ABSTRACT
Methanol is commonly used in the oil industry as a thermodynamic hydrate inhibitor. In world regions with extended winter seasons, significant amounts of methanol are injected into wells and pipelines. Previous studies have demonstrated that the presence of methanol in sour conditions can lead to higher corrosion rates and the increased susceptibility to sulfide stress cracking (SSC) in pipeline steel. Another problem is oxygen intrusion by dissolution in methanol, increasing corrosion rates and potential for localized corrosion. This paper studies the effect of oxygen in methanol on the structures and growth kinetics of iron sulfide scales. Gravimetric weight analysis was used to evaluate the corrosion mechanisms and rates. Scanning Electron Microscope/ Energy Dispersive X-ray Spectrometry (SEM/EDX), Optical Microscope and X-ray Diffraction (XRD) were used to analyze the scale.
INTRODUCTION
At certain methanol (MeOH) concentrations, the iron sulfide (FeS) structure can change, causing an increased risk of localized corrosion
Oxygen (O2) is more soluble in methanol than it is in water; if MeOH is used in large amounts, the dissolved oxygen can produce elemental sulfur, which increases the risk of localized corrosion
Under sour gas conditions, the presence of methanol can increase the risk of sulfide stress cracking (SSC) and stress orientated hydrogen induced cracking (SOHIC)
Methanol can increase the rate of vapor phase corrosion, which directly increases the risk of a top-of-the-line corrosion failure
High quantities of methanol may reduce the success of a corrosion inhibitor treatment program
Methanol is injected into pipelines within Canada and other cold climate locations in large quantities due to the extremely low temperatures in order to alleviate or prevent hydrate formation. Unfortunately, the introduction of methanol into sour gas lines, which are common in Alberta, can increase the risk of corrosion by a number of factors. These factors can include the following: Although corrosion mitigation is used in conjunction with MeOH injection, as an industry wide and commonly accepted practice, there is very little literature on the subject. The Canadian Association of Petroleum Producers (CAPP) guidelines1 state, "there is no clearly defined boundary where methanol becomes a corrosion contributor. Industry experience is that continuous MeOH injection should be limited to a 1:1 methanol/water ratio or the amount required for hydrate inhibition." This rule-of-thumb is widely applied, but there is little literature to support it. CAPP further recognizes the risks associated with the introduction of oxygen and the potential for the diminished efficiency of corrosion inhibitors.
A recent study using de-oxygenated methanol under sour conditions showed that localized corrosion increases significantly as the methanol/water ratio approaches 50%13. At higher methanol concentrations the FeS film thickness decreases and severe pitting takes place. At these concentrations the structure of the FeS layer tends towards cubic iron sulfide.
Methanol can contain up to 40 mg/L of dissolved oxygen at 25°C: an amount that can not only increase corrosion rates, but also change the corrosion mechanisms.
