Fit-For-Service Qualification of Existing 13Cr Tubulars in Mildly Sour Wells Based on Thermodynamic Approach

Abstract

Most of the existing wells in a giant oil offshore field in Abu Dhabi are equipped with L80-13Cr corrosion resistant alloy (CRA) tubulars to provide protection from CO2 corrosion due to sweet nature of reservoir. Recently, some of the wells are showing a presence of mild H2S due to unexpected reservoir souring or other geological changes. The presence of H2S in production fluids raises concerns about sulfide-stress-cracking (SSC) of L80-13Cr. As L80-13Cr CRA has been known to have limited SSC resistance, it is important to understand the maximum acceptable limit of H2S in production fluids for safe operation. Industry standards such as ISO15156/ NACE MR0175 and NORSOK-M-001 recommend safe acceptable limits of H2S for 13Cr tubular materials based on the partial pressure of H2S. However, these approaches do not take into account the effect of temperature, or non-ideal gas behavior of H2S at high pressure. Pressure, temperature, salinity and pH in the wellbore impact the solubility and chemical behavior of H2S in the water phase which defines the corrosive environment to which the material is exposed. Therefore, it is important to include non-ideal gas and solution behaviors in order to define the acceptable limit of H2S for fitness-for-service (FFS) material evaluations. In this work the acceptable limit of H2S in the wellbore was determined using a combination of thermodynamic modeling and field corrosion data. A molecular thermodynamics approach was used to calculate pH and dissolved H2S levels in water along the production tubing length. Shut-in and production operation scenarios were simulated to identify the worst-case scenario using thermal modeling software. Furthermore, tubing inspections were conducted using a multi-finger caliper tool to identify any corrosion damage. All of this information was used to identify the acceptable limit for H2S in the wellbore. This approach to determining acceptable H2S limits will avoid unnecessary workovers and enables cost saving through continued use of existing materials. Furthermore, it supports the development of a corrosion monitoring plan, and FFS assessment of tubulars based on the wellbore environment.