Factors affecting pulsed-cathodic protection effectiveness for deep well casings

摘要

Purpose - The purpose of this paper is to introduce a theoretical investigation of the pulse-cathodic protection (PCP) systems to show how they behave under different operating conditions. The effectiveness of the PCP system also is highlighted for a typical large-scale configuration. The principal technical objectives of this paper are to answer three questions: Are the PCP systems effective in the desert fields? Although they have been approved, what is the reason for their lack of effectiveness in some coastal areas? What are the operation recommendations for the currently installed PCP systems and their future application? Design/methodology/approach - The factors affecting the cathodic protection of well casings have been investigated theoretically by using a 3D field approach software package current distribution, electromagnetic fields, grounding and soil structure analysis. Cathodic interference with nearby well casings has been investigated thoroughly because corrosion of this kind is more serious than the anodic type. The performance of PCP systems has been analyzed with respect to obtaining better protection-current distribution along the protected well casing at reduced anode current, together with reduced stray current (corrosion) at any nearby unprotected structure(s). Findings - For uncoated well casings, protection current pulses are attenuated significantly and are smoothed out to be pure direct current after about 10 percent of the well-casing buried length. High-magnitude stray current can be found affecting any switched-off well casings and hence they can be corroded faster from the top part than unprotected/remote wells, as are deeper well casings that may sustain considerable localized corrosion attack on the upper portions of the casing. Without the formation of a natural protective coating with high resistivity, the PCP system becomes malfunctioning, i.e. its performance becomes very similar to that of the conventional cathodic protection (CP) systems. This effect has been confirmed by field measurements in Oman, where magnesium hydroxide is minimally formed (in desert areas). Research limitations/implications - In reality, some of the PCP modules at the same station can have a slight deviation in the operating frequency and/or voltage. It is planned, therefore, that the investigation will be extended to simulate such cases and take into account the effect of multi-layer soils. Practical implications - Knowing the performance of PCP systems for protecting deep well casings is a critical issue for the oil industry. Originality/value - The paper provides a sound basis on which oil producers can take decisions about the future application of the PCP systems, optimize their performance, and introduce application restrictions by studying all factors that affect PCP performance. The effectiveness of PCP in desert (sandy/rocky) soil, where calcium-carbonate deposition predominates over magnesium-hydroxide formation, has proven to be very similar to that of a conventional CP system. The reliability of artificial oil-lifting systems will be increased by reducing oil production losses ("oil deferment") and the rig mobilization, which has very high rent cost.