Paper: Matching Corrosion Needs Of Rod-Pumped Oil Well Systems With Special Properties Of Corrosion Inhibitors

Recent estimates indicate that oil producers will spend about $750 million in 1977 to replace tubing rods, pumps, etc. because of corrosion. While down time caused by the need to replace corroded equipment will not necessarily result in permanent loss of production, it certainly does result in lack of ability to produce oil now when we need it. To compound the problem, we will also import more than 40 percent of our petroleum needs in 1977. Thus, in addition to the out-of-pocket costs of replacements, oil producers also suffer from reduced income when wells are not producing. An obvious remedy is to prevent the corrosion. Not so obvious is how. Using corrosion-resistant materials is one approach, but the expense of exotic metals could exceed the cost of replacing conventional corroded equipment. Proper design can reduce corrosion, and corrosion allowance can be included in the design. Neither is totally satisfactory, but each will extend the time before replacement will be required. Production practices such as slowing pumping rates to reduce rod stress and complete exclusion of oxygen in annular spaces, especially wells with low fluid levels, help alleviate corrosion problems. Since the combined effects of corrosion and wear are greater than just the sum of both if each occurs alone, rubbing parts (rods and tubing) and solids entrained in fluids should be avoided. Corrosion can also be slowed by changing the corrosivity of the fluids by removing corrodents (H2S, CO2 or oxygen) or by altering the pH. Sometimes one or more of these can reduce corrosion to acceptable levels. Where they will not, corrosion inhibitors are used. For optimum effect (lowest cost) corrosion inhibitors have been developed to exhibit a wide variety of properties that enable them to prevent corrosion under a variety of production practices and equipment configurations. Corrosion inhibitors that are used to prevent corrosion in rod-type oil wells do so by adsorbing onto metal or metallic corrosion products such as iron sulfides, carbonates, oxides, and scales. They can prevent corrosion when as little as a monolayer of inhibitor molecules are adsorbed on the surfaces that would otherwise be exposed to the water that is necessary for corrosion reactions to proceed. In essence, one end of the inhibitor molecule adsorbs and the other end sticks out from the surface to repel the water. There are several important fundamental aspects of chemistry and physics that determine both the rates of adsorption and the quantities adsorbed. Second, there are also several important characteristics of inhibitor mixtures that influence these functions. And third, there are several physical factors related to equipment, production practice, and variables in the corrosive environment itself that affect the adsorption and subsequent corrosion protection.