Corrosion mitigation is critical for successful managed pressure drilling (MPD), underbalanced drilling (UBD), and air drilling operations. During these operations, large volumes of gas are injected to lower the hydrostatic pressure in the annulus. Most often the injected gas contains some oxygen and the gas is circulated in combination with some quantity of drilling fluid or liquid. Due to the oxygen content and aqueous environment, corrosion potential during MPD/UBD/air drilling operations is high.
Without a proper corrosion mitigation program during these operations, corrosion can lead to the catastrophic failure of drillstring components with corrosion rates 15 times or higher than the industry-acceptable rate of <2 lb/ft2/yr. These high corrosion rates can lead to numerous operational problems resulting in higher well costs and/or formation damage.
This paper discusses how to properly design, implement, and monitor a successful corrosion program to mitigate corrosion during MPD/UBD/air drilling operations.
Furthermore, it highlights a process that utilizes membrane technology as a mechanical means of reducing oxygen concentration in the gaseous media in combination with chemical corrosion inhibitors to alter the drilling fluid environment. By using this process, the corrosion rates can be controlled to satisfactory levels during these operations. Other areas of discussion include membrane technology, drilling fluid chemistry in terms of corrosion control, and the types of corrosion that occur during a MPD/UBD/air drilling operation. Laboratory data will be presented demonstrating the development and limits of the various corrosion control chemistries. And finally, the operational considerations of an effective corrosion control program with case histories will be presented.
Although corrosion mitigation is essential to a successful MPD/UBD/air drilling operation, very little published information is available on reducing corrosion in aerated drilling fluid environments. Most published literature focuses on conventional overbalanced drilling operations, which are effective for ultra-low oxygen environments, but they are not designed for the oxygen rich environments of MPD/UBD/air drilling.
With MPD/UBD/air drilling operations, a compressible gas is an essential part of the fluid system because it allows the annular pressure to be modified or managed. Although air is the most economical service gas, nitrogen is generally used with hydrocarbon fluid systems or when oil or gas is produced while drilling. Membrane nitrogen is widely used for these operations and offers a cost-effective mechanical means of lowering oxygen content, thereby reducing the risk of downhole fires. However, because membrane nitrogen still contains some oxygen content, drilling operations using gasified fluids can have a high corrosion potential if not properly controlled. Without the proper corrosion mitigation program, corrosion rates during these operations can be as much as 15 times higher than the acceptable API corrosion rate of 2 lb/ft2/yr or more. These high corrosion rates can lead to numerous operational problems, resulting in higher well costs and/or formation damage. As a result, the focus of this paper is to expand the knowledge of corrosion mitigation in high-oxygen environments, and specifically, how to mechanically reduce the oxygen content using membrane nitrogen while incorporating a chemical corrosion inhibition program for MPD/UBD/air drilling operations. By mechanically reducing oxygen and incorporating a corrosion inhibition program, corrosion rates can be controlled and drilling costs can be reduced.
It is important to note that corrosion cannot be completely eliminated; it can only be controlled or reduced to acceptable values. What is acceptable is dependent on the particular operator, but the industry acceptable corrosion rate is generally considered to be <2 lb/ft2/yr.