Corrosion behavior of 13Cr casing steel in cement-synthetic pore solution exposed to high pressure CO2 and H2S

Abstract

As deeper wells drilled for oil and gas extraction encounter higher pressures, temperatures, and concentrations in the downhole environment, it becomes increasingly important to consider the integrity of the casing that provides both support and isolation of the well. These casings are commonly cemented to provide better sealing, and the alkaline environment provided by the pore water at the cement-casing interface typically puts the steel in a passive region. However, exposure to acid gases can cause degradation of the cement and thus lower the pH at the steel surface. Further, many formation waters are brines containing high concentration of chlorides. This increases the risk of corrosion and failure of the well casing. With these issues in mind, a corrosion study was initiated to investigate the corrosion behavior of grade L-80 casing steel in cement synthetic pore solutions in contact with CO2 and H2S. Experimental approaches consisted of two stages. In the first stage, a Class H well cement was exposed to a 5 wt% NaCl brine in contact with CO2 and H2S at 85 °C for 200 hours. At the end of the exposure time, the pore water of the exposed cement was sampled and analyzed qualitatively and quantitatively. This chemical analysis was used for preparing a cement synthetic pore solution (CSPS) for corrosion testing. In situ electrochemical measurements, performed for the L-80 steel samples in the CSPS at 85 °C, included linear polarization resistance, electrochemical impedance spectroscopy, cyclic voltammetry, and Tafel analysis. Additionally, ex situ surface analyses using scanning electron microscopy, X-ray energy dispersive spectroscopy, and X-ray diffraction were performed for the corroded samples. Mass loss measurement coupons were also used to confirm the electrochemical results.