Wireline wear resistance of filled and unfilled polymeric coatings for downhole applications

Abstract

Sea water is often re-injected into oil wells through injector tubulars in order to stabilise the reservoir pressure. Injector tubulars are made from expensive chromium alloys due to the corrosiveness of sea water. However, to minimise cost, the tubulars can be made from inexpensive carbon steel and protected internally with cheap polymeric coatings to prevent the tubulars from corroding. However, in service it has been found that the polymeric coatings can be damaged by tools used during work-over and/or inspection of the oil well, which are lowered by wireline. This paper presents the results of the wireline wear study performed on various thermoplastic and thermosetting polymeric coatings, undertaken to understand the effect of wireline asperities on the tribological performance of filled and unfilled coatings. A pin-on-wire (POW) test setup, which utilizes a modified pin-on-disc rig, has been used for the purpose of this study. A true tribo-couple was used consisting of a 3.2 mm diameter “slickline” wire embedded in an aluminium disc to form a circular loop of radius 80 mm. The test conditions (sliding speed of 2.2 m/s and a load of 150 N) were chosen to simulate typical downhole wireline operating conditions. The results show that the unfilled thermoplastic coating can have the highest wear resistance while the filled thermoplastics have the worst wear resistance. The performance of coatings was found to be linked to the chemical and physical nature of the matrix material (either thermoplastic or thermosetting). The influence of filler concentration, shape, type and sizes on the wear rate and wear mechanism of the filled coatings was also investigated. The results show that filler shape, size and type have a significant effect on wear resistance of the coatings. Factors such as filler pull-out, filler fracture and poor abrasion resistance of the filler were found to be responsible for the poor wear resistance of some of the filled coatings because of the inadequacy of fillers to act as load bearing elements. Scanning electron microscopy (SEM) micrographs of the worn sample surfaces show either plastic deformation/cutting or formation of short fragile tendrils. Some desirable tribological and mechanical properties of the coatings’ microstructural compositions are identified that give good resistance to wireline wear.