ABSTRACT
Injection water/brine handling systems are important elements of the oil & gas field operations. Considering carbon steel as default material of construction for water/brine handling pipelines, a rapid determination of steels' corrosion is the key-element of the oil & gas field's integrity management. Daily, weekly or monthly- corrosion rate quantification with weight-loss coupons or permanent/periodic Ultrasonic Thickness (UT) measurements cannot provide reliable data that can be correlated to process upsets. Modern, on-line, real time corrosion monitoring technologies, including high resolution electrical resistance (ER) and multiple electrochemical methods such as Linear Polarization Resistance (LPR) or Electrochemical Noise (ECN) provide capabilities for corrosion detection in the matter of hours, minutes or seconds.
Electrical Resistance technology depends on physical destruction of the sensing element. Simple correlations between corrosion rates, sensor's span (thickness), sensitivity and service life, lead to situations when high-resolution ER technology becomes operationally impractical due to requirements for frequent replacement of sensing element. Parallel, typical electrochemical measurements facilitated by LPR can be affected by e.g. presence of iron sulfide-based surface deposits that may engender depolarization of working electrode.
Research undertaken by the authors' organization has demonstrated that simultaneous application of different electrochemical techniques ECN, Harmonic Distortion analysis (HDA) or Low Frequency Impedance (LFI), facilitates elimination of most of limitations related to traditional LPR. In this paper, research focused on evaluation of on-line, electrochemical corrosion monitoring, incorporating multi-technique approach in varied concentrations of NaCl brines, saturated with CO2 or CO2/O2 mixtures. Electrochemical corrosion data were compared with high- resolution ER technology and weight loss coupon data. This comparative study offers insights relevant to accuracy, sensitivity and responsiveness of different corrosion monitoring techniques in brine environment.
INTRODUCTION
Water injection system is an integral part of oil and gas field production due to natural decay of formation pressure as field ages. In order to maintain high throughput of oil production, the utilization of water injection system may be as early as a few years after the field production is commenced. Any downtime of water injection system not only directly impact the oil production, but also poses a risk of irreversible changes in reservoir structure and inability to maintain the previously achieved pressure level after extended period of water injection downtime. In addition, the high pressure experienced in water injection system, especially downstream of the water injection pump, represents a significant concern to the safety of personnel and equipment. Maintaining integrity of water injection system poses a significant challenge to facility/corrosion engineers. This is mainly due to the difficulty of controlling oxygen ingress and micro-organism growth in the system. For systems utilizing carbon steel as the material of construction, it is imperative to implement online, real-time corrosion monitoring techniques so that any corrosion change can be captured in timely fashion and correlated with particular system upsets. That will allow informed decision to be made by corrosion engineers and remedial measures can be taken to prevent future corrosion incidents.
