This paper outlines solutions for problems encountered due to scale deposits in Marcellus Shale wells, including approaches taken to remediate scale buildup. As wells in the Marcellus Shale produce over time, scale deposits in tubing form and have proven to be difficult to treat. These scale restrictions can cause well performance issues which limit options for artificial lift. Through sampling and lab analysis, the primary types of scale found are calcium carbonate and iron sulfide. After several years of evaluation, plunger lift equipment was unable to be installed in 25-30% of prospective wells due to scale buildup greater than tubing drift or 1.90″ in 2.375″ J-55 tubing.
Calcium carbonate and iron sulfide scale tend to become very hard when they deposit on tubing walls and, as a result, they do not broach or scrape away easily. Throughout this remediation process, several types of broaches were trialed with minimal success. The most effective method for scale remediation was found to be hydrochloric acid (HCl), however, HCl introduces safety and operational concerns. From an operational perspective, the hydrostatic pressure exerted by a column of acid in the tubing can be difficult for an older well to unload. Volume and pressure calculations help prevent reservoir damage by not killing the well and corrosion inhibitors added to hydrochloric acid reduce risk of tubular degradation. The reaction between calcium carbonate and iron sulfide with HCl can produce an elevated amount of CO2 and H2S, respectively which are not present in normal operations. Additional safety measures taken in these operationally induced sour conditions are detailed in this paper.
Through engineering efforts, along with computational fluid dynamics and field trials, several iterations of acid plungers designed specifically for scale remediation have been developed and successfully implemented. The role of the acid plunger is to provide temporary liquid holdup of the acid while simultaneously gauging the tubing walls as the acid column leaks by. The liquid holdup from the acid plunger provides sufficient contact time between the acid and scale buildup, especially in the vertical section of the well. The development of this process has resulted in the successful remediation of 62 wells to date. This includes using variations of this technique to free downhole tools stuck in scale. Additionally, there has not been reoccurring scaling deposits detected that would impede plunger lift operations.
Further research for this project included trialing a corrosion-resistant stainless-steel slickline containing large amounts of nickel. This line, in conjunction with acid, provides additional versatility and more efficient means of scale removal. Future methods of scale remediation involve utilizing synthetic "green" acids and large volume lateral acid batches