As the world-wide search is turning to deeper reservoirs an increasing number of situations are being encountered
where corrosive production environments are present. In many of these cases often significant amounts of hydrogen
sulphide, carbon dioxide and brine are present with oil and gas production. These crudes show, therefore a high
corrosivity with respect to general corrosion and stress corrosion cracking by sulphides ( SSCC ), by chloride
( CSCC) or their combined action.
In addition, other factors such high pressures and temperatures can complicate the material selection process. In fact,
the mechanical requirements for material used for production equipment increase with well depth because of the
greater hangoff loads and pressure; while the elevated temperatures have detrimental influence on mechanical
properties. Under these circumstances CRA materials may offer a valid alternative to conventional methods of
corrosion control. Based on that the use of corrosion-resistant alloy in oil field has substantially increased during the
last years.
With the term CRA is intended a metal that achieves a high corrosion resistance by means of alloying. A variety of
CRA materials are now available for tubing. Table 1 shows some of the commonly use for oil and gas production
application. Depending on the environment the CRA choice could range from AISI 420 ( 13% Chrome) for CO2
service to titanium alloys for very severe applications. The first topic of discussion will be manufacturing process,
with some discussion on how the different processes can influence the final product performances.
For manufacturing the CRA alloys there are essentially two processes. Group 1 comprises martensitic and martensicferritic
stainless steel, they are manufactured in a manner similar to carbon steel. The alloy is melted in an electric
furnace then it is cast into ingots. The ingot is forged to form a billet that is heated to a suitable forging temperature,
pierced and hot rolled to form a pipe. In order to achieve the mechanical properties, the pipe then is quenched and
tempered.
Groups 2, 3 and 4 alloys, such as duplex stainless steel and austenitic-nickel-base alloys, are fabricated in different
manner. After melting the material can mold to form an ingot or it can be continuously cast. The ingot is then forged
into billets that are extruded by the back-extrusion press. In the majority of cases these grades are required in
relatively high strengths which require the alloys to be cold worked. This cold work is performed on either cold draw
benches or in a cold pilger mill. Several passes on the draw bench may be necessary to achieve the correct strength
while in general only a sizing pass and the finishing pass are requested on the pilger mill.
The extrusion process, particularly when associated with cold working, is costly and time-consuming tube-making
process. Table 2 reassumes the various manufacturing process.
The problem of material selection may involve several factors like the high strength requirements combined with high
corrosion resistance of the material.
A chemical analyses of the produced fluids is generally required for evaluation of the corrosive components as
hydrogen sulphide, carbon dioxide and chlorides. Other components like scaling potential, water production,
temperature profile, pressure profile and stresses on the tubulars have also to be considered. If no water is present
there will be no corrosion and the material selection is simple. However, no well can be designed on the basis that it
will always be dry and therefore the material selection shall take into account the water production and the material
must be selected accordingly.
The proportion of H2S and CO2 present in the water are also important ; generally it is ignored but should be taken
into account where the well conditions are severe for a particular alloy and to make a conservative design decision
would involve the selection of a much more expensive tubular.
Other points to be considered are the potential for scale and the presence of asphaltene associated with production.
Scale will provide a barrier between the tubulars and the aggressive fluids reducing therefore the velocity of corrosion
process, but pitting and crevice can occur beneath the scale and damage the tubular in its integrity. For our scope we
assume that water and chloride are always present therefore a number of different scenarios can be discussed.
Experience has shown that manufacturing process qualification achieved by means of a pre-production has been
necessary for particular material/ process to provide evidence of the performance characteristics of the product and
the adequacy of the manufacturer to produce tubular that meet the user’s performance guidelines. Qualification of a
size and grade doesn’t mean the process is automatically qualified for all the sizes.
Pre-production discussion and a continuing dialogue with manufacturers are generally necessary to reach a
satisfactory quality level. Inspectors should be used to assure manufacturers compliance to the technical specification
during extrusion, microstucure evaluation, mechanical and NDE testing.
Some operational experience on CRA’s suggest to prepare proper storing and handling procedures to minimise the
galling during make/break of the connection.
Acidizing is another operation that can cause problems. Generally Group 1 and 2 can suffer severe corrosion attack
from mud acid even in presence of inhibitors. Extensive laboratories tests have demonstrated that superaustenitic
stainless steel is much more resistant than duplex steel during stimulation. To reduce the risk it is important to select
the appropriate inhibitor package.
Source:
- http://www.gruppofrattura.it/ocs/index.php/cigf/igf14/paper/viewFile/564/11238
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