon coupons made from material expected to be used in the field. In addition, there are a few companies working on such varied methods of laboratory evaluation as the determination of: 1. film resistivity; 2. hydrogen evolution; 3. drop size ratio, etc.

One discusser deals with static tests and sessile drop tests. He points to several theoretical and practical objections to the drop test, asserting that the drop test is unreliable because wettability criteria cannot be translated always into inhibitor efficiencies. The question of interfacial tensions between metals and fluids is considered with the conclusion that the distinction between adsorptive and oil wetting inhibitors is not clear. The discusser does not believe that contact angle measurements can be used theoretically to predict the extent of oil or water wetting of a surface or that it can be assumed that oil-wet surfaces will not corrode.

A second discusser outlines the methods used by one company in determining the efficiency and economics of inhibitors. The principal objection the discusser has to the committee's aim is that the number of variables in producing well systems are so great that laboratory tests cannot be considered conclusive until backed by field experience. Tests with wetting agents are described in which the rate of adsorption of water dispersable inhibitors may be checked. The discusser also says that the effectiveness of an inhibitor should be measured by the cost of well equipment that must be replaced, pointing out by examples several instances when the cost of equipment replaced was materially reduced as a result of the use of an inhibitor.

He concludes selection of inhibitors by the drop-size ratio or oil-wetting tests is reliable for oil and gas wells but not for gas condensate wells. Speed of the drop size ratio method and the possibility of quantitative recommendations are assets also. This factor is important as indicated by a study which showed a range of inhibitor costs per barrel from $0.009 to $0.224.