Chelating Chemicals: A Solution to End the Battle with Calcium Sulfate Scale

Abstract

AbstractCalcium sulfate inorganic scale deposition is a major challenge that can block tubulars and hinder flow assurance during hydrocarbon production and water injection operations. This scale can form when high sulfate water gets in contact with water containing high content of calcium ions. Dissolving calcium sulfate is problematic due to its low solubility in water and common inorganic acids. Many scale dissolvers exist in the industry; however, the dissolving performance varies significantly when applied to calcium sulfate field samples. The main objective of this study is to evaluate the efficacy of three commercial calcium sulfate dissolvers for possible applications in the field.Field scale samples were characterized using X-ray diffraction and scanning electron microscopy with energy dispersive spectroscopy (XRD/SEM-EDS) to determine its composition. The HPHT aging cell was used to conduct the experiments at a temperature ranging from 77 to 300°F to evaluate the performance of several calcium sulfate dissolvers. The testing involved static dissolution tests to identify the optimal dissolver, scale dissolver to inorganic scale ratio, temperature influence, surface area influence, and the appropriate soaking time. Compatibility and thermal stability tests were also explored to avoid formation damage issues during the removal treatment. Additionally, corrosion tests were performed using low carbon steel metal coupons to assess the dissolver corrosivity at 200 and 300°F.The scale dissolvers compatibility and thermal stability were presented up to 300°F. The performance of the dissolvers generally increased as the temperature and soaking time increased. The scale dissolver chemistries were tested at high pH conditions and most of the tests exhibited a low corrosion rate of < 0.05 lb/ft2 with no significant pitting at 200°F or 300°F for the duration of the soaking time. One of the tested scale dissolvers failed the corrosion test at 300°F and two dissolvers thermally degraded when exposed to high temperature.This work is derived from testing actual inorganic scale field samples and shares the difficulty of dissolving such scale samples. The work also systematically compares three commercial scale dissolvers to resolve this issue.