Optimization of materials in hydrocarbon production

Tenaris has conducted extensive research in the corrosion resistance of tubular products. Photo courtesy of Tenaris.

Operating regimes that include carbon dioxide (CO2), hydrogen sulfide (H2S), and oxygen (O2) are the leading causes of tubular failure. A universal method of preventing this corrosion is the selection of the most appropriate material for the specified application. The optimum choice of materials is governed by a number of key parameters that include adequate mechanical properties, corrosion performance, weldability (where appropriate), availability, and cost.

Operating regimes

Material choice is governed by the nature of its application and generally falls into two categories - production and injection.

Production: Subject to system corrosivity, carbon and low-alloy steels typically offer satisfactory performance, although a growing number of corrosion resistant alloys (CRAs) or non-metallics are being used.

Injection: Subject to maintaining water quality or fully dried gas, carbon and low-alloy steels have performed well over the years. However, the use of other CRAs may prove beneficial in situations where water quality cannot be maintained, oxygen removal may not be feasible, oxygen excursions may be foreseen, or for untreated water injection. In these situations, only fully passivated alloys are suitable. This excludes families of 13%Cr (chromium), which have shown poor resistance in these applications, and potentially 22%Cr duplex stainless steel.

Hydrocarbons not containing water are non-corrosive in the temperature range encountered in production. However, hydrocarbon production (oil, gas, or gas condensate) normally is co-produced with water, and contains acidic gasses and organic acids that cause corrosion and damage.

Corrosion in hydrocarbon production manifests itself in several forms. Carbon dioxide (CO2), or “sweet corrosion,” of carbon and low-alloy steels causes most of the damage in hydrocarbon production. CO2 is usually present in produced fluids and, although it does not cause the catastrophic failure mode of cracking associated with H2S corrosion, its presence in contact with an aqueous phase can result in very high corrosion rates where the mode of attack is often highly localized (mesa corrosion). Also, the implication of CO2 corrosion is gaining significance in gas injection systems where CO2 sequestration is intended as a measure against climate change.

Hydrogen sulfide (H2S), or “sour corrosion,” which arises from exposure to wet H2S, has wide-ranging implications on the integrity of materials used in the industry. The types of damage caused by H2S fall into three principal categories:

• Hydrogen internal pressure effects
• Stress corrosion cracking (SCC)
• Damage in related environments (those containing chlorides, cyanides, etc.).

In water injection systems, the corrosion rate is dominated by oxygen content, velocity, chlorine content, solids, and operating temperature. There are several methods to predict corrosion of carbon and low-alloy steels as affected by operating conditions. Among these is the Oldfield and Todd model which is used widely to predict carbon and low-alloy steel corrosion.

Laboratory assessment

A systematic approach to developing a materials optimization strategy needs to include flexibility in extending the application regime of available alloys. This is done by realistically assessing the properties. In assessing materials performance for hydrocarbon production, the principal emphasis should be on evaluating properties that reflect long-term, in-field performance. This will lead to identifying the most appropriate, safe, and cost-effective materials for a specific duty.

The procedure to assess the suitability of materials for specific service applications should incorporate all aspects of corrosion performance and mechanical integrity in three broad categories: mechanical integrity, SCC performance, and corrosion (particularly CO2 corrosion performance).

Without a reliable prediction data set, CO2 corrosion behavior needs to be established. Laboratory testing becomes critical where erosion due to the presence of particulates is a concern. There are no industry guidelines adequately covering this situation or CO2 corrosion testing. In such testing, the overlying effects of factors such as flow regime, film formation/deposition, hydrocarbon phase, steel chemistry, surface status, and corrosion inhibitor complicate the picture.

Corrosion design for injection systems relies on available prediction models. Nevertheless, reliable testing is essential to establish the performance of materials even though no industry standard or procedure is available for this type of test.

Materials options

Several categories of alloy are used in hydrocarbon production to get successful, trouble-free operations. Effective corrosion mitigation has been achieved using conventional grades of carbon and low-alloy steels, inhibited carbon and low-alloy steels, addition of corrosion allowance, 13% Cr, or other CRAs. The choice is governed by the system corrosivity as depicted by production conditions, solution chemistry, and acidic gasses.

The industry continues to lean heavily on the extended use of carbon and low-alloy steels which are readily available in the volumes required and meet the industry’s mechanical, structural, fabrication, and cost requirements. Their technology is well developed and these steels represent an economical materials choice for many applications. However, a key issue for their effective use is poor general and CO2 corrosion performance. The harsher conditions in hydrocarbon production necessitate development of new, proprietary grades that surpass the performance of standard grades (API/ISO). For effective broadened use of carbon and low-alloy steels, two important avenues must be followed: increased H2S tolerance through enhanced manufacturing and increased resistance to CO2 corrosion through microalloying.

