Phosphonate scale inhibitors (SIs) applied in downhole squeeze applications may be retained in the near-well formation through adsorption and/or precipitation mechanisms. In this paper, we focus on the properties of precipitated “mixed” calcium and magnesium phosphonate complexes formed by nine common phosphonate species. By “mixed”, we mean anionic SI bound to both calcium and magnesium divalent cations, i.e. the complex SI_Can1_Mgn2 is formed where n1 and n2 are the stoichiometric coefficients for Ca and Mg, respectively. The stoichiometry (n1 and n2 or the Ca2+/P and Mg2+/P molar ratios) in various precipitates is established experimentally and the effect of solution pH on the molar ratios of Ca2+/P and Mg2+/P in the precipitate is determined. Static precipitation tests were carried out varying the amounts of Ca2+ and Mg2+ present in the system at test temperatures ranging from 20°C to 95°:C, at a fixed [SI] = 2,000ppm. The solution molar ratio of Mg2+/Ca2+ was varied but the ionic strength of each test solution was kept constant. In addition, tests were also carried out with (i) only Ca2+ and SI present, and (ii) only Mg2+ and SI present. The molar ratios of Ca2+/P and Mg2+/P in the solid precipitates were determined by assaying for Ca2+, Mg2+ and P in the supernatant liquid under each test condition by ICP spectroscopy (Cao, Mg
Development of structural steels with improved corrosion resistance is gaining significance for use in offshore structures. Corrosion of structural steel depends not only on various influencing factors such as location of seaport or seawater corrosivity zone the steel is exposed in, but also on the chemical composition of the steel itself.
Against this background the influence of alloying additions of copper, chromium and nickel, which are known to be able to mitigate corrosion by seawater, was investigated aiming at an economic alloy modification of structural steels used for manufacture of sheet piling.
Laboratory corrosion tests were conducted with the rotating cage test setup. As a reference the corrosion performance of standard structural steel grades in the strength range of 275 MPa to 500 MPa was determined. The influence of alloying additions of copper, chromium and nickel was investigated in further tests using industrially manufactured sheet piling including a steel manufactured in accordance with ASTM A 690.
From the investigated alloy variants, the steels alloyed with about 0.3 % copper were found particularly suitable for reduction of general corrosion in seawater by up to 36 %. In order to ensure the workability of copper alloyed components, usually an adequate amount of Ni is added. However, the corrosion test results, e.g. for the copper/nickel alloyed steel ASTM A 690 showed that such a measure is not recommended if improved corrosion resistance in seawater is envisaged.
Gil, Perla Morales (The University of Manchester) | Domínguez , Juan Manuel Romero (Instituto Mexicano del Petróleo) | Lapinski, Jacek (The University of Manchester) | Lindsay, Robert (The University of Manchester)
2-mercaptobenzimidazole (MBI) has been shown to be an effective inhibitor of carbon-steel corrosion in aqueous solutions of both hydrochloric and sulfuric acid. Here, we extend such studies to examine its performance in conditions mimicking a sweet oilfield environment. Linear polarization resistance is employed to determine corrosion rates in the presence or absence of MBI as a function of both solution temperature and CO2 partial pressure. Results demonstrate that MBI effectively inhibits corrosion within the parameter space explored i.e. solution temperatures of 30°C and 55°C and total applied pressures of 1 bar and 20 bar. Potentiodynamic polarisation measurements are presented to further characterize interfacial corrosion chemistry. X-ray diffraction and scanning electron microscopy are applied for post-immersion substrate characterization.
For a given production system, the corrosion mitigation philosophy is usually established based on the fluid properties and the operating parameters pre-identified within the Basis of Design (BOD). Information in the BOD comes from reservoir simulation, process/flow modeling, and thermodynamic/ compositional fluid analyses. These data have limited accuracy and a wide variability throughout the field life. Very often and due to capital expenditure (CapEx) constraints, most of production facilities primarily utilize carbon steel lines with corrosion mitigation provided by injection of chemical inhibitors.
