Corrosion Fatigue of Grade-90 Pipe in 15% and 28% Hydrochloric Acid With and Without Inhibitor

Abstract

Coiled tubing (CT) is routinely exposed to different concentrations of acid in stimulation and cleanout jobs. Routine maintenance measures are generally taken, such as the use of inhibitors and proper cleanup and storage practices. However, a large number of CT strings suffer from corrosion and corrosion-fatigue related damage. Though it is difficult to isolate string damage solely from corrosion fatigue, this paper addresses the effect of acid with and without inhibitors on corrosion-fatigue properties of the grade-90 pipe through experimental testing. The results of testing demonstrate that exposure to acid without an inhibitor and during fatigue had the most pronounced effect on fatigue life. The simultaneous action of corrosion and fatigue produced more pits and cracks than would have occurred by corrosion or fatigue separately. CT exposure to acid before, during, and after bending can have different effects on fatigue life. This paper describes an experimental study that was conducted to observe these effects. A strip of metal, 1-in. wide and 6-in. long, was cut from a round 1.75-in. OD, 1.56-in. wall thickness of 90-ksi yield CT and underwent one-way, bent-beam fatigue testing. This paper will present S-N curves generated for samples in three environments: air, acid, and acid with inhibitor at deflection levels ranging from 0 to 40 mm. Finite element analysis (FEA) was used to correlate deflection to strain range levels of 0.5 to 4.0%. Introduction Acidizing is one of the oldest stimulation techniques that is still used today. Often, when petroleum exists in a formation but is unable to flow readily into the well because the formation has very-low permeability, acidizing can be performed to increase production. This treatment is most effective if the formation is composed of rocks, such as limestone or dolomite that dissolve readily in acid. Acidizing is usually performed by an acidizing service company and can be done before the rig is moved off of the well after drilling. Acidizing can also be performed after the rig is moved away. Conventional acid treatments involve pumping 50 to 1,000 gal of acid into the well. The acid travels down the tubing, enters the perforations, and contacts the formation. Continued pumping forces the acid into the formation, where it dissolves channels that provide a way for the formation's oil or gas to enter the well through the perforations. The use of CT for remedial well stimulation (including acidizing) is one of the more popular uses of CT today and offers several advantages over conventional acidizing techniques:○Live well-interventions can be performed without killing the well, reducing additional formation damage.○Additional operations, such as a wellbore cleanout and well-unloading, can be performed immediately before and after a stimulation treatment.○The wellbore tubulars and wellhead equipment are protected from coming into contact with corrosive treatment fluids.○Enhanced placement of the treatment can be achieved, greatly improving uniform injection coverage in the zone or areas of interest.○Pickling and flushing of the CT and wellbore can be performed before treatments, reducing the risk of formation damage caused by contaminants. Major acids used in chemical stimulation include hydrochloric (HCl) acid, hydrofluoric (HF)-HCl acid mixtures, and organic acids. The following is an explanation of the main differences between these acids.