BRINGING INTERNAL CATHODIC PROTECTION CURRENT DISCHARGE THROUGH PRODUCED WATER TO A STOP IN OIL PIPELINES.
ABSTRACT An increasing number of emerging economies are founding their new economy structures on oil production. In some regions such as the Amazon, this oil production is being developed in high consequence areas, mainly because this oil reservoirs lay under the biggest biodiversity of the entire world, and few of the remaining isolated communities. As today, if governments wish to continue the exploration and production of hydrocarbons they need to keep a strict control of international private companies that have production contracts in these high consequence areas. Rigorous integrity management programs are in placed to ensure that impact to the environment is kept as low as possible. This paper, describes the job performed by an international corrosion specialists group to ensure that cathodic protection and other corrosion mitigation systems work properly to allow this particular symbiosis between economy growth and ecological and social responsibility. The results showed that even if the present design has worked properly for the past years, recent changes in the physical and chemical properties of the extracted oil / water mix will present new challenges and eventually problems if not anticipated.
Novel mitigation technics are addressed and tested in laboratory and virtual environments to show their effectiveness and eventual use in especially sensitive environments. INTRODUCTION With oil production in continuous growth, Ecuador's economy is betting its mid and long term development on the exploitation of new proved oil fields. The new oil production will occur inside regions with one of the greatest biodiversity in the world that are inhabited by the Waorani, Tagaeri and Taromenane clans who try to keep themselves in complete isolation from the rest of the Ecuatorian population. In this delicate environment, following the most rigorous standards of the industry becomes an obligation. Maintaining facilities integrity makes cathodic protection a fundamental defense line in order to guarantee a reliable non-stop operation in an environment with high potential consequences for both the environment and Ecuador's oil industry.
Historically the amount of oil exceed the amount of water that came out of the wells, however in recent years this relation has flipped to the point that only 5% of the production is actually hydrocarbon and the rest is highly corrosive water with H2S and other gases dissolved in it. The effect in internal corrosion is clear and can be addressed using films of corrosion inhibitors as in other parts of the world. However, as a change administration is performed, and new threats have to be identified by risk assessment methodology, cathodic protection emerges as a possible source of corrosion. This works will show the deleterious effect that cathodic protection can have in production facilities such as the ones found in the amazon were current can actually be discharged to the highly conductive electrolyte that run inside pipelines.
Figure 1. Oil production occurring in symbiosis with one of
the biggest biodiversityâ€™s of the world.
Corrosion is one of the main risks associated with the long term integrity of oil pipelines. External corrosion is normally controlled through cathodic protection systems, while internal corrosion is dealt with corrosion inhibitors. Nevertheless, sometimes unconventional corrosion phenomena can ocurr that requires the insight of specialized engineers in order to discover and understand them. A good example of such unconventional corrosion processes in pipelines is when the extracted crude comes with high amounts of water, known as “produced water”, which may come from the oil field itself or from the injection of water for oil production increase. Pipelines with high amounts of produced water that are protected by cathodic protection systems can cause a rapid deterioration of pipeline sections near isolating joints and allow the current flow through isolating joints into other buried structures that need no cathodic protection. This was the case in an oil producing installation in Ecuador whose cathodic protection system had severe current discharges through different paths causing an inefficient operation of the system and generating corrosion cells that were difficult to detect. The problem was detected during an evaluation of the site’s cathodic protection system that was performed with the participation of three internationally renowned leaders in corrosion protection with the highest NACE certification on corrosion control using state of the art technology for the diagnosis and numerical modeling of cathodic protection infrastructure. CONTROLLING CORROSION WITH CATHODIC PROTECTION SYSTEMS SOMETIMES BRING UNEXPECTED PROBLEMS. The origin of corrosion of metals is based on the formation of a galvanic cell between two dissimilar metals that are electrically connected. Being distinct metals, their oxidation potential is also different. This generates an electromotive force that in certain conditions can generate an electric current fed by the oxidation of one of the metals and the reduction of chemical compounds surrounding the other metal. Figure 3 shows a schematic diagram of a galvanic cell where the electromotive force is generated by the potential difference between two metals. A galvanic cell can also be created between two sections of the same metal, which is the most common form of corrosion of buried or submerged structures. In such cases the potential difference between the two parts of the structure is due to differences in the composition of the soil or water around each of the sections that participate in the galvanic process.
Figure 2. Schematic diagram of a corrosion cell.
In cathodic protection systems an external power source induces a current between carefully designed anode beds and the metal structures to be protected. Oxidation reactions, instead of occurring on the metal surface, damaging it, occur at the anode beds. The reduction reactions, which are not harmful, occur at the surface of metallic structure. As long as the electromotive force keeps the metallic structure acting as a cathode, inducing reduction reactions at its surface, the structure will be protected from corrosion. Nevertheless, given the right conditions, sometimes the electric potential powered by the cathodic protection system can induce parasitic currents between two structures that are electrically isolated, generating undesired corrosion reactions. This phenomenon is known as interference; it may manifest itself in multiple ways, but it always presents the risk of accelerated corrosion upon the involved structures.
Figure 3. Schematic diagram of the ionic current flow through an isolating joint. The direction of the current flow goes from the side of the surface installations to the side of the protected pipeline.
The pipelines of the studied site had isolating joints whose purpose was to impede the electric current to flow into the surface installations, which would normally cause current losses in the cathodic protection system. Nevertheless, it was detected that an electrical current flow passed through the metallic oil casings. The casings were not supposed to have an electrical connection with the pipelines, and thus, no current should have passed through them. The problem was that the pipelines transported crude mixed with an important quantity of produced water that had high conductivity due to a high content of chlorides and other salts. This allowed the produced water to act as a bridge for the electrical current letting it flow through the isolating joints (figure 4).