When using carbon steel and inhibition, a successful operation requires active corrosion monitoring to keep tabs on effectiveness of the chemical inhibition program. The monitoring program is a key to proactively identify new corrosion mechanisms surfacing during the field operation. Due to the wide variety of operating conditions, it is not practical to run laboratory corrosion tests simulating every production scenario. Most of the corrosion predictions rely heavily on the laboratory test results; however, the tests have limitations and may not precisely cover all of the corrosion mechanisms in predicting field performance. Depending on new mechanisms identified by the corrosion monitoring program, corrective actions are usually taken. These actions may include additional chemical treatments and mechanical systems such as pigging to bring the corrosion under control.
This paper discusses some of the interesting corrosion mechanisms that have emerged in production systems due to changes in operating conditions during field operations. Corrosion monitoring data with fluid analyses, flow modeling, and additional laboratory testing have been effectively used to understand the corrosion mechanism and develop solutions for control. This work focuses on internal corrosion control of carbon steel in production and transportation lines with single or multiphase flow.
Li, Chong (ExxonMobil Upstream Research Company) | Xiong, Yao (ExxonMobil Upstream Research Company) | Pacheco, Jorge L. (ExxonMobil Upstream Research Company) | Cao, Fang (ExxonMobil Upstream Research Company) | Desai, Sanket K. (ExxonMobil Upstream Research Company) | Ling, Shiun (ExxonMobil Upstream Research Company)
Carbon steel corrosion in the presence of CO2 and H2S (referred to as CO2/H2S corrosion or sour corrosion) is a concern in the oil and gas industry. Presently laboratory evaluation of sour corrosion susceptibility typically involves measuring carbon steel corrosion rate under simulated field environmental conditions. In this lab testing, the wall shear stress is an important environmental parameter which describes the interaction of the flowing fluid with the steel surface. A good understanding of the wall shear stress effect is important to enable the extrapolation of lab test results to assess sour corrosion behavior in the field. However, currently there is a lack of such understanding on the effect of wall shear stress on sour general corrosion as well as on localized / pitting corrosion.
To develop our understanding in this technical area, lab corrosion experiments have been conducted. Weight loss of the testing coupons was measured to calculate the general sour corrosion rate. The corrosion scales were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and transmission electron microscopy (TEM) in order to generate mechanistic insights. In addition, localized / pitting corrosion was evaluated through detailed interferometric microscopy that generated 3D surface profile of the corrosion coupon surface cleaned of the corrosion scale. In this paper, autoclave lab testing results together with the corrosion scale and coupon surface analysis results will be presented, and the effects of wall shear stress on carbon steel sour general corrosion and localized / pitting corrosion will be discussed.
Ungaro, María Laura (Comisión Nacional de Energía Atómica) | Carranza, Ricardo M. (Comisión Nacional de Energía Atómica) | Rodríguez, Martín A. (Comisión Nacional de Energía Atómica) | Rebak, Raul B. (GE Global Research)
Ni-Cr-Mo alloys show a remarkable corrosion resistance in a wide variety of environments. However, these alloys are susceptible to crevice corrosion in aggressive conditions, such as high concentrations of chlorides, low pH and high temperatures. The aim of this study was to use the electrochemical noise technique to study the crevice corrosion process on alloys 22 (UNS N06022) and 625 (UNS N06625). Potential and current noise records were obtained by using asymmetric electrodes assembly which consisted of a creviced specimen of alloy N06022 or N06625 and a second non-creviced specimen of the same alloy or platinum. The obtained records were analyzed in the time domain and through statistical parameters. Noise records showed a clear potential drop along with a current increase. The platinum electrode produced higher currents and potentials and a shorter induction time compared with alloy N06022 and N06625 electrodes. Cathodic kinetics was faster on platinum than on the passive alloys surfaces. Alloy N06022 showed better corrosion resistance than alloy N06625.
