Summary
Erosion-corrosion is a severe problem in oil and gas producing wells because
of high flow velocity and sand particles entrained in the system. Using
corrosion inhibitors to reduce erosion-corrosion has recently been proposed as
an economically viable solution for carbon steel piping systems (Smart 1991).
In the literature, most studies using inhibitors to mitigate corrosion have
been conducted under stagnant or low flow rate (<1 m/s) conditions.
However, severe erosion-corrosion may occur when inhibitor films are removed
from the steel by high liquid-shear forces under actual oil and gas production
conditions.
The inhibiting effect of a commercial inhibitor [named inhibitor A (Wang et
al. 2005)] on the behavior of carbon steel in CO2-saturated media
has been examined under high flow velocity with sand by electrochemical and
gravimetric measurements. Experiments were performed using a rotating cylinder
electrode (RCE). The AC impedance technique was used to study the effect of
inhibitor adsorption onto the metal surface. The study clearly reveals that the
addition of the inhibitor moves the corrosion potential towards positive values
and reduces the corrosion rate. Changes in impedance parameters
(Rct and Cdl ) are indicative of adsorption
of inhibitor on the metal surface leading to the formation of protective
films.
Introduction
Carbon dioxide (CO2) corrosion is one of the most prevalent
attack species causing corrosion in oil and gas production. CO2
dissolves in water to form carbonic acid (H2CO3), which
is corrosive to carbon steel. Sand can also be entrained into to a system,
resulting in erosion-corrosion attack of the pipe steel. Even small amounts of
sand can increase the corrosion rate dramatically (Mclaury et al. 1995). The
erosion-corrosion behavior of carbon steel in slurry conditions is, therefore,
a major industrial practical significant.
Erosion-corrosion is a tribo-corrosion material-loss mechanism. There are
mechanical, electrochemical, and interactive processes (between mechanical and
electrochemical) involved. Therefore, the material loss includes chemical
dissolution (which can be increased by mass transfer increases at the surface),
mechanical erosion (caused by fluid flow and /or impingement of particles on
the pipe wall), and electrochemical corrosion enhanced erosion and vice versa
(Burstein and Sasaki 2001; Neville and Hu 2001).
It is possible to reduce erosion-corrosion by using corrosion-resistant
materials (such as stainless steel tubing), but the initial cost is very high.
Improving the effectiveness and efficiency of corrosion inhibitors would be a
cost-effective option. Some organic compounds have shown good inhibition
properties in protection of steel against corrosion and erosion-corrosion in
CO2 media (Dougherty 1998; Mclaury et al. 1995). The life of
pipeline can be extended with the use of effective organic inhibitors.
Organic, adsorption-type corrosion inhibitors intended for use in the oil
and gas industry, generally act by adsorption on the metal surface. This
phenomenon is strongly dependent on the nature and surface charge of the metal,
the type of aggressive electrolyte, and the chemical structure of the inhibitor
(de Damborenea et al. 1997). The most important prerequisite for compounds to
be efficient inhibitors under erosion-corrosion condition is that they should
chemisorb on the metal surface forming a barrier layer (Durnie et al.
2001).
Corrosion products produced by CO2 corrosion are important in the
mechanism, kinetics, and pattern of CO2 corrosion. The
FeCO3 corrosion product film has been intensively investigated
during the last two decades (Scmitt 1984; Palacios and Shadley 1991). Corrosion
products scales with higher crystallinity and smaller crystal size give better
protection (Tan et al. 1994). One question is what effect the inhibitor can
have if it is incorporated with the corrosion products leading to a more
densely-packed network with less porosity and higher stability.
AC impedance has been widely used as an appropriate method for corrosion
studies for the determination of corrosion rates and provides important
information to characterize the corrosion processes (Tan et al. 1994; Cao
1996). However, most studies are on the basis of static or low flow rate
conditions; it is not common for AC impedance to be used to study inhibitors
under multiphase turbulent erosion-corrosion conditions. It has been shown by
different researchers that in the presence of an inhibitor film, the electrode
behavior depends on the hydrodynamic conditions (Nesic et al. 1997; Heeg et al.
1998). There is less information of hydrodynamic parameter for multiphase flow
with sand. The expression by Gabe and Walsh (1983) for hydrodynamic
characteristics of RCE of liquid phase was used to produce reference
hydrodynamic parameters. In this work, the impedance technique is employed
along with DC linear polarisation (LPR) to determine the corrosion component of
erosion-corrosion. DC LPR is used to assess the changes of impedance parameters
that are vital in understanding the mechanism of inhibition. The results show
that under the experimental condition of this paper, Nyquist plots mainly
exhibit poorly separated semicircles indicating that the corrosion of steel is
mainly controlled by charge transfer process.
Previous work showed that inhibitors can provide protection to carbon steel
under erosion-corrosion conditions (Wang et al. 1991). This paper is an
extension to the previous paper and focuses on corrosion studies of one of the
inhibitors under erosion-corrosion conditions. The aim of this work is to
provide an understanding of the influence of high fluid shear stresses and
multiple phase flow with sand on the efficiency of the inhibitor and to
elucidate the inhibition mechanism of corrosion of carbon steel under
CO2-saturated slurry conditions.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
13 June 2006
- Meeting paper published:
30 May 2006
- Manuscript approved:
7 November 2006
- Version of record:
20 May 2008
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