The interference that was present at the isolating joint generated a corrosion cell where the pipeline of one side of the joint acting as the anode, the produced water acting as the electrolyte, and the pipeline at the other side of the joint acting as a cathode. In addition to the corrosion problem, the interference caused at the isolating joint generated a current loss of more than 57 % of the current spent to protect the pipeline (figure 5).
The current flow through an isolating joint is not easily detected without the proper equipment. If electric potentials are measured at both sides of the joint using a fixed reference cell, different potential values may be found; this would give the impression that the joint is working properly. By using a radiofrequency meter, it will become clear that the joint is not working as it should. But it is only after a high diameter clamp ammeter is used, that the problem is revealed. This was the case for the studied site; even though the joints succeeded in impeding electrical currents from flowing through the pipeline flanges, there was still an electric current measured at the supposedly isolated side. The only way this could be possible is by a current flow through the fluid that is transported inside the pipeline.
Figure 4. Distribution of the protective current and the current lost towards the surface installations and well casings.
Following the path of least resistance, the current flowed from the anode beds to the oil casings, then through the surface installations until it reached pipeline that was supposedly isolated with the isolating joints. The pipeline was connected to the cathodic protection system, closing the electrical circuit (figure 6). Figure 6. Current consumed by high current demand facilities.
An electrical current of 4.9 A was measured through the joint. This was equivalent to the dissolution 44.7 kg/year of iron from the pipeline! If nothing was done to prevent this, the affected pipelines would certainly develop leaks that would need to be repaired. This has already happened at the site; the evidence can be seen as a repair patch on the lower half of the pipeline near an isolating joint (figure 8).
Figure 5. Schematic diagram of the current flow through the well casings, the surface installations and the protected pipeline. A ionic current flows from the anode beds to the well casings (yellow arrow); the well casings acting as a cathode, conduct an electrical current through the installations until the isolating joint is reached (red arrows). By means of a corrosion cell, the current passes from one side of the joint to the other, reaching the pipeline that is directly connected to the power source of the cathodic protection system, closing the electrical circuit.
DETECTING INTERFERENCE CAN BE TRICKY
Figure 7. Current discharge (A) y leakage repair (B)
INTERFERING UPON INTERFERENCE Besides the presence of highly conductive produced water that generates a localized corrosion cell, other variables that contribute to the interference phenomenon are the geometry of the system and the soil properties. Neither of both are variables
that can be modified in order to prevent the interference. In addition, the exact position of the casings along the wells is difficult to determine due to the fact that the oil wells were not drilled straight and vertical, but directionally. Nevertheless, there are other ways to stop the interference, two of which are presented below. 1. Isolating oil well casings Well casings are basically large buried steel pipes that are in direct contact with the soil. By installing electrical isolation elements at the oil well discharge lines, the current flow through the casings is blocked. This is a common practice for similar cases. Although there are other structures that may still behave like interference electrodes such as grounding systems and structure foundations, stopping the current flow through the well casings should significantly reduce the interference current.
transport under safe operating conditions, especially in environmentally and socially sensitive areas. Nevertheless, there are sometimes that unexpected phenomena appear on cathodically protected structures that present additional corrosion risks. Such cases demand the expertise of corrosion engineers, with the help of the adequate instrumentation, in order to elucidate the origin of the anomalies and be in the best position to solve the problems. ACKNOWLEDGE We thank Juan Carlos Andrade, legal representative of Integridad para Transporte de Hidrocarburos in Ecuador, for his invaluable support in the development of this project. REFERENCES 1.
Dr. Thomas J. Barlo, “Field Testing the Criteria for Cathodic Protection,” Research sponsored by Pipeline Research Committee of American Gas Association, SAIC Interim Report, Dec. 1987, Cat. No. L51546 (Hoffman Estates, IL: AGA) 1988.
J. Cantó, E. Rodríguez Betancourt, C. G. Lopez Andrade, H. C. Albaya, Norberto Pece, L. M. Martínez-de la Escalera, H. Rivera, A. Godoy, J. A. Ascencio and L. Martínez-GÓMEZ “5,000 km row cp survey analysis and the potential advantages of the 100 mV polarization criterion for the CP of aged coating oil and gas pipelines in Gulf and North of Mexico”.
Lorenzo M. Martínez de la Escalera, Jorge Cantó Ibáñez, Hernán Rivera Ramos, Arturo Godoy Simón, Lorenzo Martínez Gómez. “Control de la corrosión exterior de tuberías de pozos para beneficio del medio ambiente y la productividad de la industria petrolera de México”. Consejo Consultivo de Ciencias de la Presidencia de la República. (2011)
W. W. von Baechmann and W. Schwenk, Handbook of Cathodic Protection, (Surrey, England: Portcullis Press Ltd., 1975)
U. R. Evans, The Corrosion and Oxidation of Metals (Edward Arnold Publishers, Ltd., 1960)
2. Internal cathodic protection at the isolating joints Another alternative is to install a cathodic protection system inside the oil pipeline in order to polarize the affected section of the pipeline forcing it to act as a cathode. An anode would be installed in the interior of the pipeline having direct contact with the produced water that flows inside, a current rectifier i.e. the power source, would be connected to the anode and to the pipeline section to be protected from corrosion. In addition, a reference cell would be necessary in order to monitor and control the power applied to the system (figure 9). This alternative solution must be considered if a significant interference current is still present after installing isolating joints at the oil well discharge lines. The size of the anodes and the power source will depend on the magnitude of the remaining interference current.
Figure 9. Schematic diagram of a cathodic protection system installed at the interior of a pipeline section next to an isolating joint.
CONCLUSIONS Protecting oil pipelines from corrosion is a well understood engineering practice that keeps crude and refined hydrocarbons