Summary
The ability to optimize the use of carbon steel in corrosive service
presents many economic advantages, including minimizing the use of expensive
corrosion-resistant alloys (CRAs), reducing well count by enabling optimized
large-bore completions, and eliminating unnecessary offshore pipelines and
facilities. An integrated approach to corrosion modeling and testing can
enable reliable extension of carbon steel application.
The integrated approach to predicting corrosion has five primary
elements:
- Rigorously establish the environmental conditions by conducting
thermodynamic and compositional hydraulic analyses, and characterize how these
conditions are expected to change over time.
- Identify and model the local environmental conditions and the types of
corrosion that are expected to occur (e.g., weight loss, pitting, environmental
cracking), including sensitivity and upset cases.
- Conduct realistic corrosion tests under the identified field conditions by
simulating brine chemistry, dissolved acid gas concentrations, hydrocarbon
effects, fluid shear stresses, and flow regimes in appropriate laboratory
equipment. Specialized laboratory test apparatus, such as a
large-diameter sour multiphase flow loop and large-volume high-pressure
high-temperature autoclave test cells, has been designed and constructed to
ensure proper replication of field conditions.
- Mathematically extrapolate the results of the laboratory tests to the
field, enabling calculation of expected tubular life.
- Conduct life-cycle cost analysis.
This paper will describe how this integrated approach to predicting
corrosion has been used to evaluate the use of carbon steel in oil and gas
production environments. Emphasis will be placed on the prediction of
pitting corrosion in H2S-containing environments.
Introduction
From the material selection perspective, the design decision for corrosive
service is generally between CRAs and carbon steel, with or without
inhibition. The ability to optimize the use of carbon steel in corrosive
service often presents economic advantages, primarily through a significant
reduction in capital expenditures, albeit with higher operating costs. The
ability to run wet gas and full wellstream pipelines and flowlines can further
reduce costs by eliminating offshore dehydration facilities. Carbon steel
often has other advantages, such as availability in a larger variety of product
sizes, grades, and forms. However, the technical feasibility of carbon
steel needs to be confidently established in order for it to be considered a
viable option. The methods by which carbon steel (with or without
inhibition) and other material options are evaluated are critical to ensure
long-term reliability with minimal life-cycle cost.
An integrated approach to materials and corrosion engineering is necessary
to enable the identification of high-impact opportunities and the optimization
of both capital expenditures (CAPEX) and operating expenditures (OPEX) over an
asset's life. This approach requires a scientific methodology based on
high-quality data that ensures replication of field conditions as nearly as
possible in the laboratory and properly considers all primary material
degradation mechanisms. A key element of this integrated approach is to
have multidisciplinary teams evaluate opportunities to select carbon steel for
nontraditional applications. Collectively, reservoir, drilling,
subsurface, and facility engineers provide input data, such as reservoir or
process modeling results, to establish the basis for the corrosion prediction
and materials selection study, consistent with the overall project development
plan. Materials and corrosion engineers and chemists work in concert to
develop and execute carefully designed laboratory test programs, the results of
which are interpreted by the multidisciplinary team. Both design and
operations personnel participate in the final material selection decision.
© 2007. Society of Petroleum Engineers
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