Many pipelines used to transport high pressure gas to long distances are made from carbon steel plates by spiral welding. There are instances in which such pipelines gave a long service in sour atmospheres. But, when they failed, they failed catastrophically and sometimes repeated failures occurred.
In one such failure instance, the pipeline in the subject case gave a service life of about 12 years in sour gas service. In year 2012, the pipeline failed suddenly by cracking along the heat affected zone (H.A.Z.) of the spiral weld. Onwards, frequent failures began to occur in identical fashion at different locations. This prompted for a failure investigation to be carried out. Mechanical, metallurgical and chemical tests and analyses have been carried out to understand the natures of the failures.
The paper presents the findings and a discussion on the correlation of mechanical, metallurgical and chemistry parameters to the type of failure that has occurred.
As part of continued efforts by an operating company to address specific corrosion mitigation needs and utilize improved integrity chemicals such as corrosion inhibitor, scale inhibitor, oxygen scavenger, biocides as well as operational chemicals such as antifoam, coagulants, anionic and cationic polyelectrolytes etc. offered by different chemical suppliers, it is necessary to conduct field trials at the existing facilities. Such chemical field trials pose specific challenges to the extent of causing even un-planned shutdown of the facilities.
This paper outlines some of the typical problems encountered by facility operators and corrosion monitoring personnel right from shortlisting of chemical vendors, performing compatibility tests, establishing evaluation criteria, until the issue of the field trial report to the asset owners. Issues highlighted in this paper are based on field trials conducted at the Kuwait Oil Company (KOC) for their Seawater Treatment Plant (SWTP) supplying treated seawater to the Central Injection Plant Facility (CIPF). Products supplied by three different chemical suppliers were utilized in the trials and unique challenges were faced in each case.
According to NACE MR 0175/ISO 15156 standard UNS S31803 duplex stainless steel can be used up to 0.1 bar H2S and 232°C/450°F regardless of the chloride concentration and the pH. These limits based on past studies are very conservative because they do not include any limits of pH and chloride concentrations.
The objective of this paper is to share some recent laboratory data dealing with Sulphide Stress Cracking (SSC) and Stress Corrosion Cracking (SCC) resistance of S31803 above the Standard H2S partial pressure limit. Corrosion resistance of the duplex material was assessed in environments representative of formation waters containing up to 120 g/l chlorides at pH=4.5. Various H2S partial pressures were considered (0.5 to 1 bar) as well as different testing temperatures (24°C/75°F, 90°C/194°F and 150°C/392°F). Constant load tests were performed using specimens coming from three industrial heats.
Results confirmed the strong influence of chloride concentration and temperature on UNS S31803 corrosion resistance. It was also demonstrated that UNS S31803 is resistant to SSC/SCC phenomena up to 0.5 bar H2S under restricted conditions of pH and chloride concentrations which is significantly above NACE MR 0175/ISO 15156 requirements.
This paper discusses a generalized approach to conduct an integrity assessment of a crude oil pipeline, where the limited availability of current and historical operating data makes the full use of NACE Internal Corrosion Direct Assessment (ICDA) procedures, as well as commonly available corrosion rate models, unsuitable. The methodology developed allowed for a considered and detailed integrity assessment to be performed despite a paucity of operational data. Various aspects from ICDA documentation were considered, in addition to data from in-line inspections (ILIs) and ultrasonic thickness (UT) field verifications. After completion of the assessment, it was possible to: verify corrosion patterns from ILIs by assessing areas of the pipeline at risk of significant water hold-up, estimate a realistic ongoing corrosion rate based on different approaches to the examination of available inspection data, advise on subsequent ILI frequencies and other operational practices, as well as provide “fitness-for-service” (FFS) assessments based on the existing and projected condition of metal loss features for different operational parameters.