This paper will review distinctive corrosive wellbore environments that can be detrimental to the performance of (High Strength Low Alloy) carbon steel coiled tubing which is typically used for workover/intervention and completion applications. The paper will demonstrate how these corrosive environments form the technical push and potential market pull for the development of coiled tubing suitable for specific corrosive applications. The product development of a corrosion resistant coiled tubing will be reviewed, indicating initial design input, verification data depicting corrosion tests, mechanical properties; inclusive of strength, hardness, full body low cycle fatigue and surface property attributes. The product range depicting diametrical and wall thickness ranges will be reviewed.
The paper will include manufacturing issues such as forming, welding and inspection techniques. The paper will conclude with a review of the strings manufactured to date with application case histories.
There has been an ongoing interest in corrosion resistant alloy (CRA) coiled tubing for a period of time. Ref. 1 The problem has been defining a market large enough to support an extended development program and commitment to capital to produce a CRA coiled tubing product.
As the industry has progressed, used strings have found second homes as velocity strings to economically extend a well's natural producing life. Operators wishing to improve reliability in the performance of these velocity strings began buying new strings of tubing. As the practice expanded, wells with corrosive environments became candidates for velocity strings. Unfortunately, existing coiled tubing alloys are carbon steel and many corrode at unacceptable rates, resulting in non-favorable production system economics. Many of these applications were in wells producing water and CO2 laden gas. The rate at which CO2 can corrode and dissolve carbon steel tubing is proportional to the partial pressure of the CO2 in the water phase. A rule of thumb for CO2 corrosion in a sweet gas condensate, wells with a partial CO2 pressure of 0 to 7 psi will most likely have a very slow, if not non-existent corrosion rate. In partial pressures ranging from 8–30 psi corrosion is possible, while in well bore environments in excess of 30 psi partial pressure, corrosion is almost certain. Ref. 2 Partial pressure of CO2 is the percentage of the total pressure represented by the CO2 percentage in the gas. For example, a well pressure of 4800 psi with 2% CO2 by volume would have a 96 psi partial pressure of CO2. Using an industry accepted corrosion prediction model to look at a simplified well condition producing 800,000 cubic feet per day of gas, 1 barrel per day of water at 175°F, with no chlorides, bicarbonates or H2S would be predicted to have the corrosion rates shown in Figure 1 for 1%, 3% and 5% CO2 at various well pressures. A typical 1 3/4" O. D. by 0.109" wall velocity string predicted to corrode at 100 mils per year would be expected to be consumed in just over a year. Unfortunately, corrosion prediction models assume a uniform general corrosion rate. In down-hole production environments, CO2 corrodes by localized pitting at penetration rates normally above those predicted by models. It is clear to see there are a large number of these wells that could become candidates for corrosion resistant coiled tubing.
Coiled tubing for these wells must exhibit substantially improved corrosion resistance to the well's produced environment. In addition the tubing material must be capable of being manufactured into coiled tubing and survive being spooled and injected into a well. The finished tubing must equal or exceed all the mechanical and quality properties of existing carbon steel coiled tubing grades used for velocity string applications. Of paramount importance, the tubing must perform reliably in the intended application and at a cost to the end user that will allow economic justification for CRA velocity string projects.