Temporary Coating Selection for Dissolvable Magnesium in High Temperature Hydrochloric Acid: A Balancing Act

The paper seeks to discuss the difficulties in using magnesium components for acid fracturing in hydrochloric acid due to aggressive corrosion. A coating is required that can resist up to 15% HCl in 150°C for four hours, but quickly fail thereafter to allow for dissolution of the components after the completion of a downhole oil and gas fracturing operation. Further, the study assesses the edge effect of coatings for component protection.

Four coating systems, 2 metallic and 2 polymeric, were assessed for HCl protection after the standard silicone-grease system proved to only provide 5 minutes of delay. Coupons with radiused ends were prepared with dimensions of 1x3x0.25”. Initial screening was conducted in glass beakers of 95°C 15% HCl using a waterbath. If samples showed little/no signs of damage, a coupon and a complex component were tested in 2L of 150°C 15% HCl within an autoclave. Samples were visually and microscopically assessed for damage, with predictions made on expected lifetime and delay.

The baseline coating failed to provide protection. Metallic coating A possessed extensive holidays that led to abrupt failure. Metallic coating B demonstrated resistance in the screen testing. However, upon 150°C testing, coating quickly failed as evidenced by a spark spike in gauge pressure due to the H2 gas released as a byproduct. Further investigation into the coating showed an edge effect where coating thinned along the edge and corners, causing failure. Polymeric coating C was tested in several thicknesses, all of which passed the minimum threshold. However, they did not show any sign of degradation, thus not balancing out with a temporary protection. Polymeric coating D was tested with several thicknesses, of which the 10um thickness met the requirements. It suffered from the edge effect, which provided a pathway to failure after the 4 hour minimum protection was achieved.