Table 1 outlines basic compositional chemistries and minimum yield strength of each grade. The results are the outcome of tests carried out in NACE TM0177 Solution A and clearly demonstrate discriminatory evidence on the performance and suitability of proprietary grade steels.

The new generation of low-carbon, 3%Cr steels offers improved CO2 corrosion resistance and an economical alternative to conventional carbon steels for downhole applications. Key attributes of this series of steel are its superior CO2 corrosion performance with a larger application window compared to conventional grades of carbon and low-alloy steels through the formation of a semi-protective chrome-rich corrosion layer; attainment of a steady-state of corrosion soon after exposure, and much earlier than that achieved, if at all, by carbon and low-alloy steels due to the formation of a semi-protective corrosion layer; and cost-effectiveness.

For years, 13%Cr stainless steels have been used extensively for corrosion control in sweet and mildly sour production environments. They generally are selected for their relatively high strength and moderate resistance to corrosion, and normally contain 13%Cr steel to provide a degree of passivity in hydrocarbon production and therefore, a low corrosion rate.

Supermartensitic grade (Super 13%Cr) steels recently have entered the downhole tubular market. This alloy results from adding the alloying elements of nickel, molybdenum, and copper to conventional API 5CT grade 13%Cr steel. These grades combine high strength and low-temperature toughness with improved corrosion resistance in sweet production conditions compared to 13%Cr.

Other corrosion-resistant alloy steels include several categories of alloy containing varying amounts of chromium, nickel, molybdenum, iron, and other alloying elements. They offer superior corrosion performance compared to other steels and rely on the formation of a passive film to render them corrosion-resistant. These include duplex stainless steels, high alloyed austenitic stainless steels, nickel-based alloys, and others. They invariably cost more than carbon and low-alloy steels, although their selection may be essential for critical applications or where other low-grade alloys do not offer adequate performance.

Materials optimization strategy

A materials optimization strategy requires the integration of key parameters to allow selection of the most suitable, safe, and economical materials option and corrosion-control procedure. The parameters captured in the present strategy take advantage of past successes and use innovative means and materials for progressive solutions. Several parameters are included in the overall strategy, which surpasses other available models. A notable example is the following:

• Corrosion Risk is defined taking into account historical data, trends of water cut, inspection and monitoring data, flow dynamics, materials data, pipe inclination, and the influence of phase ratios on the onset of oil-in-water emulsion breakout to allow significantly better indication of potential risk
• Operating Conditions cover the most influential parameters which affect materials choice, including design life, logistics of work over, temperature and pressure, strength requirements, environmental conditions, and production rate
• Corrosivity Assessment for both sweet and sour production and water injection is built through cross-referencing available models. Corrosivity of production scenarios is defined combining both dWM and CORMED numerical models thereby taking advantage of all approaches incorporating laboratory evaluations, theoretical calculations with extensive field experience, and the influential role of organic acids. Corrosivity in injection facilities is differentiated and prediction is carried out using modified models
• Erosional Velocity is included to ensure the operating regime does not lead to velocities beyond which erosion is likely
• Methodical Approach to Performance Evaluation through provision of a flexible structure is added to allow realistic testing to enable input on complementary data to provide further confidence on their application
• Window of Application encapsulates the domains within which alloys have proven track records by considering available industry-wide data, information on proprietary grades, and reliable laboratory evidence
• Whole Life Costing provides the best means to optimize capital expenditure for the highest rate of return on investment, taking account of costs associated with lost production, replacement, and residual value.

The overall approach to the optimization strategy captures these necessary steps in finalizing the materials choice. This follows a methodical route to highlight options and the most appropriate and cost-effective materials. While most of these parameters have been discussed extensively, the overriding element is the combined influence of the hydrocarbon phase and flow dynamic in which the onset of emulsion breakout may be predicted.

The onset of emulsion breakout occurs when a transition from water-in-oil emulsion to an oil-in-water phase occurs. Furthermore, the influence of organic acid in the strategy has enabled a more realistic picture of the performance in CO2-containing environments.

Conclusion

An overall materials optimization strategy combines several influential parameters. Key attributes emerging from this strategy and notable conclusions are:

1. Only through a methodical approach to material selection can a combination of suitability, appropriateness, and economy be achieved
2. A strategy that integrates past successes, present understanding of corrosion processes in hydrocarbon production, whole-life costing, and an application regime of conventional as well as proprietary grades, will allow the selection of the most suitable, safe, and economical material option and corrosion control procedure
3. Development of proprietary grades using stringent manufacturing routes has produced higher strength steels with superior tolerance to H2S.
4. Introduction of low carbon 3%Cr steels with improved CO2 corrosion resistance offers a significant commercial and industrial advantage by enlarging the window of application compared to conventional grades of carbon and low-alloy steels.