Microbial Influenced Corrosion of Coil Tubing Milling Strings in the Eagle Ford Shale
Authors: Sherman Scott , Brownlee Duane , Kakadjian Sarkis , Luft Bernard 
Published: March 2015

Microbial influenced corrosion (MIC) has been implicated in many corrosion related challenges in the well service industry in the past. Recently, the industry is observing an influx of MIC related equipment damage. The recurrence of MIC is coincidental with the switch to unconventional water sources. As fresh water for fracturing operations and well interventions becomes less available, operators are forced to use alternative water sources such as recycled flowback water, produced water, recycled frac water, and even 'grey water' from wastewater treatment plants. Regardless of the water source for a particular well treatment operation, the same water-hauling equipment and tanks are used for successive hydraulic fracturing operations.

This 'communal' use of water hauling and temporary water storage equipment is an ideal situation for bacteria to move from one water repository to another. Even if the water source used to supply water for oilfield operations is free from harmful bacteria, it may become contaminated - in transport or temporary storage vessels - before it is pumped downhole. This paper will highlight a recent discovery of MIC in coil tubing that has been used for milling frac plugs in the Eagle Ford Shale play in Texas. In this case, MIC had significantly decreased the life span of the affected coil tubing strings. Metallurgical analysis was conducted to determine if the corrosion was the result of sulfur-reducing bacteria (SRB) or anaerobic acid producing bacteria (APB) - two specific types of several microbes that can lead to MIC.

Bend fatigue testing was conducted on MIC-affected coil tubing samples, which showed a significantly reduced coil fatigue life when compared to non-affected coil tubing. This paper will explore the potential source(s) of the bacteria, the impact to the equipment that was exposed to the bacteria, as well as what is being done to mitigate the problem.

Introduction

Representing the largest portion of the Earth's overall biomass, bacteria are ubiquitous on the planet's surface. They have been recovered from subterranean rock to depths in excess of 13,000 ft (4,000 m) (Moser, 2006). Microbial species have been shown to survive temperatures in excess of 120°C (Breeder, 1994) and pressures in excess of 130 MPa (Jun, 2011). Thriving bacteria can exist in either a free floating (planktonic) or static (sessile) state wherein they are attached to a solid surface of nearly any sort. In this attached or sessile state, the bacteria commonly coexist with other microbes such as other bacterial, archaeal or fungal species. These microbial communites create their own protective environment made of biopolymers of virtually all types. These sessile communities are commonly refered to as a biofilm, which is the state the overwhelming majority of microbes exist in (Costerton, 2007).

Microbial-influenced corrosion, also known as biological corrosion, bacterial corrosion, bio-corrosion, biofouling, biotic corrosion, microbiologically-influenced corrosion, or MIC is a type of corrosioncaused, promoted or accelerated bymicroorganisms. MIC has been shown to greatly accelerate corrosion relative to the rates that occur abiotically and is thought to contribute roughly 20% to the overall cost of corrosion (Little, 2007). MIC in anaerobic conditions is most often associated with sulfate-reducing bacteria (SRB) that produce sulfide species (H2S and/or FeS) as part of their metabolic process, which can contribute to sulfide stress cracking (SCC) in addition to MIC (Cheng, 2013). In addition to SRB, thiosulfate-reducing bacteria (TRB), nitrate-reducing bacteria (NRB), sulfite-producing bacteria (SuRB), acid-producing bacteria (APB), methanogens- and exopolymeric substances (EPS)-producing baceteria, and iron-oxidizing bacteria (IOB) have also been implicated in MIC (Little, 2007).