Many oil and gas fields are being operated under severe conditions with the presence of corrosive gases such as Hydrogen Sulfide (H2S) and Carbon Dioxide (CO2). To mitigate the negative impact on operation, CRAs (Corrosion Resistant Alloys) are commonly used and getting more and more popularity. However, the selection of an appropriate CRA for a given corrosive environment requires much engineering effort, particularly for high temperature gas wells with high concentration of H2S and CO2.
In this research program, the corrosion resistance of many types of CRAs (13Cr, super 13Cr, Duplex stainless, Ni base alloy), carbon steel (L80) and low alloy (SM90SS) was evaluated in an actual gas well stream in UAE by using a Dynamic Field Tester (DFT). The DFT can permit an optimum selection of materials for tubing strings and flowlines for use in corrosive environments. Also, concerning to these candidate materials, 4 point bent beam tests were carried out in the laboratory at elevated temperatures in 25% NaCl test solutions which were saturated with the produced gas that had been sampled from the well to evaluate the resistance of the materials to stress corrosion cracking (SCC) and general and localized corrosion.
Long-term DFT tests showed that the corrosion rate of carbon steel and low alloy steel was measured to be less than 3.1MPY and those of CRAs were less than 1MPY. Namely, all the tested materials showed lower corrosion rate than the popular criterions of an acceptable corrosion rate that are 4MPY for carbon and low alloy steel and 2MPY for CRAs. Long term laboratories tests in wet H2S environment showed that carbon steel and low alloy steel suffered from corrosion severely and corroded out partly. These tests also showed that only super 13Cr steel suffered from SCC and the other materials did not suffer SCC.
Our phase behavior study on the produced water and condensate suggested that even carbon and low alloy steels would hardly corrode when the metal surfaces were predominantly wetted with oil. However, those steels are considered to corrode severely when the metal surfaces are wetted with water in this particular UAE gas-condensate field. Therefore, there is a possibility that carbon steel is suitable material for the typical gas well in UAE, if an environment is not water wet. However, if an environment is water wet, SM2535 (25%Cr-35%Ni-3%Mo alloy) and higher grade materials are suitable material for the well.
Materials optimization for down-hole tubing and pipeline is important to improve cost efficiency. To optimize materials, it is necessary to understand its performance against CO2 corrosion, H2S corrosion and corrosion in CO2, H2S and water co-existing environments. Resistance to CO2 corrosion and SCC (stress corrosion cracking) are often the principle factor affecting the select of materials for CO2, H2S and water coexisting environments, because these occurrences can result in severe corrosion such as ring worm corrosion, and, catastrophic and potentially hazardous failures. The lack of knowledge on corrosion resistance under actual well condition often leads to the selection of high alloyed, expensive materials1.
Ikeda et al. reported that a Dynamic Field Tester (DFT) is suitable for studying corrosion phenomena in the CO2 gas filed2. Therefore, U.A.E, which consists of ADNOC and ADCO, and Japan Joint Study Team, which consists of Japan National Oil Corporation, Japan Oil Development Co., Ltd. and Sumitomo Metal Industries, Ltd., carried out field tests by using a Dynamic Field Tester (DFT) to evaluate corrosion rate and resistance to localized corrosion of materials. 4-point beam test in an actual gas sampled from the well that is in a typical gas field in UAE also conducted by using an autoclave to evaluate resistance to SCC, general and localized corrosion of materials. In this paper, these test results have been reported and the material optimization for the typical gas field in UAE has been discussed.