The unstable oil market demand in the petroleum industry, the oil financial crisis and the Covid-19 pandemic have pushed the Oil and Gas operators to look for optimized, cost effective and more reliable strategies of design and operation. Large part of CAPEX is invested in the well materials. Hence, materials of Oil and Gas wells should be selected to withstand both internal and external threats to ensure integrity and availability while maintaining on focus on affordability over the lifecycle.¹ 

Coiled tubing is defined as a continuous tubular product that is used for oil and gas well interventions. Its popularity continues to grow due to its versatility and speed of operation. Though superior grades of metal alloys exist in terms of corrosion resistance, coiled tubing operations primarily employ high-strength low-alloy steels because of their availability, lower cost and weldability. The low-alloy steel can also be thermo-mechanically controlled to elicit specific material properties, such as yield strength and ductility. These coiled tubing steels are often introduced into potentially corrosive downhole conditions, therefore proper testing must be completed to ensure adequate corrosion protection prior to job execution. Downhole corrosive conditions often encountered include; oxygen saturated fluids, elevated temperatures, exposure to oxidizing agents, hydrochloric acid and highly concentrated brines. Often these fluids will be recirculated in a closed loop system, consistently re-exposing equipment to potentially damaging conditions. Frequently, these challenging conditions faced are tested individually with pressurized mass loss coupon testing at bottom hole conditions. However, due to a recent coiled tubing incident in which the coiled tubing pipe had completely parted downhole, the post-job incident investigation involving SEM and metallographic analysis revealed pitting corrosion throughout the tubing, despite the pre-job testing performed indicating adequate acid corrosion protection for the entirety of the job. A literature review indicated very little research was available involving the possible interaction of brine solutions and diluted acid on coiled tubing carbon steels. This paper aims to investigate the possible corrosive interactions between salt brines and inhibited acid blends at elevated temperatures on high grade coiled tubing coupon samples through metallographic examinations and mass loss tests in pressurized heated cells. Coiled tubing coupons will be exposed to a variety of acid blends diluted with a 10% brine (8% wt NaCl and 2% wt CaCl2) or fresh water to investigate the possibility of corrosion enhancement between saline fluids in a diluted acid system. 

Coiled tubing is a long (sometimes more than 25000 feet) electrical resistance welded pipes that is uncoiled for the deployment of oil and gas wells to perform a variety of jobs that involve subjecting the pipe to high pressure, axial loads, and contact with acidification, and production environments among others. Since the pipe is coiled and deployed multiple times for different jobs during its lifespan, it is typically plastically deformed and fatigued.
This paper discusses the challenges of improving the performance of this welded pipe base material, as well as its multiple welds, when facing an inhibition failure that could cause exposure to corrosive and/or embrittlement environments.
The product development process resulted in the complete modification of steel chemistry, its welding procedures and heat treatment process. The testing program includes, among other tests, the exposure of the pipe to different environments, followed by fatigue testing to determine the remaining material resistance after such exposure.