Abstract

This paper discusses an approach used to assess liquid film erosion/corrosion effects in the tubing strings of sour, high-rate, wet gas producers. This was done as an alternative to API RP 14E, which utilises an empirical erosional velocity factor "C" to estimate maximum velocity limits to minimise the potential for tubing metal loss from erosional effects.

Many RasGas wells are completed with a full L-80 carbon steel or a combination L-80/Corrosion Resistant Alloy (CRA) production string. Once on production, a thin iron sulfide scale develops on the tubing wall significantly retarding the rate of metal loss due to internal corrosion. However, shear stresses generated from the condensate/water film flowing along the tubing wall could potentially remove this protective iron sulfide coating and expose fresh metal to much higher corrosion rates. This paper describes the approach adopted to assess the magnitude of shear stress created across a range of flow conditions including well production rates, fluid properties, and completion sizes using transient 1D flow simulation and more detailed 3D computational fluid dynamics modelling. The results will be used to design future laboratory experiments to assess the effect of these stresses on the integrity and effectiveness of the iron sulfide scale in reducing corrosion rates.

Introduction

RasGas Company Limited initiated a study to investigate the potential effect of high flow velocity on the stability of iron sulfide scales formed in carbon steel tubing during late field life when the wells will be required to maintain high rates at low wellhead pressures. Higher flow velocities will generate higher shear stresses that might be capable of stripping iron sulfide scale leading to high general corrosion rate and/or severe localised corrosion. The former will be expected in the case of uniform scale erosion while the latter is expected in the case of non-uniform scale erosion.

In order to meet the study objective, the first step is to understand the flow behaviour (wall shear stress, flow regime and temperature distribution) in the tubing string. Four different tubing strings were investigated: a 7-inch monobore-vertical (MB-V) and deviated (MB-D), a 9-5/8 inch x 7 inch bigbore-vertical (BB-V) and deviated (BB-D). The completion schemes for MB and BB wells are illustrated in Figure 1.

1-D flow simulations using a Transient Flow Simulation Software (TFSS) were conducted to determine the flow regime, wall shear stress, and temperature distribution and the portion of liquid flowing as wall film in the tubing strings. 3-D Computational Fluid Dynamics (CFD) simulations were also conducted to determine the wall shear stress at specific locations (X-overs) where there is a change in internal diameter of the tubing string that is greater than 10%. The objective of these simulations was to obtain shear stress predictions that would be utilised in a customised test loop to establish the stability of the iron sulfide film. If this film is not stable at the predicted shear stresses, then the wellbore integrity management programme would have to be focused on corrosion protection since historical well corrosion monitoring data has established high corrosion rate in tubing strings where the iron sulfide film has not been formed.

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